feat: introduce codex_execpolicy crate for defining "safe" commands (#634)

As described in detail in `codex-rs/execpolicy/README.md` introduced in
this PR, `execpolicy` is a tool that lets you define a set of _patterns_
used to match [`execv(3)`](https://linux.die.net/man/3/execv)
invocations. When a pattern is matched, `execpolicy` returns the parsed
version in a structured form that is amenable to static analysis.

The primary use case is to define patterns match commands that should be
auto-approved by a tool such as Codex. This supports a richer pattern
matching mechanism that the sort of prefix-matching we have done to
date, e.g.:


5e40d9d221/codex-cli/src/approvals.ts (L333-L354)

Note we are still playing with the API and the `system_path` option in
particular still needs some work.

codex-rs/execpolicy/src/arg_matcher.rs

@@ -0,0 +1,118 @@
+#![allow(clippy::needless_lifetimes)]
+
+use crate::arg_type::ArgType;
+use crate::starlark::values::ValueLike;
+use allocative::Allocative;
+use derive_more::derive::Display;
+use starlark::any::ProvidesStaticType;
+use starlark::values::starlark_value;
+use starlark::values::string::StarlarkStr;
+use starlark::values::AllocValue;
+use starlark::values::Heap;
+use starlark::values::NoSerialize;
+use starlark::values::StarlarkValue;
+use starlark::values::UnpackValue;
+use starlark::values::Value;
+
+/// Patterns that lists of arguments should be compared against.
+#[derive(Clone, Debug, Display, Eq, PartialEq, NoSerialize, ProvidesStaticType, Allocative)]
+#[display("{}", self)]
+pub enum ArgMatcher {
+    /// Literal string value.
+    Literal(String),
+
+    /// We cannot say what type of value this should match, but it is *not* a file path.
+    OpaqueNonFile,
+
+    /// Required readable file.
+    ReadableFile,
+
+    /// Required writeable file.
+    WriteableFile,
+
+    /// Non-empty list of readable files.
+    ReadableFiles,
+
+    /// Non-empty list of readable files, or empty list, implying readable cwd.
+    ReadableFilesOrCwd,
+
+    /// Positive integer, like one that is required for `head -n`.
+    PositiveInteger,
+
+    /// Bespoke matcher for safe sed commands.
+    SedCommand,
+
+    /// Matches an arbitrary number of arguments without attributing any
+    /// particular meaning to them. Caller is responsible for interpreting them.
+    UnverifiedVarargs,
+}
+
+impl ArgMatcher {
+    pub fn cardinality(&self) -> ArgMatcherCardinality {
+        match self {
+            ArgMatcher::Literal(_)
+            | ArgMatcher::OpaqueNonFile
+            | ArgMatcher::ReadableFile
+            | ArgMatcher::WriteableFile
+            | ArgMatcher::PositiveInteger
+            | ArgMatcher::SedCommand => ArgMatcherCardinality::One,
+            ArgMatcher::ReadableFiles => ArgMatcherCardinality::AtLeastOne,
+            ArgMatcher::ReadableFilesOrCwd | ArgMatcher::UnverifiedVarargs => {
+                ArgMatcherCardinality::ZeroOrMore
+            }
+        }
+    }
+
+    pub fn arg_type(&self) -> ArgType {
+        match self {
+            ArgMatcher::Literal(value) => ArgType::Literal(value.clone()),
+            ArgMatcher::OpaqueNonFile => ArgType::OpaqueNonFile,
+            ArgMatcher::ReadableFile => ArgType::ReadableFile,
+            ArgMatcher::WriteableFile => ArgType::WriteableFile,
+            ArgMatcher::ReadableFiles => ArgType::ReadableFile,
+            ArgMatcher::ReadableFilesOrCwd => ArgType::ReadableFile,
+            ArgMatcher::PositiveInteger => ArgType::PositiveInteger,
+            ArgMatcher::SedCommand => ArgType::SedCommand,
+            ArgMatcher::UnverifiedVarargs => ArgType::Unknown,
+        }
+    }
+}
+
+pub enum ArgMatcherCardinality {
+    One,
+    AtLeastOne,
+    ZeroOrMore,
+}
+
+impl ArgMatcherCardinality {
+    pub fn is_exact(&self) -> Option<usize> {
+        match self {
+            ArgMatcherCardinality::One => Some(1),
+            ArgMatcherCardinality::AtLeastOne => None,
+            ArgMatcherCardinality::ZeroOrMore => None,
+        }
+    }
+}
+
+impl<'v> AllocValue<'v> for ArgMatcher {
+    fn alloc_value(self, heap: &'v Heap) -> Value<'v> {
+        heap.alloc_simple(self)
+    }
+}
+
+#[starlark_value(type = "ArgMatcher")]
+impl<'v> StarlarkValue<'v> for ArgMatcher {
+    type Canonical = ArgMatcher;
+}
+
+impl<'v> UnpackValue<'v> for ArgMatcher {
+    type Error = starlark::Error;
+
+    fn unpack_value_impl(value: Value<'v>) -> starlark::Result<Option<Self>> {
+        if let Some(str) = value.downcast_ref::<StarlarkStr>() {
+            Ok(Some(ArgMatcher::Literal(str.as_str().to_string())))
+        } else {
+            Ok(value.downcast_ref::<ArgMatcher>().cloned())
+        }
+    }
+}

codex-rs/execpolicy/src/arg_resolver.rs

@@ -0,0 +1,194 @@
+use serde::Serialize;
+
+use crate::arg_matcher::ArgMatcher;
+use crate::arg_matcher::ArgMatcherCardinality;
+use crate::error::Error;
+use crate::error::Result;
+use crate::valid_exec::MatchedArg;
+
+#[derive(Clone, Debug, Eq, PartialEq, Serialize)]
+pub struct PositionalArg {
+    pub index: usize,
+    pub value: String,
+}
+
+pub fn resolve_observed_args_with_patterns(
+    program: &str,
+    args: Vec<PositionalArg>,
+    arg_patterns: &Vec<ArgMatcher>,
+) -> Result<Vec<MatchedArg>> {
+    // Naive matching implementation. Among `arg_patterns`, there is allowed to
+    // be at most one vararg pattern. Assuming `arg_patterns` is non-empty, we
+    // end up with either:
+    //
+    // - all `arg_patterns` in `prefix_patterns`
+    // - `arg_patterns` split across `prefix_patterns` (which could be empty),
+    //   one `vararg_pattern`, and `suffix_patterns` (which could also empty).
+    //
+    // From there, we start by matching everything in `prefix_patterns`.
+    // Then we calculate how many positional args should be matched by
+    // `suffix_patterns` and use that to determine how many args are left to
+    // be matched by `vararg_pattern` (which could be zero).
+    //
+    // After assocating positional args with `vararg_pattern`, we match the
+    // `suffix_patterns` with the remaining args.
+    let ParitionedArgs {
+        num_prefix_args,
+        num_suffix_args,
+        prefix_patterns,
+        suffix_patterns,
+        vararg_pattern,
+    } = partition_args(program, arg_patterns)?;
+
+    let mut matched_args = Vec::<MatchedArg>::new();
+
+    let prefix = get_range_checked(&args, 0..num_prefix_args)?;
+    let mut prefix_arg_index = 0;
+    for pattern in prefix_patterns {
+        let n = pattern.cardinality().is_exact().unwrap();
+        for positional_arg in &prefix[prefix_arg_index..prefix_arg_index + n] {
+            let matched_arg = MatchedArg::new(
+                positional_arg.index,
+                pattern.arg_type(),
+                &positional_arg.value.clone(),
+            )?;
+            matched_args.push(matched_arg);
+        }
+        prefix_arg_index += n;
+    }
+
+    if num_suffix_args > args.len() {
+        return Err(Error::NotEnoughArgs {
+            program: program.to_string(),
+            args,
+            arg_patterns: arg_patterns.clone(),
+        });
+    }
+
+    let initial_suffix_args_index = args.len() - num_suffix_args;
+    if prefix_arg_index > initial_suffix_args_index {
+        return Err(Error::PrefixOverlapsSuffix {});
+    }
+
+    if let Some(pattern) = vararg_pattern {
+        let vararg = get_range_checked(&args, prefix_arg_index..initial_suffix_args_index)?;
+        match pattern.cardinality() {
+            ArgMatcherCardinality::One => {
+                return Err(Error::InternalInvariantViolation {
+                    message: "vararg pattern should not have cardinality of one".to_string(),
+                });
+            }
+            ArgMatcherCardinality::AtLeastOne => {
+                if vararg.is_empty() {
+                    return Err(Error::VarargMatcherDidNotMatchAnything {
+                        program: program.to_string(),
+                        matcher: pattern,
+                    });
+                } else {
+                    for positional_arg in vararg {
+                        let matched_arg = MatchedArg::new(
+                            positional_arg.index,
+                            pattern.arg_type(),
+                            &positional_arg.value.clone(),
+                        )?;
+                        matched_args.push(matched_arg);
+                    }
+                }
+            }
+            ArgMatcherCardinality::ZeroOrMore => {
+                for positional_arg in vararg {
+                    let matched_arg = MatchedArg::new(
+                        positional_arg.index,
+                        pattern.arg_type(),
+                        &positional_arg.value.clone(),
+                    )?;
+                    matched_args.push(matched_arg);
+                }
+            }
+        }
+    }
+
+    let suffix = get_range_checked(&args, initial_suffix_args_index..args.len())?;
+    let mut suffix_arg_index = 0;
+    for pattern in suffix_patterns {
+        let n = pattern.cardinality().is_exact().unwrap();
+        for positional_arg in &suffix[suffix_arg_index..suffix_arg_index + n] {
+            let matched_arg = MatchedArg::new(
+                positional_arg.index,
+                pattern.arg_type(),
+                &positional_arg.value.clone(),
+            )?;
+            matched_args.push(matched_arg);
+        }
+        suffix_arg_index += n;
+    }
+
+    if matched_args.len() < args.len() {
+        let extra_args = get_range_checked(&args, matched_args.len()..args.len())?;
+        Err(Error::UnexpectedArguments {
+            program: program.to_string(),
+            args: extra_args.to_vec(),
+        })
+    } else {
+        Ok(matched_args)
+    }
+}
+
+#[derive(Default)]
+struct ParitionedArgs {
+    num_prefix_args: usize,
+    num_suffix_args: usize,
+    prefix_patterns: Vec<ArgMatcher>,
+    suffix_patterns: Vec<ArgMatcher>,
+    vararg_pattern: Option<ArgMatcher>,
+}
+
+fn partition_args(program: &str, arg_patterns: &Vec<ArgMatcher>) -> Result<ParitionedArgs> {
+    let mut in_prefix = true;
+    let mut partitioned_args = ParitionedArgs::default();
+
+    for pattern in arg_patterns {
+        match pattern.cardinality().is_exact() {
+            Some(n) => {
+                if in_prefix {
+                    partitioned_args.prefix_patterns.push(pattern.clone());
+                    partitioned_args.num_prefix_args += n;
+                } else {
+                    partitioned_args.suffix_patterns.push(pattern.clone());
+                    partitioned_args.num_suffix_args += n;
+                }
+            }
+            None => match partitioned_args.vararg_pattern {
+                None => {
+                    partitioned_args.vararg_pattern = Some(pattern.clone());
+                    in_prefix = false;
+                }
+                Some(existing_pattern) => {
+                    return Err(Error::MultipleVarargPatterns {
+                        program: program.to_string(),
+                        first: existing_pattern,
+                        second: pattern.clone(),
+                    });
+                }
+            },
+        }
+    }
+
+    Ok(partitioned_args)
+}
+
+fn get_range_checked<T>(vec: &[T], range: std::ops::Range<usize>) -> Result<&[T]> {
+    if range.start > range.end {
+        Err(Error::RangeStartExceedsEnd {
+            start: range.start,
+            end: range.end,
+        })
+    } else if range.end > vec.len() {
+        Err(Error::RangeEndOutOfBounds {
+            end: range.end,
+            len: vec.len(),
+        })
+    } else {
+        Ok(&vec[range])
+    }
+}

codex-rs/execpolicy/src/arg_type.rs

@@ -0,0 +1,87 @@
+#![allow(clippy::needless_lifetimes)]
+
+use crate::error::Error;
+use crate::error::Result;
+use crate::sed_command::parse_sed_command;
+use allocative::Allocative;
+use derive_more::derive::Display;
+use serde::Serialize;
+use starlark::any::ProvidesStaticType;
+use starlark::values::starlark_value;
+use starlark::values::StarlarkValue;
+
+#[derive(Debug, Clone, Display, Eq, PartialEq, ProvidesStaticType, Allocative, Serialize)]
+#[display("{}", self)]
+pub enum ArgType {
+    Literal(String),
+    /// We cannot say what this argument represents, but it is *not* a file path.
+    OpaqueNonFile,
+    /// A file (or directory) that can be expected to be read as part of this command.
+    ReadableFile,
+    /// A file (or directory) that can be expected to be written as part of this command.
+    WriteableFile,
+    /// Positive integer, like one that is required for `head -n`.
+    PositiveInteger,
+    /// Bespoke arg type for a safe sed command.
+    SedCommand,
+    /// Type is unknown: it may or may not be a file.
+    Unknown,
+}
+
+impl ArgType {
+    pub fn validate(&self, value: &str) -> Result<()> {
+        match self {
+            ArgType::Literal(literal_value) => {
+                if value != *literal_value {
+                    Err(Error::LiteralValueDidNotMatch {
+                        expected: literal_value.clone(),
+                        actual: value.to_string(),
+                    })
+                } else {
+                    Ok(())
+                }
+            }
+            ArgType::ReadableFile => {
+                if value.is_empty() {
+                    Err(Error::EmptyFileName {})
+                } else {
+                    Ok(())
+                }
+            }
+            ArgType::WriteableFile => {
+                if value.is_empty() {
+                    Err(Error::EmptyFileName {})
+                } else {
+                    Ok(())
+                }
+            }
+            ArgType::OpaqueNonFile | ArgType::Unknown => Ok(()),
+            ArgType::PositiveInteger => match value.parse::<u64>() {
+                Ok(0) => Err(Error::InvalidPositiveInteger {
+                    value: value.to_string(),
+                }),
+                Ok(_) => Ok(()),
+                Err(_) => Err(Error::InvalidPositiveInteger {
+                    value: value.to_string(),
+                }),
+            },
+            ArgType::SedCommand => parse_sed_command(value),
+        }
+    }
+
+    pub fn might_write_file(&self) -> bool {
+        match self {
+            ArgType::WriteableFile | ArgType::Unknown => true,
+            ArgType::Literal(_)
+            | ArgType::OpaqueNonFile
+            | ArgType::PositiveInteger
+            | ArgType::ReadableFile
+            | ArgType::SedCommand => false,
+        }
+    }
+}
+
+#[starlark_value(type = "ArgType")]
+impl<'v> StarlarkValue<'v> for ArgType {
+    type Canonical = ArgType;
+}

codex-rs/execpolicy/src/default.policy

@@ -0,0 +1,202 @@
+"""
+define_program() supports the following arguments:
+- program: the name of the program
+- system_path: list of absolute paths on the system where program can likely be found
+- option_bundling (PLANNED): whether to allow bundling of options (e.g. `-al` for `-a -l`)
+- combine_format (PLANNED): whether to allow `--option=value` (as opposed to `--option value`)
+- options: the command-line flags/options: use flag() and opt() to define these
+- args: the rules for what arguments are allowed that are not "options"
+- should_match: list of command-line invocations that should be matched by the rule
+- should_not_match: list of command-line invocations that should not be matched by the rule
+"""
+
+define_program(
+    program="ls",
+    system_path=["/bin/ls", "/usr/bin/ls"],
+    options=[
+        flag("-1"),
+        flag("-a"),
+        flag("-l"),
+    ],
+    args=[ARG_RFILES_OR_CWD],
+)
+
+define_program(
+    program="cat",
+    options=[
+        flag("-b"),
+        flag("-n"),
+        flag("-t"),
+    ],
+    system_path=["/bin/cat", "/usr/bin/cat"],
+    args=[ARG_RFILES],
+    should_match=[
+        ["file.txt"],
+        ["-n", "file.txt"],
+        ["-b", "file.txt"],
+    ],
+    should_not_match=[
+        # While cat without args is valid, it will read from stdin, which
+        # does not seem appropriate for our current use case.
+        [],
+        # Let's not auto-approve advisory locking.
+        ["-l", "file.txt"],
+    ]
+)
+
+define_program(
+    program="cp",
+    options=[
+        flag("-r"),
+        flag("-R"),
+        flag("--recursive"),
+    ],
+    args=[ARG_RFILES, ARG_WFILE],
+    system_path=["/bin/cp", "/usr/bin/cp"],
+    should_match=[
+        ["foo", "bar"],
+    ],
+    should_not_match=[
+        ["foo"],
+    ],
+)
+
+define_program(
+    program="head",
+    system_path=["/bin/head", "/usr/bin/head"],
+    options=[
+        opt("-c", ARG_POS_INT),
+        opt("-n", ARG_POS_INT),
+    ],
+    args=[ARG_RFILES],
+)
+
+printenv_system_path = ["/usr/bin/printenv"]
+
+# Print all environment variables.
+define_program(
+    program="printenv",
+    args=[],
+    system_path=printenv_system_path,
+    # This variant of `printenv` only allows zero args.
+    should_match=[[]],
+    should_not_match=[["PATH"]],
+)
+
+# Print a specific environment variable.
+define_program(
+    program="printenv",
+    args=[ARG_OPAQUE_VALUE],
+    system_path=printenv_system_path,
+    # This variant of `printenv` only allows exactly one arg.
+    should_match=[["PATH"]],
+    should_not_match=[[], ["PATH", "HOME"]],
+)
+
+# Note that `pwd` is generally implemented as a shell built-in. It does not
+# accept any arguments.
+define_program(
+    program="pwd",
+    options=[
+        flag("-L"),
+        flag("-P"),
+    ],
+    args=[],
+)
+
+define_program(
+    program="rg",
+    options=[
+        opt("-A", ARG_POS_INT),
+        opt("-B", ARG_POS_INT),
+        opt("-C", ARG_POS_INT),
+        opt("-d", ARG_POS_INT),
+        opt("--max-depth", ARG_POS_INT),
+        opt("-g", ARG_OPAQUE_VALUE),
+        opt("--glob", ARG_OPAQUE_VALUE),
+        opt("-m", ARG_POS_INT),
+        opt("--max-count", ARG_POS_INT),
+
+        flag("-n"),
+        flag("-i"),
+        flag("-l"),
+        flag("--files"),
+        flag("--files-with-matches"),
+        flag("--files-without-match"),
+    ],
+    args=[ARG_OPAQUE_VALUE, ARG_RFILES_OR_CWD],
+    should_match=[
+        ["-n", "init"],
+        ["-n", "init", "."],
+        ["-i", "-n", "init", "src"],
+        ["--files", "--max-depth", "2", "."],
+    ],
+    should_not_match=[
+        ["-m", "-n", "init"],
+        ["--glob", "src"],
+    ],
+    # TODO(mbolin): Perhaps we need a way to indicate that we expect `rg` to be
+    # bundled with the host environment and we should be using that verison.
+    system_path=[],
+)
+
+# Unfortunately, `sed` is difficult to secure because GNU sed supports an `e`
+# flag where `s/pattern/replacement/e` would run `replacement` as a shell
+# command every time `pattern` is matched. For example, try the following on
+# Ubuntu (which uses GNU sed, unlike macOS):
+#
+# ```shell
+# $ yes | head -n 4 > /tmp/yes.txt
+# $ sed 's/y/echo hi/e' /tmp/yes.txt
+# hi
+# hi
+# hi
+# hi
+# ```
+#
+# As you can see, `echo hi` got executed four times. In order to support some
+# basic sed functionality, we implement a bespoke `ARG_SED_COMMAND` that matches
+# only "known safe" sed commands.
+common_sed_flags = [
+    # We deliberately do not support -i or -f.
+    flag("-n"),
+    flag("-u"),
+]
+sed_system_path = ["/usr/bin/sed"]
+
+# When -e is not specified, the first argument must be a valid sed command.
+define_program(
+    program="sed",
+    options=common_sed_flags,
+    args=[ARG_SED_COMMAND, ARG_RFILES],
+    system_path=sed_system_path,
+)
+
+# When -e is required, all arguments are assumed to be readable files.
+define_program(
+    program="sed",
+    options=common_sed_flags + [
+        opt("-e", ARG_SED_COMMAND, required=True),
+    ],
+    args=[ARG_RFILES],
+    system_path=sed_system_path,
+)
+
+define_program(
+    program="which",
+    options=[
+        flag("-a"),
+        flag("-s"),
+    ],
+    # Surprisingly, `which` takes more than one argument.
+    args=[ARG_RFILES],
+    should_match=[
+        ["python3"],
+        ["-a", "python3"],
+        ["-a", "python3", "cargo"],
+    ],
+    should_not_match=[
+        [],
+    ],
+    system_path=["/bin/which", "/usr/bin/which"],
+)

codex-rs/execpolicy/src/error.rs

@@ -0,0 +1,96 @@
+use std::path::PathBuf;
+
+use serde::Serialize;
+
+use crate::arg_matcher::ArgMatcher;
+use crate::arg_resolver::PositionalArg;
+use serde_with::serde_as;
+use serde_with::DisplayFromStr;
+
+pub type Result<T> = std::result::Result<T, Error>;
+
+#[serde_as]
+#[derive(Debug, Eq, PartialEq, Serialize)]
+#[serde(tag = "type")]
+pub enum Error {
+    NoSpecForProgram {
+        program: String,
+    },
+    OptionMissingValue {
+        program: String,
+        option: String,
+    },
+    OptionFollowedByOptionInsteadOfValue {
+        program: String,
+        option: String,
+        value: String,
+    },
+    UnknownOption {
+        program: String,
+        option: String,
+    },
+    UnexpectedArguments {
+        program: String,
+        args: Vec<PositionalArg>,
+    },
+    DoubleDashNotSupportedYet {
+        program: String,
+    },
+    MultipleVarargPatterns {
+        program: String,
+        first: ArgMatcher,
+        second: ArgMatcher,
+    },
+    RangeStartExceedsEnd {
+        start: usize,
+        end: usize,
+    },
+    RangeEndOutOfBounds {
+        end: usize,
+        len: usize,
+    },
+    PrefixOverlapsSuffix {},
+    NotEnoughArgs {
+        program: String,
+        args: Vec<PositionalArg>,
+        arg_patterns: Vec<ArgMatcher>,
+    },
+    InternalInvariantViolation {
+        message: String,
+    },
+    VarargMatcherDidNotMatchAnything {
+        program: String,
+        matcher: ArgMatcher,
+    },
+    EmptyFileName {},
+    LiteralValueDidNotMatch {
+        expected: String,
+        actual: String,
+    },
+    InvalidPositiveInteger {
+        value: String,
+    },
+    MissingRequiredOptions {
+        program: String,
+        options: Vec<String>,
+    },
+    SedCommandNotProvablySafe {
+        command: String,
+    },
+    ReadablePathNotInReadableFolders {
+        file: PathBuf,
+        folders: Vec<PathBuf>,
+    },
+    WriteablePathNotInWriteableFolders {
+        file: PathBuf,
+        folders: Vec<PathBuf>,
+    },
+    CannotCheckRelativePath {
+        file: PathBuf,
+    },
+    CannotCanonicalizePath {
+        file: String,
+        #[serde_as(as = "DisplayFromStr")]
+        error: std::io::ErrorKind,
+    },
+}

codex-rs/execpolicy/src/exec_call.rs

@@ -0,0 +1,28 @@
+use std::fmt::Display;
+
+use serde::Serialize;
+
+#[derive(Clone, Debug, Eq, PartialEq, Serialize)]
+pub struct ExecCall {
+    pub program: String,
+    pub args: Vec<String>,
+}
+
+impl ExecCall {
+    pub fn new(program: &str, args: &[&str]) -> Self {
+        Self {
+            program: program.to_string(),
+            args: args.iter().map(|&s| s.into()).collect(),
+        }
+    }
+}
+
+impl Display for ExecCall {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}", self.program)?;
+        for arg in &self.args {
+            write!(f, " {}", arg)?;
+        }
+        Ok(())
+    }
+}

codex-rs/execpolicy/src/execv_checker.rs

@@ -0,0 +1,263 @@
+use std::ffi::OsString;
+use std::path::Path;
+use std::path::PathBuf;
+
+use crate::ArgType;
+use crate::Error::CannotCanonicalizePath;
+use crate::Error::CannotCheckRelativePath;
+use crate::Error::ReadablePathNotInReadableFolders;
+use crate::Error::WriteablePathNotInWriteableFolders;
+use crate::ExecCall;
+use crate::MatchedExec;
+use crate::Policy;
+use crate::Result;
+use crate::ValidExec;
+use path_absolutize::*;
+use std::os::unix::fs::PermissionsExt;
+
+macro_rules! check_file_in_folders {
+    ($file:expr, $folders:expr, $error:ident) => {
+        if !$folders.iter().any(|folder| $file.starts_with(folder)) {
+            return Err($error {
+                file: $file.clone(),
+                folders: $folders.to_vec(),
+            });
+        }
+    };
+}
+
+pub struct ExecvChecker {
+    execv_policy: Policy,
+}
+
+impl ExecvChecker {
+    pub fn new(execv_policy: Policy) -> Self {
+        Self { execv_policy }
+    }
+
+    pub fn r#match(&self, exec_call: &ExecCall) -> Result<MatchedExec> {
+        self.execv_policy.check(exec_call)
+    }
+
+    /// The caller is responsible for ensuring readable_folders and
+    /// writeable_folders are in canonical form.
+    pub fn check(
+        &self,
+        valid_exec: ValidExec,
+        cwd: &Option<OsString>,
+        readable_folders: &[PathBuf],
+        writeable_folders: &[PathBuf],
+    ) -> Result<String> {
+        for (arg_type, value) in valid_exec
+            .args
+            .into_iter()
+            .map(|arg| (arg.r#type, arg.value))
+            .chain(
+                valid_exec
+                    .opts
+                    .into_iter()
+                    .map(|opt| (opt.r#type, opt.value)),
+            )
+        {
+            match arg_type {
+                ArgType::ReadableFile => {
+                    let readable_file = ensure_absolute_path(&value, cwd)?;
+                    check_file_in_folders!(
+                        readable_file,
+                        readable_folders,
+                        ReadablePathNotInReadableFolders
+                    );
+                }
+                ArgType::WriteableFile => {
+                    let writeable_file = ensure_absolute_path(&value, cwd)?;
+                    check_file_in_folders!(
+                        writeable_file,
+                        writeable_folders,
+                        WriteablePathNotInWriteableFolders
+                    );
+                }
+                ArgType::OpaqueNonFile
+                | ArgType::Unknown
+                | ArgType::PositiveInteger
+                | ArgType::SedCommand
+                | ArgType::Literal(_) => {
+                    continue;
+                }
+            }
+        }
+
+        let mut program = valid_exec.program.to_string();
+        for system_path in valid_exec.system_path {
+            if is_executable_file(&system_path) {
+                program = system_path.to_string();
+                break;
+            }
+        }
+
+        Ok(program)
+    }
+}
+
+fn ensure_absolute_path(path: &str, cwd: &Option<OsString>) -> Result<PathBuf> {
+    let file = PathBuf::from(path);
+    let result = if file.is_relative() {
+        match cwd {
+            Some(cwd) => file.absolutize_from(cwd),
+            None => return Err(CannotCheckRelativePath { file }),
+        }
+    } else {
+        file.absolutize()
+    };
+    result
+        .map(|path| path.into_owned())
+        .map_err(|error| CannotCanonicalizePath {
+            file: path.to_string(),
+            error: error.kind(),
+        })
+}
+
+fn is_executable_file(path: &str) -> bool {
+    let file_path = Path::new(path);
+
+    if let Ok(metadata) = std::fs::metadata(file_path) {
+        let permissions = metadata.permissions();
+        // Check if the file is executable (by checking the executable bit for the owner)
+        return metadata.is_file() && (permissions.mode() & 0o111 != 0);
+    }
+
+    false
+}
+
+#[cfg(test)]
+mod tests {
+    use tempfile::TempDir;
+
+    use super::*;
+    use crate::MatchedArg;
+    use crate::PolicyParser;
+
+    fn setup(fake_cp: &Path) -> ExecvChecker {
+        let source = format!(
+            r#"
+define_program(
+program="cp",
+args=[ARG_RFILE, ARG_WFILE],
+system_path=[{fake_cp:?}]
+)
+"#
+        );
+        let parser = PolicyParser::new("#test", &source);
+        let policy = parser.parse().unwrap();
+        ExecvChecker::new(policy)
+    }
+
+    #[test]
+    fn test_check_valid_input_files() -> Result<()> {
+        let temp_dir = TempDir::new().unwrap();
+
+        // Create an executable file that can be used with the system_path arg.
+        let fake_cp = temp_dir.path().join("cp");
+        let fake_cp_file = std::fs::File::create(&fake_cp).unwrap();
+        let mut permissions = fake_cp_file.metadata().unwrap().permissions();
+        permissions.set_mode(0o755);
+        std::fs::set_permissions(&fake_cp, permissions).unwrap();
+
+        // Create root_path and reference to files under the root.
+        let root_path = temp_dir.path().to_path_buf();
+        let source_path = root_path.join("source");
+        let dest_path = root_path.join("dest");
+
+        let cp = fake_cp.to_str().unwrap().to_string();
+        let root = root_path.to_str().unwrap().to_string();
+        let source = source_path.to_str().unwrap().to_string();
+        let dest = dest_path.to_str().unwrap().to_string();
+
+        let cwd = Some(root_path.clone().into());
+
+        let checker = setup(&fake_cp);
+        let exec_call = ExecCall {
+            program: "cp".into(),
+            args: vec![source.clone(), dest.clone()],
+        };
+        let valid_exec = match checker.r#match(&exec_call)? {
+            MatchedExec::Match { exec } => exec,
+            unexpected => panic!("Expected a safe exec but got {unexpected:?}"),
+        };
+
+        // No readable or writeable folders specified.
+        assert_eq!(
+            checker.check(valid_exec.clone(), &cwd, &[], &[]),
+            Err(ReadablePathNotInReadableFolders {
+                file: source_path.clone(),
+                folders: vec![]
+            }),
+        );
+
+        // Only readable folders specified.
+        assert_eq!(
+            checker.check(valid_exec.clone(), &cwd, &[root_path.clone()], &[]),
+            Err(WriteablePathNotInWriteableFolders {
+                file: dest_path.clone(),
+                folders: vec![]
+            }),
+        );
+
+        // Both readable and writeable folders specified.
+        assert_eq!(
+            checker.check(
+                valid_exec.clone(),
+                &cwd,
+                &[root_path.clone()],
+                &[root_path.clone()]
+            ),
+            Ok(cp.clone()),
+        );
+
+        // Args are the readable and writeable folders, not files within the
+        // folders.
+        let exec_call_folders_as_args = ExecCall {
+            program: "cp".into(),
+            args: vec![root.clone(), root.clone()],
+        };
+        let valid_exec_call_folders_as_args = match checker.r#match(&exec_call_folders_as_args)? {
+            MatchedExec::Match { exec } => exec,
+            _ => panic!("Expected a safe exec"),
+        };
+        assert_eq!(
+            checker.check(
+                valid_exec_call_folders_as_args,
+                &cwd,
+                &[root_path.clone()],
+                &[root_path.clone()]
+            ),
+            Ok(cp.clone()),
+        );
+
+        // Specify a parent of a readable folder as input.
+        let exec_with_parent_of_readable_folder = ValidExec {
+            program: "cp".into(),
+            args: vec![
+                MatchedArg::new(
+                    0,
+                    ArgType::ReadableFile,
+                    root_path.parent().unwrap().to_str().unwrap(),
+                )?,
+                MatchedArg::new(1, ArgType::WriteableFile, &dest)?,
+            ],
+            ..Default::default()
+        };
+        assert_eq!(
+            checker.check(
+                exec_with_parent_of_readable_folder,
+                &cwd,
+                &[root_path.clone()],
+                &[dest_path.clone()]
+            ),
+            Err(ReadablePathNotInReadableFolders {
+                file: root_path.parent().unwrap().to_path_buf(),
+                folders: vec![root_path.clone()]
+            }),
+        );
+        Ok(())
+    }
+}

codex-rs/execpolicy/src/lib.rs

@@ -0,0 +1,45 @@
+#![allow(clippy::type_complexity)]
+#![allow(clippy::too_many_arguments)]
+#[macro_use]
+extern crate starlark;
+
+mod arg_matcher;
+mod arg_resolver;
+mod arg_type;
+mod error;
+mod exec_call;
+mod execv_checker;
+mod opt;
+mod policy;
+mod policy_parser;
+mod program;
+mod sed_command;
+mod valid_exec;
+
+pub use arg_matcher::ArgMatcher;
+pub use arg_resolver::PositionalArg;
+pub use arg_type::ArgType;
+pub use error::Error;
+pub use error::Result;
+pub use exec_call::ExecCall;
+pub use execv_checker::ExecvChecker;
+pub use opt::Opt;
+pub use policy::Policy;
+pub use policy_parser::PolicyParser;
+pub use program::Forbidden;
+pub use program::MatchedExec;
+pub use program::NegativeExamplePassedCheck;
+pub use program::PositiveExampleFailedCheck;
+pub use program::ProgramSpec;
+pub use sed_command::parse_sed_command;
+pub use valid_exec::MatchedArg;
+pub use valid_exec::MatchedFlag;
+pub use valid_exec::MatchedOpt;
+pub use valid_exec::ValidExec;
+
+const DEFAULT_POLICY: &str = include_str!("default.policy");
+
+pub fn get_default_policy() -> starlark::Result<Policy> {
+    let parser = PolicyParser::new("#default", DEFAULT_POLICY);
+    parser.parse()
+}

codex-rs/execpolicy/src/main.rs

@@ -0,0 +1,166 @@
+use anyhow::Result;
+use clap::Parser;
+use clap::Subcommand;
+use codex_execpolicy::get_default_policy;
+use codex_execpolicy::ExecCall;
+use codex_execpolicy::MatchedExec;
+use codex_execpolicy::Policy;
+use codex_execpolicy::PolicyParser;
+use codex_execpolicy::ValidExec;
+use serde::de;
+use serde::Deserialize;
+use serde::Serialize;
+use std::path::PathBuf;
+use std::str::FromStr;
+
+const MATCHED_BUT_WRITES_FILES_EXIT_CODE: i32 = 12;
+const MIGHT_BE_SAFE_EXIT_CODE: i32 = 13;
+const FORBIDDEN_EXIT_CODE: i32 = 14;
+
+#[derive(Parser, Deserialize, Debug)]
+#[command(version, about, long_about = None)]
+pub struct Args {
+    /// If the command fails the policy, exit with 13, but print parseable JSON
+    /// to stdout.
+    #[clap(long)]
+    pub require_safe: bool,
+
+    /// Path to the policy file.
+    #[clap(long, short = 'p')]
+    pub policy: Option<PathBuf>,
+
+    #[command(subcommand)]
+    pub command: Command,
+}
+
+#[derive(Clone, Debug, Deserialize, Subcommand)]
+pub enum Command {
+    /// Checks the command as if the arguments were the inputs to execv(3).
+    Check {
+        #[arg(trailing_var_arg = true)]
+        command: Vec<String>,
+    },
+
+    /// Checks the command encoded as a JSON object.
+    #[clap(name = "check-json")]
+    CheckJson {
+        /// JSON object with "program" (str) and "args" (list[str]) fields.
+        #[serde(deserialize_with = "deserialize_from_json")]
+        exec: ExecArg,
+    },
+}
+
+#[derive(Clone, Debug, Deserialize)]
+pub struct ExecArg {
+    pub program: String,
+
+    #[serde(default)]
+    pub args: Vec<String>,
+}
+
+fn main() -> Result<()> {
+    env_logger::init();
+
+    let args = Args::parse();
+    let policy = match args.policy {
+        Some(policy) => {
+            let policy_source = policy.to_string_lossy().to_string();
+            let unparsed_policy = std::fs::read_to_string(policy)?;
+            let parser = PolicyParser::new(&policy_source, &unparsed_policy);
+            parser.parse()
+        }
+        None => get_default_policy(),
+    };
+    let policy = policy.map_err(|err| err.into_anyhow())?;
+
+    let exec = match args.command {
+        Command::Check { command } => match command.split_first() {
+            Some((first, rest)) => ExecArg {
+                program: first.to_string(),
+                args: rest.iter().map(|s| s.to_string()).collect(),
+            },
+            None => {
+                eprintln!("no command provided");
+                std::process::exit(1);
+            }
+        },
+        Command::CheckJson { exec } => exec,
+    };
+
+    let (output, exit_code) = check_command(&policy, exec, args.require_safe);
+    let json = serde_json::to_string(&output)?;
+    println!("{}", json);
+    std::process::exit(exit_code);
+}
+
+fn check_command(
+    policy: &Policy,
+    ExecArg { program, args }: ExecArg,
+    check: bool,
+) -> (Output, i32) {
+    let exec_call = ExecCall { program, args };
+    match policy.check(&exec_call) {
+        Ok(MatchedExec::Match { exec }) => {
+            if exec.might_write_files() {
+                let exit_code = if check {
+                    MATCHED_BUT_WRITES_FILES_EXIT_CODE
+                } else {
+                    0
+                };
+                (Output::Match { r#match: exec }, exit_code)
+            } else {
+                (Output::Safe { r#match: exec }, 0)
+            }
+        }
+        Ok(MatchedExec::Forbidden { reason, cause }) => {
+            let exit_code = if check { FORBIDDEN_EXIT_CODE } else { 0 };
+            (Output::Forbidden { reason, cause }, exit_code)
+        }
+        Err(err) => {
+            let exit_code = if check { MIGHT_BE_SAFE_EXIT_CODE } else { 0 };
+            (Output::Unverified { error: err }, exit_code)
+        }
+    }
+}
+
+#[derive(Debug, Serialize)]
+#[serde(tag = "result")]
+pub enum Output {
+    /// The command is verified as safe.
+    #[serde(rename = "safe")]
+    Safe { r#match: ValidExec },
+
+    /// The command has matched a rule in the policy, but the caller should
+    /// decide whether it is "safe" given the files it wants to write.
+    #[serde(rename = "match")]
+    Match { r#match: ValidExec },
+
+    /// The user is forbidden from running the command.
+    #[serde(rename = "forbidden")]
+    Forbidden {
+        reason: String,
+        cause: codex_execpolicy::Forbidden,
+    },
+
+    /// The safety of the command could not be verified.
+    #[serde(rename = "unverified")]
+    Unverified { error: codex_execpolicy::Error },
+}
+
+fn deserialize_from_json<'de, D>(deserializer: D) -> Result<ExecArg, D::Error>
+where
+    D: de::Deserializer<'de>,
+{
+    let s = String::deserialize(deserializer)?;
+    let decoded = serde_json::from_str(&s)
+        .map_err(|e| serde::de::Error::custom(format!("JSON parse error: {e}")))?;
+    Ok(decoded)
+}
+
+impl FromStr for ExecArg {
+    type Err = anyhow::Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        serde_json::from_str(s).map_err(|e| e.into())
+    }
+}

codex-rs/execpolicy/src/opt.rs

@@ -0,0 +1,77 @@
+#![allow(clippy::needless_lifetimes)]
+
+use crate::starlark::values::ValueLike;
+use crate::ArgType;
+use allocative::Allocative;
+use derive_more::derive::Display;
+use starlark::any::ProvidesStaticType;
+use starlark::values::starlark_value;
+use starlark::values::AllocValue;
+use starlark::values::Heap;
+use starlark::values::NoSerialize;
+use starlark::values::StarlarkValue;
+use starlark::values::UnpackValue;
+use starlark::values::Value;
+
+/// Command line option that takes a value.
+#[derive(Clone, Debug, Display, PartialEq, Eq, ProvidesStaticType, NoSerialize, Allocative)]
+#[display("opt({})", opt)]
+pub struct Opt {
+    /// The option as typed on the command line, e.g., `-h` or `--help`. If
+    /// it can be used in the `--name=value` format, then this should be
+    /// `--name` (though this is subject to change).
+    pub opt: String,
+    pub meta: OptMeta,
+    pub required: bool,
+}
+
+/// When defining an Opt, use as specific an OptMeta as possible.
+#[derive(Clone, Debug, Display, PartialEq, Eq, ProvidesStaticType, NoSerialize, Allocative)]
+#[display("{}", self)]
+pub enum OptMeta {
+    /// Option does not take a value.
+    Flag,
+
+    /// Option takes a single value matching the specified type.
+    Value(ArgType),
+}
+
+impl Opt {
+    pub fn new(opt: String, meta: OptMeta, required: bool) -> Self {
+        Self {
+            opt,
+            meta,
+            required,
+        }
+    }
+
+    pub fn name(&self) -> &str {
+        &self.opt
+    }
+}
+
+#[starlark_value(type = "Opt")]
+impl<'v> StarlarkValue<'v> for Opt {
+    type Canonical = Opt;
+}
+
+impl<'v> UnpackValue<'v> for Opt {
+    type Error = starlark::Error;
+
+    fn unpack_value_impl(value: Value<'v>) -> starlark::Result<Option<Self>> {
+        // TODO(mbolin): It fels like this should be doable without cloning?
+        // Cannot simply consume the value?
+        Ok(value.downcast_ref::<Opt>().cloned())
+    }
+}
+
+impl<'v> AllocValue<'v> for Opt {
+    fn alloc_value(self, heap: &'v Heap) -> Value<'v> {
+        heap.alloc_simple(self)
+    }
+}
+
+#[starlark_value(type = "OptMeta")]
+impl<'v> StarlarkValue<'v> for OptMeta {
+    type Canonical = OptMeta;
+}

codex-rs/execpolicy/src/policy.rs

@@ -0,0 +1,103 @@
+use multimap::MultiMap;
+use regex::Error as RegexError;
+use regex::Regex;
+
+use crate::error::Error;
+use crate::error::Result;
+use crate::policy_parser::ForbiddenProgramRegex;
+use crate::program::PositiveExampleFailedCheck;
+use crate::ExecCall;
+use crate::Forbidden;
+use crate::MatchedExec;
+use crate::NegativeExamplePassedCheck;
+use crate::ProgramSpec;
+
+pub struct Policy {
+    programs: MultiMap<String, ProgramSpec>,
+    forbidden_program_regexes: Vec<ForbiddenProgramRegex>,
+    forbidden_substrings_pattern: Option<Regex>,
+}
+
+impl Policy {
+    pub fn new(
+        programs: MultiMap<String, ProgramSpec>,
+        forbidden_program_regexes: Vec<ForbiddenProgramRegex>,
+        forbidden_substrings: Vec<String>,
+    ) -> std::result::Result<Self, RegexError> {
+        let forbidden_substrings_pattern = if forbidden_substrings.is_empty() {
+            None
+        } else {
+            let escaped_substrings = forbidden_substrings
+                .iter()
+                .map(|s| regex::escape(s))
+                .collect::<Vec<_>>()
+                .join("|");
+            Some(Regex::new(&format!("({escaped_substrings})"))?)
+        };
+        Ok(Self {
+            programs,
+            forbidden_program_regexes,
+            forbidden_substrings_pattern,
+        })
+    }
+
+    pub fn check(&self, exec_call: &ExecCall) -> Result<MatchedExec> {
+        let ExecCall { program, args } = &exec_call;
+        for ForbiddenProgramRegex { regex, reason } in &self.forbidden_program_regexes {
+            if regex.is_match(program) {
+                return Ok(MatchedExec::Forbidden {
+                    cause: Forbidden::Program {
+                        program: program.clone(),
+                        exec_call: exec_call.clone(),
+                    },
+                    reason: reason.clone(),
+                });
+            }
+        }
+
+        for arg in args {
+            if let Some(regex) = &self.forbidden_substrings_pattern {
+                if regex.is_match(arg) {
+                    return Ok(MatchedExec::Forbidden {
+                        cause: Forbidden::Arg {
+                            arg: arg.clone(),
+                            exec_call: exec_call.clone(),
+                        },
+                        reason: format!("arg `{}` contains forbidden substring", arg),
+                    });
+                }
+            }
+        }
+
+        let mut last_err = Err(Error::NoSpecForProgram {
+            program: program.clone(),
+        });
+        if let Some(spec_list) = self.programs.get_vec(program) {
+            for spec in spec_list {
+                match spec.check(exec_call) {
+                    Ok(matched_exec) => return Ok(matched_exec),
+                    Err(err) => {
+                        last_err = Err(err);
+                    }
+                }
+            }
+        }
+        last_err
+    }
+
+    pub fn check_each_good_list_individually(&self) -> Vec<PositiveExampleFailedCheck> {
+        let mut violations = Vec::new();
+        for (_program, spec) in self.programs.flat_iter() {
+            violations.extend(spec.verify_should_match_list());
+        }
+        violations
+    }
+
+    pub fn check_each_bad_list_individually(&self) -> Vec<NegativeExamplePassedCheck> {
+        let mut violations = Vec::new();
+        for (_program, spec) in self.programs.flat_iter() {
+            violations.extend(spec.verify_should_not_match_list());
+        }
+        violations
+    }
+}

codex-rs/execpolicy/src/policy_parser.rs

@@ -0,0 +1,222 @@
+#![allow(clippy::needless_lifetimes)]
+
+use crate::arg_matcher::ArgMatcher;
+use crate::opt::OptMeta;
+use crate::Opt;
+use crate::Policy;
+use crate::ProgramSpec;
+use log::info;
+use multimap::MultiMap;
+use regex::Regex;
+use starlark::any::ProvidesStaticType;
+use starlark::environment::GlobalsBuilder;
+use starlark::environment::LibraryExtension;
+use starlark::environment::Module;
+use starlark::eval::Evaluator;
+use starlark::syntax::AstModule;
+use starlark::syntax::Dialect;
+use starlark::values::list::UnpackList;
+use starlark::values::none::NoneType;
+use starlark::values::Heap;
+use std::cell::RefCell;
+use std::collections::HashMap;
+
+pub struct PolicyParser {
+    policy_source: String,
+    unparsed_policy: String,
+}
+
+impl PolicyParser {
+    pub fn new(policy_source: &str, unparsed_policy: &str) -> Self {
+        Self {
+            policy_source: policy_source.to_string(),
+            unparsed_policy: unparsed_policy.to_string(),
+        }
+    }
+
+    pub fn parse(&self) -> starlark::Result<Policy> {
+        let mut dialect = Dialect::Extended.clone();
+        dialect.enable_f_strings = true;
+        let ast = AstModule::parse(&self.policy_source, self.unparsed_policy.clone(), &dialect)?;
+        let globals = GlobalsBuilder::extended_by(&[LibraryExtension::Typing])
+            .with(policy_builtins)
+            .build();
+        let module = Module::new();
+
+        let heap = Heap::new();
+
+        module.set("ARG_OPAQUE_VALUE", heap.alloc(ArgMatcher::OpaqueNonFile));
+        module.set("ARG_RFILE", heap.alloc(ArgMatcher::ReadableFile));
+        module.set("ARG_WFILE", heap.alloc(ArgMatcher::WriteableFile));
+        module.set("ARG_RFILES", heap.alloc(ArgMatcher::ReadableFiles));
+        module.set(
+            "ARG_RFILES_OR_CWD",
+            heap.alloc(ArgMatcher::ReadableFilesOrCwd),
+        );
+        module.set("ARG_POS_INT", heap.alloc(ArgMatcher::PositiveInteger));
+        module.set("ARG_SED_COMMAND", heap.alloc(ArgMatcher::SedCommand));
+        module.set(
+            "ARG_UNVERIFIED_VARARGS",
+            heap.alloc(ArgMatcher::UnverifiedVarargs),
+        );
+
+        let policy_builder = PolicyBuilder::new();
+        {
+            let mut eval = Evaluator::new(&module);
+            eval.extra = Some(&policy_builder);
+            eval.eval_module(ast, &globals)?;
+        }
+        let policy = policy_builder.build();
+        policy.map_err(|e| starlark::Error::new_kind(starlark::ErrorKind::Other(e.into())))
+    }
+}
+
+#[derive(Debug)]
+pub struct ForbiddenProgramRegex {
+    pub regex: regex::Regex,
+    pub reason: String,
+}
+
+#[derive(Debug, ProvidesStaticType)]
+struct PolicyBuilder {
+    programs: RefCell<MultiMap<String, ProgramSpec>>,
+    forbidden_program_regexes: RefCell<Vec<ForbiddenProgramRegex>>,
+    forbidden_substrings: RefCell<Vec<String>>,
+}
+
+impl PolicyBuilder {
+    fn new() -> Self {
+        Self {
+            programs: RefCell::new(MultiMap::new()),
+            forbidden_program_regexes: RefCell::new(Vec::new()),
+            forbidden_substrings: RefCell::new(Vec::new()),
+        }
+    }
+
+    fn build(self) -> Result<Policy, regex::Error> {
+        let programs = self.programs.into_inner();
+        let forbidden_program_regexes = self.forbidden_program_regexes.into_inner();
+        let forbidden_substrings = self.forbidden_substrings.into_inner();
+        Policy::new(programs, forbidden_program_regexes, forbidden_substrings)
+    }
+
+    fn add_program_spec(&self, program_spec: ProgramSpec) {
+        info!("adding program spec: {:?}", program_spec);
+        let name = program_spec.program.clone();
+        let mut programs = self.programs.borrow_mut();
+        programs.insert(name.clone(), program_spec);
+    }
+
+    fn add_forbidden_substrings(&self, substrings: &[String]) {
+        let mut forbidden_substrings = self.forbidden_substrings.borrow_mut();
+        forbidden_substrings.extend_from_slice(substrings);
+    }
+
+    fn add_forbidden_program_regex(&self, regex: Regex, reason: String) {
+        let mut forbidden_program_regexes = self.forbidden_program_regexes.borrow_mut();
+        forbidden_program_regexes.push(ForbiddenProgramRegex { regex, reason });
+    }
+}
+
+#[starlark_module]
+fn policy_builtins(builder: &mut GlobalsBuilder) {
+    fn define_program<'v>(
+        program: String,
+        system_path: Option<UnpackList<String>>,
+        option_bundling: Option<bool>,
+        combined_format: Option<bool>,
+        options: Option<UnpackList<Opt>>,
+        args: Option<UnpackList<ArgMatcher>>,
+        forbidden: Option<String>,
+        should_match: Option<UnpackList<UnpackList<String>>>,
+        should_not_match: Option<UnpackList<UnpackList<String>>>,
+        eval: &mut Evaluator,
+    ) -> anyhow::Result<NoneType> {
+        let option_bundling = option_bundling.unwrap_or(false);
+        let system_path = system_path.map_or_else(Vec::new, |v| v.items.to_vec());
+        let combined_format = combined_format.unwrap_or(false);
+        let options = options.map_or_else(Vec::new, |v| v.items.to_vec());
+        let args = args.map_or_else(Vec::new, |v| v.items.to_vec());
+
+        let mut allowed_options = HashMap::<String, Opt>::new();
+        for opt in options {
+            let name = opt.name().to_string();
+            if allowed_options
+                .insert(opt.name().to_string(), opt)
+                .is_some()
+            {
+                return Err(anyhow::format_err!("duplicate flag: {name}"));
+            }
+        }
+
+        let program_spec = ProgramSpec::new(
+            program,
+            system_path,
+            option_bundling,
+            combined_format,
+            allowed_options,
+            args,
+            forbidden,
+            should_match
+                .map_or_else(Vec::new, |v| v.items.to_vec())
+                .into_iter()
+                .map(|v| v.items.to_vec())
+                .collect(),
+            should_not_match
+                .map_or_else(Vec::new, |v| v.items.to_vec())
+                .into_iter()
+                .map(|v| v.items.to_vec())
+                .collect(),
+        );
+        let policy_builder = eval
+            .extra
+            .as_ref()
+            .unwrap()
+            .downcast_ref::<PolicyBuilder>()
+            .unwrap();
+        policy_builder.add_program_spec(program_spec);
+        Ok(NoneType)
+    }
+
+    fn forbid_substrings(
+        strings: UnpackList<String>,
+        eval: &mut Evaluator,
+    ) -> anyhow::Result<NoneType> {
+        let policy_builder = eval
+            .extra
+            .as_ref()
+            .unwrap()
+            .downcast_ref::<PolicyBuilder>()
+            .unwrap();
+        policy_builder.add_forbidden_substrings(&strings.items.to_vec());
+        Ok(NoneType)
+    }
+
+    fn forbid_program_regex(
+        regex: String,
+        reason: String,
+        eval: &mut Evaluator,
+    ) -> anyhow::Result<NoneType> {
+        let policy_builder = eval
+            .extra
+            .as_ref()
+            .unwrap()
+            .downcast_ref::<PolicyBuilder>()
+            .unwrap();
+        let compiled_regex = regex::Regex::new(&regex)?;
+        policy_builder.add_forbidden_program_regex(compiled_regex, reason);
+        Ok(NoneType)
+    }
+
+    fn opt(name: String, r#type: ArgMatcher, required: Option<bool>) -> anyhow::Result<Opt> {
+        Ok(Opt::new(
+            name,
+            OptMeta::Value(r#type.arg_type()),
+            required.unwrap_or(false),
+        ))
+    }
+
+    fn flag(name: String) -> anyhow::Result<Opt> {
+        Ok(Opt::new(name, OptMeta::Flag, false))
+    }
+}

codex-rs/execpolicy/src/program.rs

@@ -0,0 +1,247 @@
+use serde::Serialize;
+use std::collections::HashMap;
+use std::collections::HashSet;
+
+use crate::arg_matcher::ArgMatcher;
+use crate::arg_resolver::resolve_observed_args_with_patterns;
+use crate::arg_resolver::PositionalArg;
+use crate::error::Error;
+use crate::error::Result;
+use crate::opt::Opt;
+use crate::opt::OptMeta;
+use crate::valid_exec::MatchedFlag;
+use crate::valid_exec::MatchedOpt;
+use crate::valid_exec::ValidExec;
+use crate::ArgType;
+use crate::ExecCall;
+
+#[derive(Debug)]
+pub struct ProgramSpec {
+    pub program: String,
+    pub system_path: Vec<String>,
+    pub option_bundling: bool,
+    pub combined_format: bool,
+    pub allowed_options: HashMap<String, Opt>,
+    pub arg_patterns: Vec<ArgMatcher>,
+    forbidden: Option<String>,
+    required_options: HashSet<String>,
+    should_match: Vec<Vec<String>>,
+    should_not_match: Vec<Vec<String>>,
+}
+
+impl ProgramSpec {
+    pub fn new(
+        program: String,
+        system_path: Vec<String>,
+        option_bundling: bool,
+        combined_format: bool,
+        allowed_options: HashMap<String, Opt>,
+        arg_patterns: Vec<ArgMatcher>,
+        forbidden: Option<String>,
+        should_match: Vec<Vec<String>>,
+        should_not_match: Vec<Vec<String>>,
+    ) -> Self {
+        let required_options = allowed_options
+            .iter()
+            .filter_map(|(name, opt)| {
+                if opt.required {
+                    Some(name.clone())
+                } else {
+                    None
+                }
+            })
+            .collect();
+        Self {
+            program,
+            system_path,
+            option_bundling,
+            combined_format,
+            allowed_options,
+            arg_patterns,
+            forbidden,
+            required_options,
+            should_match,
+            should_not_match,
+        }
+    }
+}
+
+#[derive(Clone, Debug, Eq, PartialEq, Serialize)]
+pub enum MatchedExec {
+    Match { exec: ValidExec },
+    Forbidden { cause: Forbidden, reason: String },
+}
+
+#[derive(Clone, Debug, Eq, PartialEq, Serialize)]
+pub enum Forbidden {
+    Program {
+        program: String,
+        exec_call: ExecCall,
+    },
+    Arg {
+        arg: String,
+        exec_call: ExecCall,
+    },
+    Exec {
+        exec: ValidExec,
+    },
+}
+
+impl ProgramSpec {
+    // TODO(mbolin): The idea is that there should be a set of rules defined for
+    // a program and the args should be checked against the rules to determine
+    // if the program should be allowed to run.
+    pub fn check(&self, exec_call: &ExecCall) -> Result<MatchedExec> {
+        let mut expecting_option_value: Option<(String, ArgType)> = None;
+        let mut args = Vec::<PositionalArg>::new();
+        let mut matched_flags = Vec::<MatchedFlag>::new();
+        let mut matched_opts = Vec::<MatchedOpt>::new();
+
+        for (index, arg) in exec_call.args.iter().enumerate() {
+            if let Some(expected) = expecting_option_value {
+                // If we are expecting an option value, then the next argument
+                // should be the value for the option.
+                // This had better not be another option!
+                let (name, arg_type) = expected;
+                if arg.starts_with("-") {
+                    return Err(Error::OptionFollowedByOptionInsteadOfValue {
+                        program: self.program.clone(),
+                        option: name,
+                        value: arg.clone(),
+                    });
+                }
+
+                matched_opts.push(MatchedOpt::new(&name, arg, arg_type)?);
+                expecting_option_value = None;
+            } else if arg == "--" {
+                return Err(Error::DoubleDashNotSupportedYet {
+                    program: self.program.clone(),
+                });
+            } else if arg.starts_with("-") {
+                match self.allowed_options.get(arg) {
+                    Some(opt) => {
+                        match &opt.meta {
+                            OptMeta::Flag => {
+                                matched_flags.push(MatchedFlag { name: arg.clone() });
+                                // A flag does not expect an argument: continue.
+                                continue;
+                            }
+                            OptMeta::Value(arg_type) => {
+                                expecting_option_value = Some((arg.clone(), arg_type.clone()));
+                                continue;
+                            }
+                        }
+                    }
+                    None => {
+                        // It could be an --option=value style flag...
+                    }
+                }
+
+                return Err(Error::UnknownOption {
+                    program: self.program.clone(),
+                    option: arg.clone(),
+                });
+            } else {
+                args.push(PositionalArg {
+                    index,
+                    value: arg.clone(),
+                });
+            }
+        }
+
+        if let Some(expected) = expecting_option_value {
+            let (name, _arg_type) = expected;
+            return Err(Error::OptionMissingValue {
+                program: self.program.clone(),
+                option: name,
+            });
+        }
+
+        let matched_args =
+            resolve_observed_args_with_patterns(&self.program, args, &self.arg_patterns)?;
+
+        // Verify all required options are present.
+        let matched_opt_names: HashSet<String> = matched_opts
+            .iter()
+            .map(|opt| opt.name().to_string())
+            .collect();
+        if !matched_opt_names.is_superset(&self.required_options) {
+            let mut options = self
+                .required_options
+                .difference(&matched_opt_names)
+                .map(|s| s.to_string())
+                .collect::<Vec<_>>();
+            options.sort();
+            return Err(Error::MissingRequiredOptions {
+                program: self.program.clone(),
+                options,
+            });
+        }
+
+        let exec = ValidExec {
+            program: self.program.clone(),
+            flags: matched_flags,
+            opts: matched_opts,
+            args: matched_args,
+            system_path: self.system_path.clone(),
+        };
+        match &self.forbidden {
+            Some(reason) => Ok(MatchedExec::Forbidden {
+                cause: Forbidden::Exec { exec },
+                reason: reason.clone(),
+            }),
+            None => Ok(MatchedExec::Match { exec }),
+        }
+    }
+
+    pub fn verify_should_match_list(&self) -> Vec<PositiveExampleFailedCheck> {
+        let mut violations = Vec::new();
+        for good in &self.should_match {
+            let exec_call = ExecCall {
+                program: self.program.clone(),
+                args: good.clone(),
+            };
+            match self.check(&exec_call) {
+                Ok(_) => {}
+                Err(error) => {
+                    violations.push(PositiveExampleFailedCheck {
+                        program: self.program.clone(),
+                        args: good.clone(),
+                        error,
+                    });
+                }
+            }
+        }
+        violations
+    }
+
+    pub fn verify_should_not_match_list(&self) -> Vec<NegativeExamplePassedCheck> {
+        let mut violations = Vec::new();
+        for bad in &self.should_not_match {
+            let exec_call = ExecCall {
+                program: self.program.clone(),
+                args: bad.clone(),
+            };
+            if self.check(&exec_call).is_ok() {
+                violations.push(NegativeExamplePassedCheck {
+                    program: self.program.clone(),
+                    args: bad.clone(),
+                });
+            }
+        }
+        violations
+    }
+}
+
+#[derive(Debug, Eq, PartialEq)]
+pub struct PositiveExampleFailedCheck {
+    pub program: String,
+    pub args: Vec<String>,
+    pub error: Error,
+}
+
+#[derive(Debug, Eq, PartialEq)]
+pub struct NegativeExamplePassedCheck {
+    pub program: String,
+    pub args: Vec<String>,
+}

codex-rs/execpolicy/src/sed_command.rs

@@ -0,0 +1,17 @@
+use crate::error::Error;
+use crate::error::Result;
+
+pub fn parse_sed_command(sed_command: &str) -> Result<()> {
+    // For now, we parse only commands like `122,202p`.
+    if let Some(stripped) = sed_command.strip_suffix("p") {
+        if let Some((first, rest)) = stripped.split_once(",") {
+            if first.parse::<u64>().is_ok() && rest.parse::<u64>().is_ok() {
+                return Ok(());
+            }
+        }
+    }
+
+    Err(Error::SedCommandNotProvablySafe {
+        command: sed_command.to_string(),
+    })
+}

codex-rs/execpolicy/src/valid_exec.rs

@@ -0,0 +1,95 @@
+use crate::arg_type::ArgType;
+use crate::error::Result;
+use serde::Serialize;
+
+/// exec() invocation that has been accepted by a `Policy`.
+#[derive(Clone, Debug, Default, Eq, PartialEq, Serialize)]
+pub struct ValidExec {
+    pub program: String,
+    pub flags: Vec<MatchedFlag>,
+    pub opts: Vec<MatchedOpt>,
+    pub args: Vec<MatchedArg>,
+
+    /// If non-empty, a prioritized list of paths to try instead of `program`.
+    /// For example, `/bin/ls` is harder to compromise than whatever `ls`
+    /// happens to be in the user's `$PATH`, so `/bin/ls` would be included for
+    /// `ls`. The caller is free to disregard this list and use `program`.
+    pub system_path: Vec<String>,
+}
+
+impl ValidExec {
+    pub fn new(program: &str, args: Vec<MatchedArg>, system_path: &[&str]) -> Self {
+        Self {
+            program: program.to_string(),
+            flags: vec![],
+            opts: vec![],
+            args,
+            system_path: system_path.iter().map(|&s| s.to_string()).collect(),
+        }
+    }
+
+    /// Whether a possible side effect of running this command includes writing
+    /// a file.
+    pub fn might_write_files(&self) -> bool {
+        self.opts.iter().any(|opt| opt.r#type.might_write_file())
+            || self.args.iter().any(|opt| opt.r#type.might_write_file())
+    }
+}
+
+#[derive(Clone, Debug, Eq, PartialEq, Serialize)]
+pub struct MatchedArg {
+    pub index: usize,
+    pub r#type: ArgType,
+    pub value: String,
+}
+
+impl MatchedArg {
+    pub fn new(index: usize, r#type: ArgType, value: &str) -> Result<Self> {
+        r#type.validate(value)?;
+        Ok(Self {
+            index,
+            r#type,
+            value: value.to_string(),
+        })
+    }
+}
+
+/// A match for an option declared with opt() in a .policy file.
+#[derive(Clone, Debug, Eq, PartialEq, Serialize)]
+pub struct MatchedOpt {
+    /// Name of the option that was matched.
+    pub name: String,
+    /// Value supplied for the option.
+    pub value: String,
+    /// Type of the value supplied for the option.
+    pub r#type: ArgType,
+}
+
+impl MatchedOpt {
+    pub fn new(name: &str, value: &str, r#type: ArgType) -> Result<Self> {
+        r#type.validate(value)?;
+        Ok(Self {
+            name: name.to_string(),
+            value: value.to_string(),
+            r#type,
+        })
+    }
+
+    pub fn name(&self) -> &str {
+        &self.name
+    }
+}
+
+#[derive(Clone, Debug, Eq, PartialEq, Serialize)]
+pub struct MatchedFlag {
+    /// Name of the flag that was matched.
+    pub name: String,
+}
+
+impl MatchedFlag {
+    pub fn new(name: &str) -> Self {
+        Self {
+            name: name.to_string(),
+        }
+    }
+}