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/tests/ls.rs

@@ -0,0 +1,166 @@
+extern crate codex_execpolicy;
+
+use codex_execpolicy::get_default_policy;
+use codex_execpolicy::ArgType;
+use codex_execpolicy::Error;
+use codex_execpolicy::ExecCall;
+use codex_execpolicy::MatchedArg;
+use codex_execpolicy::MatchedExec;
+use codex_execpolicy::MatchedFlag;
+use codex_execpolicy::Policy;
+use codex_execpolicy::Result;
+use codex_execpolicy::ValidExec;
+
+fn setup() -> Policy {
+    get_default_policy().expect("failed to load default policy")
+}
+
+#[test]
+fn test_ls_no_args() {
+    let policy = setup();
+    let ls = ExecCall::new("ls", &[]);
+    assert_eq!(
+        Ok(MatchedExec::Match {
+            exec: ValidExec::new("ls", vec![], &["/bin/ls", "/usr/bin/ls"])
+        }),
+        policy.check(&ls)
+    );
+}
+
+#[test]
+fn test_ls_dash_a_dash_l() {
+    let policy = setup();
+    let args = &["-a", "-l"];
+    let ls_a_l = ExecCall::new("ls", args);
+    assert_eq!(
+        Ok(MatchedExec::Match {
+            exec: ValidExec {
+                program: "ls".into(),
+                flags: vec![MatchedFlag::new("-a"), MatchedFlag::new("-l")],
+                system_path: ["/bin/ls".into(), "/usr/bin/ls".into()].into(),
+                ..Default::default()
+            }
+        }),
+        policy.check(&ls_a_l)
+    );
+}
+
+#[test]
+fn test_ls_dash_z() {
+    let policy = setup();
+
+    // -z is currently an invalid option for ls, but it has so many options,
+    // perhaps it will get added at some point...
+    let ls_z = ExecCall::new("ls", &["-z"]);
+    assert_eq!(
+        Err(Error::UnknownOption {
+            program: "ls".into(),
+            option: "-z".into()
+        }),
+        policy.check(&ls_z)
+    );
+}
+
+#[test]
+fn test_ls_dash_al() {
+    let policy = setup();
+
+    // This currently fails, but it should pass once option_bundling=True is implemented.
+    let ls_al = ExecCall::new("ls", &["-al"]);
+    assert_eq!(
+        Err(Error::UnknownOption {
+            program: "ls".into(),
+            option: "-al".into()
+        }),
+        policy.check(&ls_al)
+    );
+}
+
+#[test]
+fn test_ls_one_file_arg() -> Result<()> {
+    let policy = setup();
+
+    let ls_one_file_arg = ExecCall::new("ls", &["foo"]);
+    assert_eq!(
+        Ok(MatchedExec::Match {
+            exec: ValidExec::new(
+                "ls",
+                vec![MatchedArg::new(0, ArgType::ReadableFile, "foo")?],
+                &["/bin/ls", "/usr/bin/ls"]
+            )
+        }),
+        policy.check(&ls_one_file_arg)
+    );
+    Ok(())
+}
+
+#[test]
+fn test_ls_multiple_file_args() -> Result<()> {
+    let policy = setup();
+
+    let ls_multiple_file_args = ExecCall::new("ls", &["foo", "bar", "baz"]);
+    assert_eq!(
+        Ok(MatchedExec::Match {
+            exec: ValidExec::new(
+                "ls",
+                vec![
+                    MatchedArg::new(0, ArgType::ReadableFile, "foo")?,
+                    MatchedArg::new(1, ArgType::ReadableFile, "bar")?,
+                    MatchedArg::new(2, ArgType::ReadableFile, "baz")?,
+                ],
+                &["/bin/ls", "/usr/bin/ls"]
+            )
+        }),
+        policy.check(&ls_multiple_file_args)
+    );
+    Ok(())
+}
+
+#[test]
+fn test_ls_multiple_flags_and_file_args() -> Result<()> {
+    let policy = setup();
+
+    let ls_multiple_flags_and_file_args = ExecCall::new("ls", &["-l", "-a", "foo", "bar", "baz"]);
+    assert_eq!(
+        Ok(MatchedExec::Match {
+            exec: ValidExec {
+                program: "ls".into(),
+                flags: vec![MatchedFlag::new("-l"), MatchedFlag::new("-a")],
+                args: vec![
+                    MatchedArg::new(2, ArgType::ReadableFile, "foo")?,
+                    MatchedArg::new(3, ArgType::ReadableFile, "bar")?,
+                    MatchedArg::new(4, ArgType::ReadableFile, "baz")?,
+                ],
+                system_path: ["/bin/ls".into(), "/usr/bin/ls".into()].into(),
+                ..Default::default()
+            }
+        }),
+        policy.check(&ls_multiple_flags_and_file_args)
+    );
+    Ok(())
+}
+
+#[test]
+fn test_flags_after_file_args() -> Result<()> {
+    let policy = setup();
+
+    // TODO(mbolin): While this is "safe" in that it will not do anything bad
+    // to the user's machine, it will fail because apparently `ls` does not
+    // allow flags after file arguments (as some commands do). We should
+    // extend define_program() to make this part of the configuration so that
+    // this command is disallowed.
+    let ls_flags_after_file_args = ExecCall::new("ls", &["foo", "-l"]);
+    assert_eq!(
+        Ok(MatchedExec::Match {
+            exec: ValidExec {
+                program: "ls".into(),
+                flags: vec![MatchedFlag::new("-l")],
+                args: vec![MatchedArg::new(0, ArgType::ReadableFile, "foo")?],
+                system_path: ["/bin/ls".into(), "/usr/bin/ls".into()].into(),
+                ..Default::default()
+            }
+        }),
+        policy.check(&ls_flags_after_file_args)
+    );
+    Ok(())
+}