fix(linux-sandbox): prefer system /usr/bin/bwrap when available (#14963)

## Problem
Ubuntu/AppArmor hosts started failing in the default Linux sandbox path
after the switch to vendored/default bubblewrap in `0.115.0`.

The clearest report is in
[#14919](https://github.com/openai/codex/issues/14919), especially [this
investigation
comment](https://github.com/openai/codex/issues/14919#issuecomment-4076504751):
on affected Ubuntu systems, `/usr/bin/bwrap` works, but a copied or
vendored `bwrap` binary fails with errors like `bwrap: setting up uid
map: Permission denied` or `bwrap: loopback: Failed RTM_NEWADDR:
Operation not permitted`.

The root cause is Ubuntu's `/etc/apparmor.d/bwrap-userns-restrict`
profile, which grants `userns` access specifically to `/usr/bin/bwrap`.
Once Codex started using a vendored/internal bubblewrap path, that path
was no longer covered by the distro AppArmor exception, so sandbox
namespace setup could fail even when user namespaces were otherwise
enabled and `uidmap` was installed.

## What this PR changes
- prefer system `/usr/bin/bwrap` whenever it is available
- keep vendored bubblewrap as the fallback when `/usr/bin/bwrap` is
missing
- when `/usr/bin/bwrap` is missing, surface a Codex startup warning
through the app-server/TUI warning path instead of printing directly
from the sandbox helper with `eprintln!`
- use the same launcher decision for both the main sandbox execution
path and the `/proc` preflight path
- document the updated Linux bubblewrap behavior in the Linux sandbox
and core READMEs

## Why this fix
This still fixes the Ubuntu/AppArmor regression from
[#14919](https://github.com/openai/codex/issues/14919), but it keeps the
runtime rule simple and platform-agnostic: if the standard system
bubblewrap is installed, use it; otherwise fall back to the vendored
helper.

The warning now follows that same simple rule. If Codex cannot find
`/usr/bin/bwrap`, it tells the user that it is falling back to the
vendored helper, and it does so through the existing startup warning
plumbing that reaches the TUI and app-server instead of low-level
sandbox stderr.

## Testing
- `cargo test -p codex-linux-sandbox`
- `cargo test -p codex-app-server --lib`
- `cargo test -p codex-tui-app-server
tests::embedded_app_server_start_failure_is_returned`
- `cargo clippy -p codex-linux-sandbox --all-targets`
- `cargo clippy -p codex-app-server --all-targets`
- `cargo clippy -p codex-tui-app-server --all-targets`

codex-rs/linux-sandbox/README.md

@@ -7,13 +7,20 @@ This crate is responsible for producing:
   - the `codex-exec` CLI can check if its arg0 is `codex-linux-sandbox` and, if so, execute as if it were `codex-linux-sandbox`
   - this should also be true of the `codex` multitool CLI
 
-On Linux, the bubblewrap pipeline uses the vendored bubblewrap path compiled
-into this binary.
+On Linux, the bubblewrap pipeline prefers the system `/usr/bin/bwrap` whenever
+it is available. If `/usr/bin/bwrap` is missing, the helper still falls back to
+the vendored bubblewrap path compiled into this binary.
+Codex also surfaces a startup warning when `/usr/bin/bwrap` is missing so users
+know it is falling back to the vendored helper.
 
 **Current Behavior**
 - Legacy `SandboxPolicy` / `sandbox_mode` configs remain supported.
-- Bubblewrap is the default filesystem sandbox pipeline and is standardized on
-  the vendored path.
+- Bubblewrap is the default filesystem sandbox pipeline.
+- If `/usr/bin/bwrap` is present, the helper uses it.
+- If `/usr/bin/bwrap` is missing, the helper falls back to the vendored
+  bubblewrap path.
+- If `/usr/bin/bwrap` is missing, Codex also surfaces a startup warning instead
+  of printing directly from the sandbox helper.
 - Legacy Landlock + mount protections remain available as an explicit legacy
   fallback path.
 - Set `features.use_legacy_landlock = true` (or CLI `-c use_legacy_landlock=true`)

codex-rs/linux-sandbox/src/launcher.rs

@@ -0,0 +1,134 @@
+use std::ffi::CString;
+use std::fs::File;
+use std::os::fd::AsRawFd;
+use std::os::raw::c_char;
+use std::os::unix::ffi::OsStrExt;
+use std::path::Path;
+
+use crate::vendored_bwrap::exec_vendored_bwrap;
+use codex_utils_absolute_path::AbsolutePathBuf;
+
+const SYSTEM_BWRAP_PATH: &str = "/usr/bin/bwrap";
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+enum BubblewrapLauncher {
+    System(AbsolutePathBuf),
+    Vendored,
+}
+
+pub(crate) fn exec_bwrap(argv: Vec<String>, preserved_files: Vec<File>) -> ! {
+    match preferred_bwrap_launcher() {
+        BubblewrapLauncher::System(program) => exec_system_bwrap(&program, argv, preserved_files),
+        BubblewrapLauncher::Vendored => exec_vendored_bwrap(argv, preserved_files),
+    }
+}
+
+fn preferred_bwrap_launcher() -> BubblewrapLauncher {
+    if !Path::new(SYSTEM_BWRAP_PATH).is_file() {
+        return BubblewrapLauncher::Vendored;
+    }
+
+    let system_bwrap_path = match AbsolutePathBuf::from_absolute_path(SYSTEM_BWRAP_PATH) {
+        Ok(path) => path,
+        Err(err) => panic!("failed to normalize system bubblewrap path {SYSTEM_BWRAP_PATH}: {err}"),
+    };
+    BubblewrapLauncher::System(system_bwrap_path)
+}
+
+fn exec_system_bwrap(
+    program: &AbsolutePathBuf,
+    argv: Vec<String>,
+    preserved_files: Vec<File>,
+) -> ! {
+    // System bwrap runs across an exec boundary, so preserved fds must survive exec.
+    make_files_inheritable(&preserved_files);
+
+    let program_path = program.as_path().display().to_string();
+    let program = CString::new(program.as_path().as_os_str().as_bytes())
+        .unwrap_or_else(|err| panic!("invalid system bubblewrap path: {err}"));
+    let cstrings = argv_to_cstrings(&argv);
+    let mut argv_ptrs: Vec<*const c_char> = cstrings.iter().map(|arg| arg.as_ptr()).collect();
+    argv_ptrs.push(std::ptr::null());
+
+    // SAFETY: `program` and every entry in `argv_ptrs` are valid C strings for
+    // the duration of the call. On success `execv` does not return.
+    unsafe {
+        libc::execv(program.as_ptr(), argv_ptrs.as_ptr());
+    }
+    let err = std::io::Error::last_os_error();
+    panic!("failed to exec system bubblewrap {program_path}: {err}");
+}
+
+fn argv_to_cstrings(argv: &[String]) -> Vec<CString> {
+    let mut cstrings: Vec<CString> = Vec::with_capacity(argv.len());
+    for arg in argv {
+        match CString::new(arg.as_str()) {
+            Ok(value) => cstrings.push(value),
+            Err(err) => panic!("failed to convert argv to CString: {err}"),
+        }
+    }
+    cstrings
+}
+
+fn make_files_inheritable(files: &[File]) {
+    for file in files {
+        clear_cloexec(file.as_raw_fd());
+    }
+}
+
+fn clear_cloexec(fd: libc::c_int) {
+    // SAFETY: `fd` is an owned descriptor kept alive by `files`.
+    let flags = unsafe { libc::fcntl(fd, libc::F_GETFD) };
+    if flags < 0 {
+        let err = std::io::Error::last_os_error();
+        panic!("failed to read fd flags for preserved bubblewrap file descriptor {fd}: {err}");
+    }
+    let cleared_flags = flags & !libc::FD_CLOEXEC;
+    if cleared_flags == flags {
+        return;
+    }
+
+    // SAFETY: `fd` is valid and we are only clearing FD_CLOEXEC.
+    let result = unsafe { libc::fcntl(fd, libc::F_SETFD, cleared_flags) };
+    if result < 0 {
+        let err = std::io::Error::last_os_error();
+        panic!("failed to clear CLOEXEC for preserved bubblewrap file descriptor {fd}: {err}");
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+    use tempfile::NamedTempFile;
+
+    #[test]
+    fn preserved_files_are_made_inheritable_for_system_exec() {
+        let file = NamedTempFile::new().expect("temp file");
+        set_cloexec(file.as_file().as_raw_fd());
+
+        make_files_inheritable(std::slice::from_ref(file.as_file()));
+
+        assert_eq!(fd_flags(file.as_file().as_raw_fd()) & libc::FD_CLOEXEC, 0);
+    }
+
+    fn set_cloexec(fd: libc::c_int) {
+        let flags = fd_flags(fd);
+        // SAFETY: `fd` is valid for the duration of the test.
+        let result = unsafe { libc::fcntl(fd, libc::F_SETFD, flags | libc::FD_CLOEXEC) };
+        if result < 0 {
+            let err = std::io::Error::last_os_error();
+            panic!("failed to set CLOEXEC for test fd {fd}: {err}");
+        }
+    }
+
+    fn fd_flags(fd: libc::c_int) -> libc::c_int {
+        // SAFETY: `fd` is valid for the duration of the test.
+        let flags = unsafe { libc::fcntl(fd, libc::F_GETFD) };
+        if flags < 0 {
+            let err = std::io::Error::last_os_error();
+            panic!("failed to read fd flags for test fd {fd}: {err}");
+        }
+        flags
+    }
+}

codex-rs/linux-sandbox/src/lib.rs

@@ -8,6 +8,8 @@ mod bwrap;
 #[cfg(target_os = "linux")]
 mod landlock;
 #[cfg(target_os = "linux")]
+mod launcher;
+#[cfg(target_os = "linux")]
 mod linux_run_main;
 #[cfg(target_os = "linux")]
 mod proxy_routing;

codex-rs/linux-sandbox/src/linux_run_main.rs

@@ -11,10 +11,9 @@ use crate::bwrap::BwrapNetworkMode;
 use crate::bwrap::BwrapOptions;
 use crate::bwrap::create_bwrap_command_args;
 use crate::landlock::apply_sandbox_policy_to_current_thread;
+use crate::launcher::exec_bwrap;
 use crate::proxy_routing::activate_proxy_routes_in_netns;
 use crate::proxy_routing::prepare_host_proxy_route_spec;
-use crate::vendored_bwrap::exec_vendored_bwrap;
-use crate::vendored_bwrap::run_vendored_bwrap_main;
 use codex_protocol::protocol::FileSystemSandboxPolicy;
 use codex_protocol::protocol::NetworkSandboxPolicy;
 use codex_protocol::protocol::SandboxPolicy;
@@ -434,7 +433,7 @@ fn run_bwrap_with_proc_fallback(
         command_cwd,
         options,
     );
-    exec_vendored_bwrap(bwrap_args.args, bwrap_args.preserved_files);
+    exec_bwrap(bwrap_args.args, bwrap_args.preserved_files);
 }
 
 fn bwrap_network_mode(
@@ -568,8 +567,7 @@ fn run_bwrap_in_child_capture_stderr(bwrap_args: crate::bwrap::BwrapArgs) -> Str
             close_fd_or_panic(write_fd, "close write end in bubblewrap child");
         }
 
-        let exit_code = run_vendored_bwrap_main(&bwrap_args.args, &bwrap_args.preserved_files);
-        std::process::exit(exit_code);
+        exec_bwrap(bwrap_args.args, bwrap_args.preserved_files);
     }
 
     // Parent: close the write end and read stderr while the child runs.

codex-rs/linux-sandbox/src/vendored_bwrap.rs

@@ -76,4 +76,3 @@ Notes:
 }
 
 pub(crate) use imp::exec_vendored_bwrap;
-pub(crate) use imp::run_vendored_bwrap_main;