core: add a sandbox-backed fs helper

codex-rs/Cargo.lock

@@ -1572,6 +1572,7 @@ version = "0.0.0"
 dependencies = [
  "anyhow",
  "codex-apply-patch",
+ "codex-fs-ops",
  "codex-linux-sandbox",
  "codex-shell-escalation",
  "codex-utils-home-dir",
@@ -1863,6 +1864,7 @@ dependencies = [
  "codex-execpolicy",
  "codex-features",
  "codex-file-search",
+ "codex-fs-ops",
  "codex-git",
  "codex-hooks",
  "codex-login",
@@ -2119,6 +2121,14 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "codex-fs-ops"
+version = "0.0.0"
+dependencies = [
+ "pretty_assertions",
+ "tempfile",
+]
+
 [[package]]
 name = "codex-git"
 version = "0.0.0"

codex-rs/Cargo.toml

@@ -10,8 +10,6 @@ members = [
     "debug-client",
     "apply-patch",
     "arg0",
-    "feedback",
-    "features",
     "codex-backend-openapi-models",
     "cloud-requirements",
     "cloud-tasks",
@@ -29,6 +27,9 @@ members = [
     "exec-server",
     "execpolicy",
     "execpolicy-legacy",
+    "feedback",
+    "features",
+    "fs-ops",
     "keyring-store",
     "file-search",
     "linux-sandbox",
@@ -111,9 +112,10 @@ codex-exec = { path = "exec" }
 codex-exec-server = { path = "exec-server" }
 codex-execpolicy = { path = "execpolicy" }
 codex-experimental-api-macros = { path = "codex-experimental-api-macros" }
-codex-feedback = { path = "feedback" }
 codex-features = { path = "features" }
+codex-feedback = { path = "feedback" }
 codex-file-search = { path = "file-search" }
+codex-fs-ops = { path = "fs-ops" }
 codex-git = { path = "utils/git" }
 codex-hooks = { path = "hooks" }
 codex-keyring-store = { path = "keyring-store" }

codex-rs/arg0/Cargo.toml

@@ -14,6 +14,7 @@ workspace = true
 [dependencies]
 anyhow = { workspace = true }
 codex-apply-patch = { workspace = true }
+codex-fs-ops = { workspace = true }
 codex-linux-sandbox = { workspace = true }
 codex-shell-escalation = { workspace = true }
 codex-utils-home-dir = { workspace = true }

codex-rs/arg0/src/lib.rs

@@ -4,6 +4,7 @@ use std::path::Path;
 use std::path::PathBuf;
 
 use codex_apply_patch::CODEX_CORE_APPLY_PATCH_ARG1;
+use codex_fs_ops::CODEX_CORE_FS_OPS_ARG1;
 use codex_utils_home_dir::find_codex_home;
 #[cfg(unix)]
 use std::os::unix::fs::symlink;
@@ -105,6 +106,12 @@ pub fn arg0_dispatch() -> Option<Arg0PathEntryGuard> {
         };
         std::process::exit(exit_code);
     }
+    if argv1 == CODEX_CORE_FS_OPS_ARG1 {
+        let mut stdin = std::io::stdin();
+        let mut stdout = std::io::stdout();
+        let mut stderr = std::io::stderr();
+        codex_fs_ops::run_from_args_and_exit(args, &mut stdin, &mut stdout, &mut stderr);
+    }
 
     // This modifies the environment, which is not thread-safe, so do this
     // before creating any threads/the Tokio runtime.

codex-rs/core/Cargo.toml

@@ -39,6 +39,7 @@ codex-login = { workspace = true }
 codex-shell-command = { workspace = true }
 codex-skills = { workspace = true }
 codex-execpolicy = { workspace = true }
+codex-fs-ops = { workspace = true }
 codex-file-search = { workspace = true }
 codex-git = { workspace = true }
 codex-hooks = { workspace = true }

codex-rs/core/src/exec.rs

@@ -319,6 +319,62 @@ pub(crate) async fn execute_exec_request(
     stdout_stream: Option<StdoutStream>,
     after_spawn: Option<Box<dyn FnOnce() + Send>>,
 ) -> Result<ExecToolCallOutput> {
+    let PreparedExecRequest {
+        params,
+        sandbox,
+        file_system_sandbox_policy,
+        network_sandbox_policy,
+    } = prepare_exec_request(exec_request);
+    let start = Instant::now();
+    let raw_output_result = exec(
+        params,
+        sandbox,
+        sandbox_policy,
+        &file_system_sandbox_policy,
+        network_sandbox_policy,
+        stdout_stream,
+        after_spawn,
+    )
+    .await;
+    let duration = start.elapsed();
+    Ok(normalize_exec_result(raw_output_result, sandbox, duration)?.to_utf8_lossy_output())
+}
+
+pub(crate) async fn execute_exec_request_raw_output(
+    exec_request: ExecRequest,
+    sandbox_policy: &SandboxPolicy,
+    stdout_stream: Option<StdoutStream>,
+    after_spawn: Option<Box<dyn FnOnce() + Send>>,
+) -> Result<ExecToolCallRawOutput> {
+    let PreparedExecRequest {
+        params,
+        sandbox,
+        file_system_sandbox_policy,
+        network_sandbox_policy,
+    } = prepare_exec_request(exec_request);
+    let start = Instant::now();
+    let raw_output_result = exec(
+        params,
+        sandbox,
+        sandbox_policy,
+        &file_system_sandbox_policy,
+        network_sandbox_policy,
+        stdout_stream,
+        after_spawn,
+    )
+    .await;
+    let duration = start.elapsed();
+    normalize_exec_result(raw_output_result, sandbox, duration)
+}
+
+struct PreparedExecRequest {
+    params: ExecParams,
+    sandbox: SandboxType,
+    file_system_sandbox_policy: FileSystemSandboxPolicy,
+    network_sandbox_policy: NetworkSandboxPolicy,
+}
+
+fn prepare_exec_request(exec_request: ExecRequest) -> PreparedExecRequest {
     let ExecRequest {
         command,
         cwd,
@@ -338,33 +394,24 @@ pub(crate) async fn execute_exec_request(
     } = exec_request;
     let _ = _sandbox_policy_from_env;
 
-    let params = ExecParams {
-        command,
-        cwd,
-        expiration,
-        capture_policy,
-        env,
-        network: network.clone(),
-        sandbox_permissions,
-        windows_sandbox_level,
-        windows_sandbox_private_desktop,
-        justification,
-        arg0,
-    };
-
-    let start = Instant::now();
-    let raw_output_result = exec(
-        params,
+    PreparedExecRequest {
+        params: ExecParams {
+            command,
+            cwd,
+            expiration,
+            capture_policy,
+            env,
+            network,
+            sandbox_permissions,
+            windows_sandbox_level,
+            windows_sandbox_private_desktop,
+            justification,
+            arg0,
+        },
         sandbox,
-        sandbox_policy,
-        &file_system_sandbox_policy,
         network_sandbox_policy,
-        stdout_stream,
-        after_spawn,
-    )
-    .await;
-    let duration = start.elapsed();
-    finalize_exec_result(raw_output_result, sandbox, duration)
+        file_system_sandbox_policy,
+    }
 }
 
 #[cfg(target_os = "windows")]
@@ -558,19 +605,25 @@ async fn exec_windows_sandbox(
     })
 }
 
-fn finalize_exec_result(
+fn normalize_exec_result(
     raw_output_result: std::result::Result<RawExecToolCallOutput, CodexErr>,
     sandbox_type: SandboxType,
     duration: Duration,
-) -> Result<ExecToolCallOutput> {
+) -> Result<ExecToolCallRawOutput> {
     match raw_output_result {
         Ok(raw_output) => {
-            #[allow(unused_mut)]
-            let mut timed_out = raw_output.timed_out;
+            let RawExecToolCallOutput {
+                exit_status,
+                stdout,
+                stderr,
+                aggregated_output,
+                #[cfg_attr(target_os = "windows", allow(unused_mut))]
+                mut timed_out,
+            } = raw_output;
 
             #[cfg(target_family = "unix")]
             {
-                if let Some(signal) = raw_output.exit_status.signal() {
+                if let Some(signal) = exit_status.signal() {
                     if signal == TIMEOUT_CODE {
                         timed_out = true;
                     } else {
@@ -579,15 +632,12 @@ fn finalize_exec_result(
                 }
             }
 
-            let mut exit_code = raw_output.exit_status.code().unwrap_or(-1);
+            let mut exit_code = exit_status.code().unwrap_or(-1);
             if timed_out {
                 exit_code = EXEC_TIMEOUT_EXIT_CODE;
             }
 
-            let stdout = raw_output.stdout.from_utf8_lossy();
-            let stderr = raw_output.stderr.from_utf8_lossy();
-            let aggregated_output = raw_output.aggregated_output.from_utf8_lossy();
-            let exec_output = ExecToolCallOutput {
+            let exec_output = ExecToolCallRawOutput {
                 exit_code,
                 stdout,
                 stderr,
@@ -598,13 +648,14 @@ fn finalize_exec_result(
 
             if timed_out {
                 return Err(CodexErr::Sandbox(SandboxErr::Timeout {
-                    output: Box::new(exec_output),
+                    output: Box::new(exec_output.to_utf8_lossy_output()),
                 }));
             }
 
-            if is_likely_sandbox_denied(sandbox_type, &exec_output) {
+            let string_output = exec_output.to_utf8_lossy_output();
+            if is_likely_sandbox_denied(sandbox_type, &string_output) {
                 return Err(CodexErr::Sandbox(SandboxErr::Denied {
-                    output: Box::new(exec_output),
+                    output: Box::new(string_output),
                     network_policy_decision: None,
                 }));
             }
@@ -796,6 +847,16 @@ pub struct ExecToolCallOutput {
     pub timed_out: bool,
 }
 
+#[derive(Clone, Debug)]
+pub(crate) struct ExecToolCallRawOutput {
+    pub exit_code: i32,
+    pub stdout: StreamOutput<Vec<u8>>,
+    pub stderr: StreamOutput<Vec<u8>>,
+    pub aggregated_output: StreamOutput<Vec<u8>>,
+    pub duration: Duration,
+    pub timed_out: bool,
+}
+
 impl Default for ExecToolCallOutput {
     fn default() -> Self {
         Self {
@@ -809,6 +870,19 @@ impl Default for ExecToolCallOutput {
     }
 }
 
+impl ExecToolCallRawOutput {
+    fn to_utf8_lossy_output(&self) -> ExecToolCallOutput {
+        ExecToolCallOutput {
+            exit_code: self.exit_code,
+            stdout: self.stdout.from_utf8_lossy(),
+            stderr: self.stderr.from_utf8_lossy(),
+            aggregated_output: self.aggregated_output.from_utf8_lossy(),
+            duration: self.duration,
+            timed_out: self.timed_out,
+        }
+    }
+}
+
 #[cfg_attr(not(target_os = "windows"), allow(unused_variables))]
 async fn exec(
     params: ExecParams,

codex-rs/core/src/lib.rs

@@ -121,6 +121,7 @@ pub mod default_client {
 pub mod project_doc;
 mod rollout;
 pub(crate) mod safety;
+mod sandboxed_fs;
 pub mod seatbelt;
 pub mod shell;
 pub mod shell_snapshot;

codex-rs/core/src/sandboxed_fs.rs

@@ -0,0 +1,161 @@
+use crate::codex::Session;
+use crate::codex::TurnContext;
+use crate::exec::ExecCapturePolicy;
+use crate::exec::ExecExpiration;
+use crate::exec::ExecToolCallRawOutput;
+use crate::exec::execute_exec_request_raw_output;
+use crate::sandboxing::CommandSpec;
+use crate::sandboxing::SandboxPermissions;
+use crate::sandboxing::merge_permission_profiles;
+use crate::tools::sandboxing::SandboxAttempt;
+use crate::tools::sandboxing::SandboxablePreference;
+use codex_fs_ops::CODEX_CORE_FS_OPS_ARG1;
+use codex_fs_ops::READ_FILE_OPERATION_ARG2;
+use codex_protocol::models::PermissionProfile;
+use codex_utils_absolute_path::AbsolutePathBuf;
+use std::collections::HashMap;
+use std::sync::Arc;
+use std::time::Duration;
+
+/// Reads the contents of the specified file subject to the sandbox constraints
+/// imposed by the provided session and turn context.
+///
+/// Note that this function is comparable to `cat FILE`, though unlike `cat
+/// FILE`, this function verifies that FILE is a regular file before reading,
+/// which means that if you pass `/dev/zero` as the path, it will error (rather
+/// than hang forever).
+#[allow(dead_code)]
+pub(crate) async fn read_file(
+    path: &AbsolutePathBuf,
+    session: &Arc<Session>,
+    turn: &Arc<TurnContext>,
+) -> Result<Vec<u8>, SandboxedFsError> {
+    let output = perform_operation(
+        SandboxedFsOperation::Read { path: path.clone() },
+        session,
+        turn,
+    )
+    .await?;
+    Ok(output.stdout.text)
+}
+
+/// Operations supported by the [CODEX_CORE_FS_OPS_ARG1] sandbox helper.
+enum SandboxedFsOperation {
+    Read { path: AbsolutePathBuf },
+}
+
+async fn perform_operation(
+    operation: SandboxedFsOperation,
+    session: &Arc<Session>,
+    turn: &Arc<TurnContext>,
+) -> Result<ExecToolCallRawOutput, SandboxedFsError> {
+    let exe = std::env::current_exe().map_err(|error| SandboxedFsError::ResolveExe {
+        message: error.to_string(),
+    })?;
+    let additional_permissions = effective_granted_permissions(session).await;
+    let sandbox_manager = crate::sandboxing::SandboxManager::new();
+    let attempt = SandboxAttempt {
+        sandbox: sandbox_manager.select_initial(
+            &turn.file_system_sandbox_policy,
+            turn.network_sandbox_policy,
+            SandboxablePreference::Auto,
+            turn.windows_sandbox_level,
+            /*has_managed_network_requirements*/ false,
+        ),
+        policy: &turn.sandbox_policy,
+        file_system_policy: &turn.file_system_sandbox_policy,
+        network_policy: turn.network_sandbox_policy,
+        enforce_managed_network: false,
+        manager: &sandbox_manager,
+        sandbox_cwd: &turn.cwd,
+        codex_linux_sandbox_exe: turn.codex_linux_sandbox_exe.as_ref(),
+        use_legacy_landlock: turn.features.use_legacy_landlock(),
+        windows_sandbox_level: turn.windows_sandbox_level,
+        windows_sandbox_private_desktop: turn.config.permissions.windows_sandbox_private_desktop,
+    };
+
+    let args = match operation {
+        SandboxedFsOperation::Read { ref path } => vec![
+            CODEX_CORE_FS_OPS_ARG1.to_string(),
+            READ_FILE_OPERATION_ARG2.to_string(),
+            path.to_string_lossy().to_string(),
+        ],
+    };
+
+    // `FullBuffer` reads ignore exec expiration, but `ExecRequest` still requires
+    // an `expiration` field, so keep a placeholder timeout here until that API
+    // changes.
+    let ignored_expiration = Duration::from_secs(30);
+    let exec_request = attempt
+        .env_for(
+            CommandSpec {
+                program: exe.to_string_lossy().to_string(),
+                args,
+                cwd: turn.cwd.clone(),
+                env: HashMap::new(),
+                expiration: ExecExpiration::Timeout(ignored_expiration),
+                capture_policy: ExecCapturePolicy::FullBuffer,
+                sandbox_permissions: SandboxPermissions::UseDefault,
+                additional_permissions,
+                justification: None,
+            },
+            /*network*/ None,
+        )
+        .map_err(|error| SandboxedFsError::ProcessFailed {
+            exit_code: -1,
+            message: error.to_string(),
+        })?;
+
+    let effective_policy = exec_request.sandbox_policy.clone();
+    let output = execute_exec_request_raw_output(
+        exec_request,
+        &effective_policy,
+        /*stdout_stream*/ None,
+        /*after_spawn*/ None,
+    )
+    .await
+    .map_err(|error| SandboxedFsError::ProcessFailed {
+        exit_code: 1,
+        message: error.to_string(),
+    })?;
+    if output.exit_code == 0 {
+        Ok(output)
+    } else {
+        Err(parse_helper_failure(
+            output.exit_code,
+            &output.stderr.text,
+            &output.stdout.text,
+        ))
+    }
+}
+
+async fn effective_granted_permissions(session: &Session) -> Option<PermissionProfile> {
+    let granted_session_permissions = session.granted_session_permissions().await;
+    let granted_turn_permissions = session.granted_turn_permissions().await;
+    merge_permission_profiles(
+        granted_session_permissions.as_ref(),
+        granted_turn_permissions.as_ref(),
+    )
+}
+
+fn parse_helper_failure(exit_code: i32, stderr: &[u8], stdout: &[u8]) -> SandboxedFsError {
+    let stderr = String::from_utf8_lossy(stderr);
+    let stdout = String::from_utf8_lossy(stdout);
+    let message = if !stderr.trim().is_empty() {
+        stderr.trim().to_string()
+    } else if !stdout.trim().is_empty() {
+        stdout.trim().to_string()
+    } else {
+        "no error details emitted".to_string()
+    };
+
+    SandboxedFsError::ProcessFailed { exit_code, message }
+}
+
+#[derive(Debug, thiserror::Error)]
+pub(crate) enum SandboxedFsError {
+    #[error("failed to determine codex executable: {message}")]
+    ResolveExe { message: String },
+    #[error("sandboxed fs helper exited with code {exit_code}: {message}")]
+    ProcessFailed { exit_code: i32, message: String },
+}

codex-rs/fs-ops/BUILD.bazel

@@ -0,0 +1,6 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "fs-ops",
+    crate_name = "codex_fs_ops",
+)

codex-rs/fs-ops/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "codex-fs-ops"
+edition.workspace = true
+license.workspace = true
+version.workspace = true
+
+[lib]
+name = "codex_fs_ops"
+path = "src/lib.rs"
+
+[lints]
+workspace = true
+
+[dev-dependencies]
+pretty_assertions = { workspace = true }
+tempfile = { workspace = true }

codex-rs/fs-ops/src/command.rs

@@ -0,0 +1,37 @@
+use crate::constants::READ_FILE_OPERATION_ARG2;
+use std::ffi::OsString;
+use std::path::PathBuf;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum FsCommand {
+    ReadFile { path: PathBuf },
+}
+
+pub fn parse_command_from_args(
+    mut args: impl Iterator<Item = OsString>,
+) -> Result<FsCommand, String> {
+    let Some(operation) = args.next() else {
+        return Err("missing operation".to_string());
+    };
+    let Some(operation) = operation.to_str() else {
+        return Err("operation must be valid UTF-8".to_string());
+    };
+    let Some(path) = args.next() else {
+        return Err(format!("missing path for operation `{operation}`"));
+    };
+    if args.next().is_some() {
+        return Err(format!(
+            "unexpected extra arguments for operation `{operation}`"
+        ));
+    }
+
+    let path = PathBuf::from(path);
+    match operation {
+        READ_FILE_OPERATION_ARG2 => Ok(FsCommand::ReadFile { path }),
+        _ => Err(format!("unsupported filesystem operation `{operation}`")),
+    }
+}
+
+#[cfg(test)]
+#[path = "command_tests.rs"]
+mod tests;

codex-rs/fs-ops/src/command_tests.rs

@@ -0,0 +1,21 @@
+use super::FsCommand;
+use super::READ_FILE_OPERATION_ARG2;
+use super::parse_command_from_args;
+use pretty_assertions::assert_eq;
+
+#[test]
+fn parse_read_command() {
+    let command = parse_command_from_args(
+        [READ_FILE_OPERATION_ARG2, "/tmp/example.png"]
+            .into_iter()
+            .map(Into::into),
+    )
+    .expect("command should parse");
+
+    assert_eq!(
+        command,
+        FsCommand::ReadFile {
+            path: "/tmp/example.png".into(),
+        }
+    );
+}

codex-rs/fs-ops/src/constants.rs

@@ -0,0 +1,8 @@
+/// Special argv[1] flag used when the Codex executable self-invokes to run the
+/// internal sandbox-backed filesystem helper path.
+pub const CODEX_CORE_FS_OPS_ARG1: &str = "--codex-run-as-fs-ops";
+
+/// When passed as argv[2] to the Codex filesystem helper, it should be followed
+/// by a single path argument, and the helper will read the contents of the file
+/// at that path and write it to stdout.
+pub const READ_FILE_OPERATION_ARG2: &str = "read";

codex-rs/fs-ops/src/lib.rs

@@ -0,0 +1,13 @@
+//! The codex-fs-ops crate provides a helper binary for performing various
+//! filesystem operations when `codex` is invoked with `--codex-run-as-fs-ops`
+//! as the first argument. By exposing this functionality via a CLI, this makes
+//! it possible to execute the CLI within a sandboxed context in order to ensure
+//! the filesystem restrictions of the sandbox are honored.
+
+mod command;
+mod constants;
+mod runner;
+
+pub use constants::CODEX_CORE_FS_OPS_ARG1;
+pub use constants::READ_FILE_OPERATION_ARG2;
+pub use runner::run_from_args_and_exit;

codex-rs/fs-ops/src/runner.rs

@@ -0,0 +1,66 @@
+use crate::command::FsCommand;
+use crate::command::parse_command_from_args;
+use std::ffi::OsString;
+use std::io::Read;
+use std::io::Write;
+
+/// Runs the fs-ops helper with the given arguments and I/O streams.
+pub fn run_from_args_and_exit(
+    args: impl Iterator<Item = OsString>,
+    stdin: &mut impl Read,
+    stdout: &mut impl Write,
+    stderr: &mut impl Write,
+) -> ! {
+    let exit_code = match run_from_args(args, stdin, stdout, stderr) {
+        Ok(()) => 0,
+        Err(_) => {
+            // Discard the specific error, since we already wrote it to stderr.
+            1
+        }
+    };
+    std::process::exit(exit_code);
+}
+
+/// Testable version of `run_from_args_and_exit` that returns a Result instead
+/// of exiting the process.
+fn run_from_args(
+    args: impl Iterator<Item = OsString>,
+    stdin: &mut impl Read,
+    stdout: &mut impl Write,
+    stderr: &mut impl Write,
+) -> std::io::Result<()> {
+    try_run_from_args(args, stdin, stdout).inspect_err(|error| {
+        writeln!(stderr, "error: {error}").ok();
+    })
+}
+
+fn try_run_from_args(
+    args: impl Iterator<Item = OsString>,
+    _stdin: &mut impl Read,
+    stdout: &mut impl Write,
+) -> std::io::Result<()> {
+    let command = parse_command_from_args(args)
+        .map_err(|error| std::io::Error::new(std::io::ErrorKind::InvalidInput, error))?;
+
+    match command {
+        FsCommand::ReadFile { path } => {
+            let mut file = std::fs::File::open(&path)?;
+            if !file.metadata()?.is_file() {
+                let error_message = format!(
+                    "`{path}` is not a regular file",
+                    path = path.to_string_lossy()
+                );
+                return Err(std::io::Error::new(
+                    std::io::ErrorKind::InvalidInput,
+                    error_message,
+                ));
+            }
+
+            std::io::copy(&mut file, stdout).map(|_| ())
+        }
+    }
+}
+
+#[cfg(test)]
+#[path = "runner_tests.rs"]
+mod tests;

codex-rs/fs-ops/src/runner_tests.rs

@@ -0,0 +1,114 @@
+use super::run_from_args;
+use crate::READ_FILE_OPERATION_ARG2;
+use pretty_assertions::assert_eq;
+use tempfile::tempdir;
+
+#[test]
+fn run_from_args_streams_file_bytes_to_stdout() {
+    let tempdir = tempdir().expect("tempdir");
+    let path = tempdir.path().join("image.bin");
+    let expected = b"hello\x00world".to_vec();
+    std::fs::write(&path, &expected).expect("write test file");
+
+    let mut stdin = std::io::empty();
+    let mut stdout = Vec::new();
+    let mut stderr = Vec::new();
+    run_from_args(
+        [
+            READ_FILE_OPERATION_ARG2,
+            path.to_str().expect("utf-8 test path"),
+        ]
+        .into_iter()
+        .map(Into::into),
+        &mut stdin,
+        &mut stdout,
+        &mut stderr,
+    )
+    .expect("read should succeed");
+
+    assert_eq!(stdout, expected);
+    assert_eq!(stderr, Vec::<u8>::new());
+}
+
+#[test]
+#[cfg(unix)]
+fn rejects_path_that_is_not_a_regular_file() {
+    let path = std::path::PathBuf::from("/dev/zero");
+
+    let mut stdin = std::io::empty();
+    let mut stdout = Vec::new();
+    let mut stderr = Vec::new();
+    let error = run_from_args(
+        [
+            READ_FILE_OPERATION_ARG2,
+            path.to_str().expect("utf-8 test path"),
+        ]
+        .into_iter()
+        .map(Into::into),
+        &mut stdin,
+        &mut stdout,
+        &mut stderr,
+    )
+    .expect_err(
+        r#"reading a non-regular file should fail or else
+the user risks hanging the process by trying
+to read from something like /dev/zero"#,
+    );
+
+    assert_eq!(error.kind(), std::io::ErrorKind::InvalidInput);
+    assert_eq!(stdout, Vec::<u8>::new());
+    assert_eq!(
+        "error: `/dev/zero` is not a regular file\n",
+        String::from_utf8_lossy(&stderr),
+    );
+}
+
+#[test]
+fn read_reports_directory_error() {
+    let tempdir = tempdir().expect("tempdir");
+    let mut stdin = std::io::empty();
+    let mut stdout = Vec::new();
+    let mut stderr = Vec::new();
+
+    let error = run_from_args(
+        [
+            READ_FILE_OPERATION_ARG2,
+            tempdir.path().to_str().expect("utf-8 test path"),
+        ]
+        .into_iter()
+        .map(Into::into),
+        &mut stdin,
+        &mut stdout,
+        &mut stderr,
+    )
+    .expect_err("reading a directory should fail");
+
+    #[cfg(not(windows))]
+    assert_eq!(error.kind(), std::io::ErrorKind::InvalidInput);
+    #[cfg(windows)]
+    assert_eq!(error.kind(), std::io::ErrorKind::PermissionDenied);
+}
+
+#[test]
+fn run_from_args_serializes_errors_to_stderr() {
+    let tempdir = tempdir().expect("tempdir");
+    let missing = tempdir.path().join("missing.txt");
+    let mut stdin = std::io::empty();
+    let mut stdout = Vec::new();
+    let mut stderr = Vec::new();
+
+    let result = run_from_args(
+        [
+            READ_FILE_OPERATION_ARG2,
+            missing.to_str().expect("utf-8 test path"),
+        ]
+        .into_iter()
+        .map(Into::into),
+        &mut stdin,
+        &mut stdout,
+        &mut stderr,
+    );
+
+    assert!(result.is_err(), "missing file should fail");
+    assert_eq!(stdout, Vec::<u8>::new());
+}