test(e2e): reproduce constrained dev shm failure

crates/fspy/tests/rust_std.rs

@@ -2,7 +2,7 @@ mod test_utils;
 
 use std::{
     env::current_dir,
-    fs::{File, OpenOptions},
+    fs::{self, File, OpenOptions},
     process::Stdio,
 };
 
@@ -35,6 +35,22 @@ async fn open_write() -> anyhow::Result<()> {
     Ok(())
 }
 
+#[test(tokio::test)]
+async fn metadata() -> anyhow::Result<()> {
+    let tmp_dir = tempfile::tempdir()?;
+    let tmp_path = tmp_dir.path().join("hello");
+    File::create(&tmp_path)?;
+    let tmp_path_str = tmp_path.to_str().unwrap().to_owned();
+
+    let accesses = track_fn!(tmp_path_str, |tmp_path_str: String| {
+        let _ = fs::metadata(tmp_path_str);
+    })
+    .await?;
+    assert_contains(&accesses, tmp_path.as_path(), AccessMode::READ);
+
+    Ok(())
+}
+
 #[test(tokio::test)]
 async fn readdir() -> anyhow::Result<()> {
     let tmpdir = tempfile::tempdir()?;

crates/fspy_preload_unix/src/interceptions/linux_syscall.rs

@@ -2,7 +2,10 @@ use fspy_shared::ipc::AccessMode;
 use libc::{c_char, c_int, c_long};
 
 use crate::{
-    client::{convert::PathAt, handle_open},
+    client::{
+        convert::{Fd, PathAt},
+        handle_open,
+    },
     macros::intercept,
 };
 
@@ -23,16 +26,26 @@ unsafe extern "C" fn syscall(syscall_no: c_long, mut args: ...) -> c_long {
     let a5 = unsafe { args.next_arg::<c_long>() };
 
     if syscall_no == libc::SYS_statx {
-        // c-style conversion is expected: (4294967196 -> -100 aka libc::AT_FDCWD)
+        // C-style conversions are expected for the variadic syscall arguments.
         #[expect(
             clippy::cast_possible_truncation,
-            reason = "c-style conversion is expected: (4294967196 -> -100 aka libc::AT_FDCWD)"
+            reason = "C-style conversion from c_long syscall arguments to c_int"
         )]
         let dirfd = a0 as c_int;
         let pathname = a1 as *const c_char;
-        // SAFETY: pathname is a valid pointer to a null-terminated C string provided via the syscall arguments
-        unsafe {
-            handle_open(PathAt(dirfd, pathname), AccessMode::READ);
+        #[expect(
+            clippy::cast_possible_truncation,
+            reason = "C-style conversion from c_long syscall arguments to c_int"
+        )]
+        let flags = a2 as c_int;
+        if pathname.is_null() {
+            if flags & libc::AT_EMPTY_PATH != 0 {
+                // SAFETY: dirfd is provided by the statx syscall caller.
+                unsafe { handle_open(Fd(dirfd), AccessMode::READ) };
+            }
+        } else {
+            // SAFETY: pathname is a non-null C string pointer provided by the statx syscall caller.
+            unsafe { handle_open(PathAt(dirfd, pathname), AccessMode::READ) };
         }
     }
     // SAFETY: forwarding the syscall to the original libc syscall function with the extracted arguments

crates/fspy_preload_unix/src/interceptions/stat.rs

@@ -1,6 +1,8 @@
 use fspy_shared::ipc::AccessMode;
 use libc::{c_char, c_int, stat as stat_struct};
 
+#[cfg(target_os = "linux")]
+use crate::client::convert::Fd;
 use crate::{
     client::{convert::PathAt, handle_open},
     macros::intercept,
@@ -41,3 +43,40 @@ unsafe extern "C" fn fstatat(
     // SAFETY: calling the original libc fstatat() with the same arguments forwarded from the interposed function
     unsafe { fstatat::original()(dirfd, pathname, buf, flags) }
 }
+
+#[cfg(target_os = "linux")]
+intercept!(statx: unsafe extern "C" fn(
+    dirfd: c_int,
+    pathname: *const c_char,
+    flags: c_int,
+    mask: libc::c_uint,
+    statxbuf: *mut libc::statx,
+) -> c_int);
+#[cfg(target_os = "linux")]
+unsafe extern "C" fn statx(
+    dirfd: c_int,
+    pathname: *const c_char,
+    flags: c_int,
+    mask: libc::c_uint,
+    statxbuf: *mut libc::statx,
+) -> c_int {
+    let Some(original) = statx::try_original() else {
+        // Rust's standard library interprets ENOSYS from its statx availability
+        // probe as unsupported and falls back to stat64.
+        // SAFETY: __errno_location returns the calling thread's errno storage on Linux.
+        unsafe { *libc::__errno_location() = libc::ENOSYS };
+        return -1;
+    };
+
+    if pathname.is_null() {
+        if flags & libc::AT_EMPTY_PATH != 0 {
+            // SAFETY: dirfd is provided by the statx caller.
+            unsafe { handle_open(Fd(dirfd), AccessMode::READ) };
+        }
+    } else {
+        // SAFETY: pathname is a non-null C string pointer provided by the statx caller.
+        unsafe { handle_open(PathAt(dirfd, pathname), AccessMode::READ) };
+    }
+    // SAFETY: calling the original libc statx() with the same arguments forwarded from the interposed function
+    unsafe { original(dirfd, pathname, flags, mask, statxbuf) }
+}

crates/fspy_preload_unix/src/macros/linux.rs

@@ -28,7 +28,10 @@ macro_rules! intercept {
             #[cfg(test)]
             #[test]
             fn symbol_64_does_not_exist() {
-               ::core::assert_eq!($crate::macros::symbol_exists(::core::stringify!($name)), false);
+                ::core::assert_eq!(
+                    $crate::macros::symbol_exists(::core::concat!(::core::stringify!($name), 64)),
+                    false,
+                );
             }
         }
     };
@@ -47,7 +50,7 @@ pub fn symbol_exists(name: &str) -> bool {
 }
 
 macro_rules! intercept_inner {
-    ($name: ident: $fn_sig: ty; $test_fn: item ) => {
+    ($name: ident: $fn_sig: ty; $test_fn: item) => {
         const _: $fn_sig = $name;
         const _: $fn_sig = $crate::libc::$name;
 
@@ -66,17 +69,44 @@ macro_rules! intercept_inner {
             #[expect(clippy::allow_attributes, reason = "using allow because unused_imports may or may not fire depending on macro expansion")]
             #[allow(unused_imports, reason = "glob import brings types into scope for macro-generated code")]
             use super::*;
+            #[expect(
+                clippy::allow_attributes,
+                reason = "using allow because dead_code only fires for optional original symbols"
+            )]
+            #[allow(
+                dead_code,
+                reason = "not every interposer forwards to its generated original function"
+            )]
             pub unsafe fn original() -> $fn_sig {
-                static LAZY: std::sync::LazyLock<$fn_sig> = std::sync::LazyLock::new(||
-                    // SAFETY: dlsym with RTLD_NEXT returns the next symbol in the dynamic linking order,
-                    // and transmute converts the resulting function pointer to the expected function signature.
-                    // The caller guarantees the symbol name matches the expected function signature via the macro invocation.
-                    unsafe {
-                    ::core::mem::transmute(::libc::dlsym(
-                        ::libc::RTLD_NEXT,
-                        ::core::concat!(::core::stringify!($name), "\0").as_ptr().cast(),
+                try_original().unwrap_or_else(|| {
+                    panic!(::core::concat!(
+                        "original symbol not found: ",
+                        ::core::stringify!($name)
                     ))
-                });
+                })
+            }
+            pub fn try_original() -> ::core::option::Option<$fn_sig> {
+                static LAZY: std::sync::LazyLock<::core::option::Option<$fn_sig>> =
+                    std::sync::LazyLock::new(|| {
+                        // SAFETY: dlsym with RTLD_NEXT returns the next symbol in the dynamic
+                        // linking order. A non-null pointer has the signature checked by the
+                        // macro invocation.
+                        let symbol = unsafe {
+                            ::libc::dlsym(
+                                ::libc::RTLD_NEXT,
+                                ::core::concat!(::core::stringify!($name), "\0").as_ptr().cast(),
+                            )
+                        };
+                        if symbol.is_null() {
+                            ::core::option::Option::None
+                        } else {
+                            // SAFETY: the symbol name and function signature are paired by the
+                            // macro invocation, and null was checked above.
+                            ::core::option::Option::Some(unsafe {
+                                ::core::mem::transmute::<*mut ::libc::c_void, $fn_sig>(symbol)
+                            })
+                        }
+                    });
                 *LAZY
             }
             $test_fn

crates/vite_task_bin/Cargo.toml

@@ -31,6 +31,9 @@ vite_str = { workspace = true }
 vite_task = { workspace = true }
 which = { workspace = true }
 
+[target.'cfg(target_os = "linux")'.dependencies]
+nix = { workspace = true, features = ["mount", "sched", "user"] }
+
 [dev-dependencies]
 cow-utils = { workspace = true }
 cp_r = { workspace = true }

crates/vite_task_bin/src/vtt/main.rs

@@ -23,7 +23,10 @@ mod print_file;
 mod read_stdin;
 mod replace_file_content;
 mod rm;
+#[cfg(target_os = "linux")]
+mod small_dev_shm;
 mod stat_file;
+mod stat_long_filename;
 mod touch_file;
 mod write_file;
 
@@ -32,7 +35,7 @@ fn main() {
     if args.len() < 2 {
         eprintln!("Usage: vtt <subcommand> [args...]");
         eprintln!(
-            "Subcommands: barrier, check-tty, cp, exit, exit-on-ctrlc, grep-file, list-dir, mkdir, pipe-stdin, print, print-color, print-cwd, print-env, print-file, read-stdin, replace-file-content, rm, stat-file, touch-file, write-file"
+            "Subcommands: barrier, check-tty, cp, exit, exit-on-ctrlc, grep-file, list-dir, mkdir, pipe-stdin, print, print-color, print-cwd, print-env, print-file, read-stdin, replace-file-content, rm, small_dev_shm, stat-file, stat_long_filename, touch-file, write-file"
         );
         std::process::exit(1);
     }
@@ -64,10 +67,15 @@ fn main() {
         "read-stdin" => read_stdin::run(),
         "replace-file-content" => replace_file_content::run(&args[2..]),
         "rm" => rm::run(&args[2..]),
+        #[cfg(target_os = "linux")]
+        "small_dev_shm" => small_dev_shm::run(&args[2..]).map_err(Into::into),
+        #[cfg(not(target_os = "linux"))]
+        "small_dev_shm" => Err("vtt small_dev_shm is only supported on Linux".into()),
         "stat-file" => {
             stat_file::run(&args[2..]);
             Ok(())
         }
+        "stat_long_filename" => stat_long_filename::run(&args[2..]),
         "touch-file" => touch_file::run(&args[2..]),
         "write-file" => write_file::run(&args[2..]),
         other => {

crates/vite_task_bin/src/vtt/small_dev_shm.rs

@@ -0,0 +1,58 @@
+#![cfg(target_os = "linux")]
+
+use std::{os::unix::process::ExitStatusExt as _, process::Command};
+
+use anyhow::{Context as _, Result};
+use nix::{
+    mount::{MsFlags, mount},
+    sched::{CloneFlags, unshare},
+    unistd::{Gid, Uid},
+};
+
+const USAGE: &str = "Usage: vtt small_dev_shm <command> [args...]";
+
+pub fn run(args: &[String]) -> Result<()> {
+    let (program, command_args) = parse_command(args)?;
+    run_platform(program, command_args)
+}
+
+fn parse_command(args: &[String]) -> Result<(&str, &[String])> {
+    args.split_first().map(|(program, args)| (program.as_str(), args)).context(USAGE)
+}
+
+fn run_platform(program: &str, command_args: &[String]) -> Result<()> {
+    let uid = Uid::current().as_raw();
+    let gid = Gid::current().as_raw();
+
+    unshare(CloneFlags::CLONE_NEWUSER | CloneFlags::CLONE_NEWNS)
+        .context("unshare user and mount namespaces")?;
+
+    std::fs::write("/proc/self/uid_map", format!("0 {uid} 1\n"))
+        .context("write /proc/self/uid_map")?;
+    match std::fs::write("/proc/self/setgroups", "deny") {
+        Ok(()) => {}
+        Err(error) if error.kind() == std::io::ErrorKind::NotFound => {}
+        Err(error) => return Err(error).context("write /proc/self/setgroups"),
+    }
+    std::fs::write("/proc/self/gid_map", format!("0 {gid} 1\n"))
+        .context("write /proc/self/gid_map")?;
+
+    mount(None::<&str>, "/", None::<&str>, MsFlags::MS_REC | MsFlags::MS_PRIVATE, None::<&str>)
+        .context("make / recursively private")?;
+
+    mount(
+        Some("tmpfs"),
+        "/dev/shm",
+        Some("tmpfs"),
+        MsFlags::empty(),
+        Some("nr_blocks=1,huge=never"),
+    )
+    .context("mount one-page tmpfs at /dev/shm")?;
+
+    let status = Command::new(program)
+        .args(command_args)
+        .status()
+        .context("run command with constrained /dev/shm")?;
+    let code = status.code().unwrap_or_else(|| status.signal().map_or(1, |signal| 128 + signal));
+    std::process::exit(code);
+}

crates/vite_task_bin/src/vtt/stat_long_filename.rs

@@ -0,0 +1,39 @@
+use std::{error::Error, io};
+
+const USAGE: &str = "Usage: vtt stat_long_filename <count>";
+
+pub fn run(args: &[String]) -> Result<(), Box<dyn Error>> {
+    let count = parse_count(args)?;
+    access_generated_path(count, metadata)?;
+    Ok(())
+}
+
+fn parse_count(args: &[String]) -> Result<usize, String> {
+    let [count] = args else { return Err(USAGE.to_owned()) };
+    count.parse().map_err(|_| USAGE.to_owned())
+}
+
+fn generated_path(count: usize) -> String {
+    "x".repeat(count)
+}
+
+fn access_generated_path(
+    count: usize,
+    mut metadata: impl FnMut(&str) -> io::Result<()>,
+) -> io::Result<()> {
+    let path = generated_path(count);
+    match metadata(&path) {
+        Ok(()) => Ok(()),
+        Err(error)
+            if error.kind() == io::ErrorKind::NotFound
+                || error.raw_os_error() == Some(libc::ENAMETOOLONG) =>
+        {
+            Ok(())
+        }
+        Err(error) => Err(error),
+    }
+}
+
+fn metadata(path: &str) -> io::Result<()> {
+    std::fs::metadata(path).map(|_| ())
+}

crates/vite_task_bin/tests/e2e_snapshots/fixtures/constrained_dev_shm/package.json

@@ -0,0 +1 @@
+{}

crates/vite_task_bin/tests/e2e_snapshots/fixtures/constrained_dev_shm/snapshots.toml

@@ -0,0 +1,8 @@
+[[e2e]]
+name = "constrained_dev_shm"
+comment = """
+Mounting a one-page `/dev/shm` reproduces the SIGBUS seen before fspy moved its shared-memory backing to memfd.
+"""
+platform = "linux-gnu"
+ignore = true
+steps = [["vtt", "small_dev_shm", "vt", "run", "stress"]]

crates/vite_task_bin/tests/e2e_snapshots/fixtures/constrained_dev_shm/snapshots/constrained_dev_shm.md

@@ -0,0 +1,11 @@
+# constrained_dev_shm
+
+Mounting a one-page `/dev/shm` reproduces the SIGBUS seen before fspy moved its shared-memory backing to memfd.
+
+## `vtt small_dev_shm vt run stress`
+
+**Exit code:** 135
+
+```
+$ vtt stat_long_filename 1048576
+```

crates/vite_task_bin/tests/e2e_snapshots/fixtures/constrained_dev_shm/vite-task.json

@@ -0,0 +1,8 @@
+{
+  "tasks": {
+    "stress": {
+      "command": "vtt stat_long_filename 1048576",
+      "cache": true
+    }
+  }
+}