fix(fspy): forward calls with -1 as the directory descriptor

[magurotuna]

Motivation

In a cached task, the ps -p <pid> command aborts with the SIGABRT signal if the process <pid> has already exited. Outside of vp run, the same command exits with status 1.

Some file system calls, such as openat, take a directory descriptor: an open file descriptor of a directory, which the call uses to resolve a relative path. Starting with version 4.0.6 (5e4886ae), ps from procps-ng opens /proc/<pid> as a directory descriptor and then reads the status file relative to it. If the process has exited, the open fails and returns -1. ps does not check the result, and calls openat(-1, "status", O_RDONLY). This call is harmless by itself: the kernel fails it with the EBADF error.

On Linux and macOS, fspy injects interceptors into tracked programs. An interceptor is a function that replaces a libc function, such as openat, records the path, and then calls the original function. The interceptor converted the directory descriptor with the BorrowedFd::borrow_raw function, which panics on -1. The injected library aborts on panic, so the panic aborted ps.

Any tracked program that passes -1 as a directory descriptor aborted in the same way. The ps command is the reported case.

Fixes voidzero-dev/vite-plus#2954.

Changes

• Add the BorrowedFd::try_borrow_raw function, which returns None for -1. The interceptors use this function, so that they forward a call with -1 to the original function instead of panicking.
• If the path is absolute, the kernel ignores the directory descriptor, so fspy still records the path. If the path is relative, the call fails, so fspy does not record the path.
• The changed interceptors are openat, fstatat, faccessat, statx, syscall(SYS_statx), fdopendir, getdirentries, and execveat, and on macOS also openat$NOCANCEL and __getdirentries64.
• This change does not affect the seccomp path, which fspy uses for static programs and musl builds. The seccomp handler already ignores a failed descriptor lookup.
• Add the crates/fspy/tests/invalid_dirfd.rs tests. The tests call statx with the syscall function, and call the glibc statx and execveat functions through the dlsym function, so that the tests build for glibc 2.17 and musl.