Bug: Mismatches in Ordinary Relative Path Resolution (`path_translate_at`)

[doanbaotrung]

1. Empty Path Bug (Bypasses ENOENT Check)

Problem

When a guest process triggers a path-based system call with an empty path "" (e.g., openat(dirfd, "", flags) without AT_EMPTY_PATH), it is expected to fail with ENOENT.

However, in path_translate_at (src/syscall/path.c), the following condition is checked to detect ordinary relative paths:

if (tx->guest_path == path && path[0] != '/') {

Because the empty string "" does not start with / (i.e. path[0] == '\0' != '/'), this condition evaluates to true.

The code then resolves the path relative to the directory descriptor dirfd using dirfd_guest_base_path(dirfd, base, sizeof(base)), which writes the path of the parent directory itself into tx->guest_buf. As a result:

• The empty path "" is translated to the directory path itself (e.g., /usr/bin/ or /).
• The subsequent host call succeeds and returns a descriptor/details for the directory, rather than returning ENOENT.

Linux/POSIX Standard Behavior

Unless AT_EMPTY_PATH is set, passing an empty path to file-based syscalls (such as openat, fstatat, fchmodat, fchownat) must fail with -ENOENT.

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2. Non-Directory dirfd Bug (Returns EBADF instead of ENOTDIR)

Problem

If a guest process calls a system call like openat(dirfd, "relative_path", ...) where dirfd is a valid file descriptor pointing to a regular file (not a directory), the call should fail with ENOTDIR.

In the new relative path resolution code, dirfd_guest_base_path is called to find the base directory. However, dirfd_guest_base_path rejects any non-directory file descriptors with EBADF:

if (snap.type != FD_DIR) {
    errno = EBADF;
    return -1;
}

As a result:

• path_translate_at returns -1 with errno = EBADF.
• The system call surfaces -EBADF (Bad File Descriptor) to the guest process instead of the correct -ENOTDIR (Not a Directory).

Linux/POSIX Standard Behavior

If dirfd is a valid, open file descriptor but does not refer to a directory, trying to resolve a relative path through it must fail with ENOTDIR. EBADF should only be returned if the file descriptor is not open or is invalid.