return-value/errno for utimensat(<filefd>, NULL, NULL, 0) mismatch across musl and glibc: bug or a feature?

[Sergei Trofimovich <slyfox@gentoo.org>]

Hi musl@ folk!

The original issue popped in https://bugs.gentoo.org/549108#c22.
There glibc's utimensat() wrapper handles one corner case differently
from musl's wrapper.

Here is the minimal reproducer:

$ cat a.c
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stddef.h>

int main() {
    int fd = open("f", O_WRONLY|O_CREAT, 0666);
    return utimensat(fd, NULL, NULL, 0);
}

On glibc (x86_64 linux-5.2-rc5):

$ gcc a.c -o a && strace -etrace=open,openat,utimensat,exit_group ./a
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
openat(AT_FDCWD, "/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
openat(AT_FDCWD, "f", O_WRONLY|O_CREAT, 0666) = 3
exit_group(-1)                          = ?
+++ exited with 255 +++

On musl (x86_64 linux-5.2-rc5):
$ gcc a.c -o a && strace -etrace=open,openat,utimensat,exit_group ./a
open("f", O_WRONLY|O_CREAT, 0666)       = 3
utimensat(3, NULL, NULL, 0)             = 0
exit_group(0)                           = ?

The difference stems from this extra check in glibc:
    https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/utimensat.c;h=04b549f360b88a7e7c1e5e617158caf73299736b;hb=HEAD#l32

int utimensat (int fd, const char *file, const struct timespec tsp[2], int flags)
{
   if (file == NULL)
     return INLINE_SYSCALL_ERROR_RETURN_VALUE (EINVAL);
   /* Avoid implicit array coercion in syscall macros.  */
   return INLINE_SYSCALL (utimensat, 4, fd, file, &tsp[0], flags);
}

while musl just calls the syscall directly:

https://git.musl-libc.org/cgit/musl/tree/src/stat/utimensat.c

int utimensat(int fd, const char *path, const struct timespec times[2], int flags)
{
	int r = __syscall(SYS_utimensat, fd, path, times, flags);
        // ...
	return __syscall_ret(r);
}

Is this divergence expected? Or maybe it's accidental? Does it make
sense to handle non-directory fds in utimensat() according to POSIX?

I wonder if we should drop the unstable test or some of libc implementations
actually deviates from the spec.

Thank you!

-- 

  Sergei