Re: [PATCH 1/2] vfs: implement fchmodat2() syscall

[Colin Walters]

On Tue, Apr 11, 2017, at 02:07 PM, Eric Blake wrote:
> 
> A good idea on the surface. But reading the man page of openat(), the
> section on O_PATH says:
>    The  file
>               itself  is not opened, and other file operations (e.g.,
> read(2),
>               write(2), fchmod(2), fchown(2), fgetxattr(2), mmap(2))
> fail with
>               the error EBADF.

Right, though more topically I'd have expected
fchmodat() (not fchmod()) to take AT_EMPTY_PATH,
just like fstatat() does.

But it doesn't appear to be supported...oh, even at
the syscall level, interesting.   Ah, I see, glibc does:

int
fchmodat (int fd, const char *file, mode_t mode, int flag)
{
  if (flag & ~AT_SYMLINK_NOFOLLOW)
    return INLINE_SYSCALL_ERROR_RETURN_VALUE (EINVAL);
...
}

And indeed the syscall doesn't have flags, bringing us back
to the start here.   Sorry, that seems obvious in retrospect,
but I was "working forwards" from the O_PATH userspace API
mindset.