[PATCH] Cygwin: fchmodat: add limited support for AT_SYMLINK_NOFOLLOW

[Ken Brown]

On 1/26/2021 4:30 PM, Ken Brown via Cygwin-patches wrote:
> Allow fchmodat with the AT_SYMLINK_NOFOLLOW flag to succeed on
> non-symlinks.  Previously it always failed, as it does on Linux.  But
> POSIX permits it to succeed on non-symlinks even if it fails on
> symlinks.
> 
> The reason for following POSIX rather than Linux is to make gnulib
> report that fchmodat works on Cygwin.  This improves the efficiency of
> packages like GNU tar that use gnulib's fchmodat module.  Previously
> such packages would use a gnulib replacement for fchmodat on Cygwin.
> ---
>   winsup/cygwin/syscalls.cc | 20 +++++++++++++++-----
>   1 file changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/winsup/cygwin/syscalls.cc b/winsup/cygwin/syscalls.cc
> index 4cc8d07f5..0983cc76a 100644
> --- a/winsup/cygwin/syscalls.cc
> +++ b/winsup/cygwin/syscalls.cc
> @@ -4787,17 +4787,27 @@ fchmodat (int dirfd, const char *pathname, mode_t mode, int flags)
>     tmp_pathbuf tp;
>     __try
>       {
> -      if (flags)
> +      if (flags & ~AT_SYMLINK_NOFOLLOW)
>   	{
> -	  /* BSD has lchmod, but Linux does not.  POSIX says
> -	     AT_SYMLINK_NOFOLLOW is allowed to fail on symlinks; but Linux
> -	     blindly fails even for non-symlinks.  */
> -	  set_errno ((flags & ~AT_SYMLINK_NOFOLLOW) ? EINVAL : EOPNOTSUPP);
> +	  set_errno (EINVAL);
>   	  __leave;
>   	}
>         char *path = tp.c_get ();
>         if (gen_full_path_at (path, dirfd, pathname))
>   	__leave;
> +      if (flags)
> +	{
> +          /* BSD has lchmod, but Linux does not.  POSIX says
> +	     AT_SYMLINK_NOFOLLOW is allowed to fail on symlinks.
> +	     Linux blindly fails even for non-symlinks, but we allow
> +	     it to succeed. */
> +	  path_conv pc (path);
> +	  if (pc.issymlink ())
> +	    {
> +	      set_errno (EOPNOTSUPP);
> +	      __leave;
> +	    }
> +	}
>         return chmod (path, mode);
>       }
>     __except (EFAULT) {}

This is wrong.  Please ignore.  I'll send a revised patch.

Ken