[PATCH 0/2] vfs: POSIX-compliant version of fchmodat with flag argument

[Andrew Ayer]

Hi,

The below patches implement a 4 argument version of fchmodat (fchmodat4)
that supports a flag argument, as specified by POSIX.  Currently, the
glibc wrapper has a flag argument, but fails with EOPNOTSUPP if you
specify AT_SYMLINK_NOFOLLOW. POSIX says that fchmodat should only fail
with EOPNOTSUPP if you specify AT_SYMLINK_NOFOLLOW and the path actually
refers to a symbolic link (and the system doesn't support changing the
mode of symbolic links).  Unfortunately glibc can't do better because
the current syscall doesn't have a flag argument.

In addition to supporting AT_SYMLINK_NOFOLLOW, this also supports the
AT_EMPTY_PATH flag, just like fchownat.

Besides the POSIX-compliance, this patch will make it possible to
ensure, in a race-free way, that you do not follow symlinks when
chmodding. Previously, you could open a file with O_NOFOLLOW and fchmod
it, but this only worked if you had read or write permissions on the
file.  Now you can open the file with O_PATH|O_NOFOLLOW and do fchmodat
with AT_EMPTY_PATH.

Any feedback is welcome.

Cheers, 
Andrew

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