Re: XFS reports lchmod failure, but changes file system contents

[Paul Eggert]

On 2/12/20 12:01 PM, Florian Weimer wrote:
> I assumed that an O_PATH descriptor was not intending to
> confer that capability.

I originally assumed the other way, as I don't see any security reason 
why fchmod should not work on O_PATH-opened descriptors. I see that the 
Linux man page says open+O_PATH doesn't work with fchmod, but that's 
just a bug in the spec.

In Android, the bionic C library has worked around this problem since 
2015 by wrapping fchmod so that it works even when the fd was 
O_PATH-opened. Bionic then uses O_PATH + fchmod to work around the 
fchmodat+AT_SYMLINK_NOFOLLOW problem[1]. glibc (and Gnulib, etc.) could 
do the same. It's the most sane way out of this mess.

[1] 
https://android.googlesource.com/platform/bionic/+/3cbc6c627fe57c9a9783c52d148078f8d52f7b96