* Re: XFS reports lchmod failure, but changes file system contents
[not found] ` <87wo8rlgml.fsf@mid.deneb.enyo.de>
@ 2020-02-12 20:38 ` Paul Eggert
0 siblings, 0 replies; only message in thread
From: Paul Eggert @ 2020-02-12 20:38 UTC (permalink / raw)
To: Florian Weimer, Al Viro
Cc: linux-xfs, Rich Felker, libc-alpha, Darrick J. Wong,
Christoph Hellwig, linux-fsdevel, Gnulib bugs
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
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