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

From: Florian Weimer <fw@deneb.enyo.de>
To: Rich Felker <dalias@libc.org>
Cc: Andreas Schwab <schwab@linux-m68k.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	"Darrick J. Wong" <darrick.wong@oracle.com>,
	Christoph Hellwig <hch@infradead.org>,
	linux-xfs@vger.kernel.org, libc-alpha@sourceware.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: XFS reports lchmod failure, but changes file system contents
Date: Wed, 12 Feb 2020 21:26:11 +0100	[thread overview]
Message-ID: <87sgjflfh8.fsf@mid.deneb.enyo.de> (raw)
In-Reply-To: <20200212201951.GC1663@brightrain.aerifal.cx> (Rich Felker's message of "Wed, 12 Feb 2020 15:19:51 -0500")

* Rich Felker:

> On Wed, Feb 12, 2020 at 09:17:41PM +0100, Andreas Schwab wrote:
>> On Feb 12 2020, Florian Weimer wrote:
>> 
>> > * Al Viro:
>> >
>> >> On Wed, Feb 12, 2020 at 08:15:08PM +0100, Florian Weimer wrote:
>> >>
>> >>> | Further, I've found some inconsistent behavior with ext4: chmod on the
>> >>> | magic symlink fails with EOPNOTSUPP as in Florian's test, but fchmod
>> >>> | on the O_PATH fd succeeds and changes the symlink mode. This is with
>> >>> | 5.4. Cany anyone else confirm this? Is it a problem?
>> >>> 
>> >>> It looks broken to me because fchmod (as an inode-changing operation)
>> >>> is not supposed to work on O_PATH descriptors.
>> >>
>> >> Why?  O_PATH does have an associated inode just fine; where does
>> >> that "not supposed to" come from?
>> >
>> > It fails on most file systems right now.  I thought that was expected.
>> > Other system calls (fsetxattr IIRC) do not work on O_PATH descriptors,
>> > either.  I assumed that an O_PATH descriptor was not intending to
>> > confer that capability.  Even openat fails.
>> 
>> According to open(2), this is expected:
>> 
>>        O_PATH (since Linux 2.6.39)
>>               Obtain a file descriptor that can be used for two  purposes:  to
>>               indicate a location in the filesystem tree and to perform opera-
>>               tions that act purely at the file descriptor  level.   The  file
>>               itself  is not opened, and other file operations (e.g., read(2),
>>               write(2), fchmod(2), fchown(2), fgetxattr(2), ioctl(2), mmap(2))
>>               fail with the error EBADF.
>
> That text is outdated and should be corrected. Fixing fchmod fchown,
> fstat, etc. to operate on O_PATH file descriptors was a very
> intentional change in the kernel.

I suppose we could do the S_ISLNK check, try fchmod, and if that
fails, go via /proc.  Is this the direction you want to go in?