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From:   Florian Weimer <fw@deneb.enyo.de>
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     "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, Rich Felker <dalias@libc.org>
Subject: Re: XFS reports lchmod failure, but changes file system contents
References: <874kvwowke.fsf@mid.deneb.enyo.de>
        <20200212161604.GP6870@magnolia>
        <20200212181128.GA31394@infradead.org>
        <20200212183718.GQ6870@magnolia> <87d0ajmxc3.fsf@mid.deneb.enyo.de>
        <20200212195118.GN23230@ZenIV.linux.org.uk>
Date:   Wed, 12 Feb 2020 21:01:22 +0100
In-Reply-To: <20200212195118.GN23230@ZenIV.linux.org.uk> (Al Viro's message of
        "Wed, 12 Feb 2020 19:51:18 +0000")
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* 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.

Although fchmod does succeed on read-only descriptors, which is a bit
strange.