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Date:   Fri, 21 Feb 2020 05:02:05 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Aleksa Sarai <cyphar@cyphar.com>
Cc:     Florian Weimer <fw@deneb.enyo.de>,
        "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
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On Fri, Feb 21, 2020 at 03:09:19PM +1100, Aleksa Sarai wrote:

>  * open(/proc/self/fd/$n) failing with ELOOP might actually be a bug
>    (the error is coming from may_open as though the lookup was done with
>    O_NOFOLLOW) -- the nd_jump_link() jump takes the namei lookup to a
>    the symlink but it looks like the normal link_path_walk et al
>    handling doesn't actually try to continue resolving it. I'll look
>    into this a bit more.

Not a bug.  Neither mount nor symlink traversal applies to destinations
of pure jumps (be it a symlink to "/" or a procfs symlink).  Both are
deliberate and both for very good reasons.  We'd discussed that last
year (and I'm going to cover that on LSF); basically, there's no
good semantics for symlink traversal in such situation.

Again, this is absolutely deliberate.  And for sanity sake, don't bother
with link_path_walk() et.al. state in mainline - see #work.namei or
#work.do_last in vfs.git; I'm going to repost that series tonight or
tomorrow.  The logics is easier to follow there.