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Date:   Wed, 13 Nov 2019 18:52:27 +1100
From:   Aleksa Sarai <cyphar@cyphar.com>
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Jeff Layton <jlayton@kernel.org>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Arnd Bergmann <arnd@arndb.de>,
        David Howells <dhowells@redhat.com>,
        Shuah Khan <shuah@kernel.org>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Jann Horn <jannh@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Eric Biederman <ebiederm@xmission.com>,
        Andy Lutomirski <luto@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Tycho Andersen <tycho@tycho.ws>,
        David Drysdale <drysdale@google.com>,
        Chanho Min <chanho.min@lge.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Christian Brauner <christian@brauner.io>,
        Aleksa Sarai <asarai@suse.de>,
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Subject: Re: [PATCH v15 6/9] namei: LOOKUP_{IN_ROOT,BENEATH}: permit limited
 ".." resolution
Message-ID: <20191113075227.lu5b5uvc2nuk76uk@yavin.dot.cyphar.com>
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On 2019-11-13, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Tue, Nov 05, 2019 at 08:05:50PM +1100, Aleksa Sarai wrote:
>=20
> > One other possible alternative (which previous versions of this patch
> > used) would be to check with path_is_under() if there was a racing
> > rename or mount (after re-taking the relevant seqlocks). While this does
> > work, it results in possible O(n*m) behaviour if there are many renames
> > or mounts occuring *anywhere on the system*.
>=20
> BTW, do you realize that open-by-fhandle (or working nfsd, for that matte=
r)
> will trigger arseloads of write_seqlock(&rename_lock) simply on d_splice_=
alias()
> bringing disconnected subtrees in contact with parent?

I wasn't aware of that -- that makes path_is_under() even less viable.
I'll reword it to be clearer that path_is_under() isn't a good idea and
why we went with -EAGAIN over an in-kernel retry.

--=20
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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