linux-kernel - Re: [PATCH v15 6/9] namei: LOOKUP_{IN_ROOT,BENEATH}: permit limited ".." resolution

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Message-ID: <20191113075227.lu5b5uvc2nuk76uk@yavin.dot.cyphar.com>
Date:   Wed, 13 Nov 2019 18:52:27 +1100
From:   Aleksa Sarai <cyphar@...har.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Tycho Andersen <tycho@...ho.ws>,
        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Aleksa Sarai <asarai@...e.de>,
        containers@...ts.linux-foundation.org, linux-alpha@...r.kernel.org,
        linux-api@...r.kernel.org, libc-alpha@...rceware.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v15 6/9] namei: LOOKUP_{IN_ROOT,BENEATH}: permit limited
 ".." resolution

On 2019-11-13, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Tue, Nov 05, 2019 at 08:05:50PM +1100, Aleksa Sarai wrote:
> 
> > 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*.
> 
> BTW, do you realize that open-by-fhandle (or working nfsd, for that matter)
> 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.

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

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