linux-kernel - Re: [PATCH] fs/exec.c: Add fast path for ENOENT on PATH search before allocating mm

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <202311081123.391A316@keescook>
Date:   Wed, 8 Nov 2023 11:25:34 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Mateusz Guzik <mjguzik@...il.com>
Cc:     Kees Cook <kees@...nel.org>, Josh Triplett <josh@...htriplett.org>,
        Eric Biederman <ebiederm@...ssion.com>,
        Alexander Viro <viro@...iv.linux.org.uk>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/exec.c: Add fast path for ENOENT on PATH search
 before allocating mm

On Wed, Nov 08, 2023 at 01:03:33AM +0100, Mateusz Guzik wrote:
> [...]
> >>@[
> >>    __pv_queued_spin_lock_slowpath+1
> >>    _raw_spin_lock_irq+43
> >>    wait_for_completion+141
> >>    stop_one_cpu+127
> >>    sched_exec+165
> >
> > There's the suspicious sched_exec() I was talking about! :)
> >
> > I think it needs to be moved, and perhaps _later_ instead of earlier?
> > Hmm...
> >
> 
> I'm getting around 3.4k execs/s. However, if I "taskset -c 3
> ./static-doexec 1" the number goes up to about 9.5k and lock
> contention disappears from the profile. So off hand looks like the
> task is walking around the box when it perhaps could be avoided -- it
> is idle apart from running the test. Again this is going to require a
> serious look instead of ad hoc pokes.

Hm, that is pretty interesting. I'll see if I can go find the original
rationale for adding sched_exec() in there...

> Side note I actually read your patch this time around instead of
> skimming through it and assuming it did what I thought.
> 
> do_filp_open is of course very expensive and kmalloc + kfree are slow.
> On top of it deallocating a file object even after a failed open was
> very expensive due to delegation to task_work (recently fixed).
> 
> What I claim should be clear-cut faster is that lookup as in the
> original patch and only messing with file allocation et al if it
> succeeds.

I'm less familiar with the VFS guts here -- I'm open to alternatives! :)

-- 
Kees Cook