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From: Kees Cook <keescook@chromium.org>
To: Josh Triplett <josh@joshtriplett.org>
Cc: Eric Biederman <ebiederm@xmission.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fs/exec.c: Add fast path for ENOENT on PATH search before allocating mm
Date: Tue, 7 Nov 2023 12:30:37 -0800	[thread overview]
Message-ID: <202311071228.27D22C00@keescook> (raw)
In-Reply-To: <5c7333ea4bec2fad1b47a8fa2db7c31e4ffc4f14.1663334978.git.josh@joshtriplett.org>

On Fri, Sep 16, 2022 at 02:41:30PM +0100, Josh Triplett wrote:
> Currently, execve allocates an mm and parses argv and envp before
> checking if the path exists. However, the common case of a $PATH search
> may have several failed calls to exec before a single success. Do a
> filename lookup for the purposes of returning ENOENT before doing more
> expensive operations.
> 
> This does not create a TOCTTOU race, because this can only happen if the
> file didn't exist at some point during the exec call, and that point is
> permitted to be when we did our lookup.
> 
> To measure performance, I ran 2000 fork and execvpe calls with a
> seven-element PATH in which the file was found in the seventh directory
> (representative of the common case as /usr/bin is the seventh directory
> on my $PATH), as well as 2000 fork and execve calls with an absolute
> path to an existing binary. I recorded the minimum time for each, to
> eliminate noise from context switches and similar.
> 
> Without fast-path:
> fork/execvpe: 49876ns
> fork/execve:  32773ns
> 
> With fast-path:
> fork/execvpe: 36890ns
> fork/execve:  32069ns
> 
> The cost of the additional lookup seems to be in the noise for a
> successful exec, but it provides a 26% improvement for the path search
> case by speeding up the six failed execs.
> 
> Signed-off-by: Josh Triplett <josh@joshtriplett.org>

*thread necromancy*

I'll snag this patch after -rc1 is out. Based on the research we both
did in the rest of this thread, this original patch is a clear win.
Let's get it into linux-next and see if anything else falls out of it.

I did, however, scratch my head over the 0-day report:
https://lore.kernel.org/lkml/202209221401.90061e56-yujie.liu@intel.com/

But I can't see why this patch could trigger those problems...

Thanks!

-Kees

-- 
Kees Cook

  parent reply	other threads:[~2023-11-07 20:30 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-16 13:41 [PATCH] fs/exec.c: Add fast path for ENOENT on PATH search before allocating mm Josh Triplett
2022-09-16 14:38 ` Kees Cook
2022-09-16 20:13   ` Josh Triplett
2022-09-17  0:11     ` Kees Cook
2022-09-17  0:50       ` Josh Triplett
2022-09-19 20:02         ` Kees Cook
2022-10-01 16:01           ` Josh Triplett
2022-09-19 14:34       ` Peter Zijlstra
2022-09-22  7:27 ` [fs/exec.c] 0a276ae2d2: BUG:workqueue_lockup-pool kernel test robot
2023-11-07 20:30 ` Kees Cook [this message]
2023-11-07 20:51   ` [PATCH] fs/exec.c: Add fast path for ENOENT on PATH search before allocating mm Mateusz Guzik
2023-11-07 21:23     ` Mateusz Guzik
2023-11-07 22:50       ` Kees Cook
2023-11-07 23:08         ` Mateusz Guzik
2023-11-07 23:39           ` Kees Cook
2023-11-08  0:03             ` Mateusz Guzik
2023-11-08 19:25               ` Kees Cook
2023-11-08 19:31               ` Kees Cook
2023-11-08 19:35                 ` Mateusz Guzik
2023-11-09  0:17                   ` Eric W. Biederman
2023-11-09 12:21                     ` Mateusz Guzik
2023-11-10  5:26                       ` Eric W. Biederman
2023-11-07 20:37 ` Kees Cook

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