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

[Kees Cook]

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@xxxxxxxxxxxxxxxx>

*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@xxxxxxxxx/

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

Thanks!

-Kees

-- 
Kees Cook