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

Kees Cook
2022-09-19T20:03:01Z

On Sat, Sep 17, 2022 at 01:50:24AM +0100, Josh Triplett wrote:
> On Fri, Sep 16, 2022 at 05:11:18PM -0700, Kees Cook wrote:
> > I don't like the idea of penalizing the _succeeding_ case, though, which
> > happens if we do the path walk twice. So, I went and refactoring the setup
> > order, moving the do_open_execat() up into alloc_bprm() instead of where
> > it was in bprm_exec(). The result makes it so it is, as you observed,
> > before the mm creation and generally expensive argument copying. The
> > difference to your patch seems to only be the allocation of the file
> > table entry, but avoids the double lookup, so I'm hoping the result is
> > actually even faster.
>
> Thanks for giving this a try; I'd wondered how feasible it would be to
> just do one lookup.
>
> However, on the same test system with the same test setup, with your
> refactor it seems to go slower:
> fork/execvpe: 38087ns
> fork/execve: 33758ns
>
> For comparison, the previous numbers (which I re-confirmed):
>
> Without fast-path:
> fork/execvpe: 49876ns
> fork/execve: 32773ns
>
> With my original separate-lookup fast-path:
> fork/execvpe: 36890ns
> fork/execve: 31551ns

Hmm, this shows as slower in the *normal* case, which I find rather
surprising -- it's the same work, just reordered.

Can you post a URL to your tests? I'd like to reproduce this and maybe
throw perf at it as well.

> I tried several runs of each, and I seem to get reasonably consistent
> results.
>
> My test program just creates a pipe once, then loops on
> clock_gettime/fork/execvpe/read, with the spawned child process doing
> clock_gettime/write/exit (in asm to minimize overhead). The test PATH is
> PATH=/usr/local/bin:/usr/local/sbin:/usr/bin:/usr/sbin:/bin:/sbin:. with
> the test program in the current directory.

I'm also curious about less synthetic testing. It'd be nice to see real
workloads with these changes, etc.

--
Kees Cook