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

[Josh Triplett <josh@joshtriplett.org>]

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


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.

- Josh Triplett