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

[Josh Triplett]

On Mon, Sep 19, 2022 at 01:02:51PM -0700, Kees Cook wrote:
> 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.

Sure. Sorry for the delay, needed to integrate some fixes (such as
aarch64 support) and factor out the bits that won't build if you don't
have a patched liburing.

https://github.com/joshtriplett/spawnbench

- Josh Triplett