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

Mateusz Guzik
2023-11-07T20:52:08Z

On Tue, Nov 07, 2023 at 12:30:37PM -0800, Kees Cook wrote:
> 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.
>

So the obvious question is why not store lookup result within bprm,
instead of doing the lookup again later.

Turns out you had very same question and even wrote a patch to sort it
out: https://lore.kernel.org/all/202209161637.9EDAF6B18@keescook/

Why do you intend to go with this patch instead?