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

[Mateusz Guzik]

On 11/7/23, Mateusz Guzik <mjguzik@xxxxxxxxx> wrote:
> 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@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.
>>
>
> 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?
>

Welp, should have read the remaining follow up discussion.

Even so, the patch which is only doing the lookup once cannot be
legitimately slower.

Note there is often perf variance between different boots of the same
kernel and I suspect this is what justifies it.

I would suggest adding a runtime knob to control whether 1. lookups
are done late (stock kernel) 2. there is one upfront lookup like in
the original patch and finally 3. stuffing file into bprm

Then a bunch of runs with the knob cycling between them could serve as
a justification.

If the patch which dodges second lookup still somehow appears slower a
flamegraph or other profile would be nice. I can volunteer to take a
look at what's going on provided above measurements will be done and
show funkyness.

-- 
Mateusz Guzik <mjguzik gmail.com>