Re: [PATCH v4 2/2] UML: add support for KASAN under x86_64

[David Gow <davidgow@google.com>]

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On Fri, Jul 1, 2022 at 5:16 PM Vincent Whitchurch
<vincent.whitchurch@axis.com> wrote:
>
> On Fri, Jul 01, 2022 at 11:08:27AM +0200, David Gow wrote:
> > On Thu, Jun 30, 2022 at 9:29 PM Andrey Konovalov <andreyknvl@gmail.com> wrote:
> > > Stack trace collection code might trigger KASAN splats when walking
> > > stack frames, but this can be resolved by using unchecked accesses.
> > > The main reason to disable instrumentation here is for performance
> > > reasons, see the upcoming patch for arm64 [1] for some details.
> > >
> > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git/commit/?id=802b91118d11
> >
> > Ah -- that does it! Using READ_ONCE_NOCHECK() in dump_trace() gets rid
> > of the nasty recursive KASAN failures we were getting in the tests.
> >
> > I'll send out v5 with those files instrumented again.
>
> Hmm, do we really want that?  In the patch Andrey linked to above he
> removed the READ_ONCE_NOCHECK() and added the KASAN_SANITIZE on the
> corresponding files for arm64, just like it's already the case in this
> patch for UML.

Personally, I'm okay with the performance overhead so far: in my tests
with a collection of ~350 KUnit tests, the total difference in runtime
was about ~.2 seconds, and was within the margin of error caused by
fluctuations in the compilation time.

As an example, without the stacktrace code instrumented:
[17:36:50] Testing complete. Passed: 364, Failed: 0, Crashed: 0,
Skipped: 47, Errors: 0
[17:36:50] Elapsed time: 15.114s total, 0.003s configuring, 8.518s
building, 6.433s running

versus with it instrumented:
[17:35:40] Testing complete. Passed: 364, Failed: 0, Crashed: 0,
Skipped: 47, Errors: 0
[17:35:40] Elapsed time: 15.497s total, 0.003s configuring, 8.691s
building, 6.640s running

That being said, I'm okay with disabling it again and adding a comment
if it's slow enough in some other usecase to cause problems (or even
just be annoying). That could either be done in a v6 of this patchset,
or a follow-up patch, depending on what people would prefer. But I'd
not have a problem with leaving it instrumented for now.

-- David

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