Re: [PATCH bpf-next v5 6/6] selftests/bpf: Add a series of tests for bpf_snprintf

[Florent Revest <revest@chromium.org>]

On Wed, May 5, 2021 at 8:55 AM Rasmus Villemoes
<linux@rasmusvillemoes.dk> wrote:
>
> On 28/04/2021 16.59, Florent Revest wrote:
> > On Tue, Apr 27, 2021 at 8:03 PM Andrii Nakryiko
> > <andrii.nakryiko@gmail.com> wrote:
> >>
> >> On Tue, Apr 27, 2021 at 2:51 AM Florent Revest <revest@chromium.org> wrote:
> >>>
> >>> On Tue, Apr 27, 2021 at 8:35 AM Rasmus Villemoes
> >>> <linux@rasmusvillemoes.dk> wrote:
> >>>> Probably the test cases are not run in parallel, but this is the kind of
> >>>> thing that would give those symptoms.
> >>>
> >>> I think it's a separate issue from what Andrii reported though because
> >>> the flaky test exercises the bpf_snprintf helper and this buf spinlock
> >>> bug you just found only affects the bpf_trace_printk helper.
> >>>
> >>> That being said, it does smell a little bit like a concurrency issue
> >>> too, indeed. The bpf_snprintf test program is a raw_tp/sys_enter so it
> >>> attaches to all syscall entries and most likely gets executed many
> >>> more times than necessary and probably on parallel CPUs. The "pad_out"
> >>> buffer they write to is unique and not locked so maybe the test's
> >>> userspace reads pad_out while another CPU is writing on it and if the
> >>> string output goes through a stage where it is "    4 0000" before
> >>> being "    4 000", we might read at the wrong time. That being said, I
> >>> would find it weird that this happens as much as 50% of the time and
> >>> always specifically on that test case.
> >>>
> >>> Andrii could you maybe try changing the prog type to
> >>> "tp/syscalls/sys_enter_nanosleep" on the machine where you can
> >>> reproduce this bug ?
> >>
> >> Yes, it helps. I can't repro it easily anymore.
> >
> > Good, so it does sound like a concurrency issue indeed
> >
> >> I think the right fix, though, should be to filter by tid, not change the tracepoint.
> >
> > Agreed, I'll send a patch for that today. :)
> >
> >> I think what's happening is we see the string right before bstr_printf
> >> does zero-termination with end[-1] = '\0'; So in some cases we see
> >> truncated string, in others we see untruncated one.
> >
> > Makes sense but I still wonder why it happens so often (50% of the
> > time is really a lot) and why it is consistently that one test case
> > that fails and not the "overflow" case for example. But I'm confident
> > that this is not a bug in the helper now and that the tid filter will
> > fix the test.
> >
>
> If the caller, or one of its sibling threads, inspects the buffer before
> (v)snprintf has returned it's very obviously a bug in the caller. As for

Absolutely

> why that particular case exposes the race: It seems to be the only one
> that "expects" an insanely long result, and it takes a very very long
> time (several cycles per byte I'd assume) for the vsnprintf code to very
> carefully go through the
>
>   if (buf < end)
>      *buf = /* '0' or ' ' or whatever it is it is emitting here */
>   buf++;
>
> 900k times. So there's simply a very large window where the buffer
> contents is "    4 0000" while number() is still 'emitting' 0s until
> control returns to vsnprintf() which does that final end[-1] = '\0'.

Aaah, of course, it makes complete sense! :) Thank you Rasmus