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

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

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:
>>>>         u64 args[MAX_TRACE_PRINTK_VARARGS] = { arg1, arg2, arg3 };
>>>> -       enum bpf_printf_mod_type mod[MAX_TRACE_PRINTK_VARARGS];
>>>> +       u32 *bin_args;
>>>>         static char buf[BPF_TRACE_PRINTK_SIZE];
>>>>         unsigned long flags;
>>>>         int ret;
>>>>
>>>> -       ret = bpf_printf_prepare(fmt, fmt_size, args, args, mod,
>>>> -                                MAX_TRACE_PRINTK_VARARGS);
>>>> +       ret = bpf_bprintf_prepare(fmt, fmt_size, args, &bin_args,
>>>> +                                 MAX_TRACE_PRINTK_VARARGS);
>>>>         if (ret < 0)
>>>>                 return ret;
>>>>
>>>> -       ret = snprintf(buf, sizeof(buf), fmt, BPF_CAST_FMT_ARG(0, args, mod),
>>>> -               BPF_CAST_FMT_ARG(1, args, mod), BPF_CAST_FMT_ARG(2, args, mod));
>>>> -       /* snprintf() will not append null for zero-length strings */
>>>> -       if (ret == 0)
>>>> -               buf[0] = '\0';
>>>> +       ret = bstr_printf(buf, sizeof(buf), fmt, bin_args);
>>>>
>>>>         raw_spin_lock_irqsave(&trace_printk_lock, flags);
>>>>         trace_bpf_trace_printk(buf);
>>>>         raw_spin_unlock_irqrestore(&trace_printk_lock, flags);
>>>>
>>>> Why isn't the write to buf[] protected by that spinlock? Or put another
>>>> way, what protects buf[] from concurrent writes?
>>>
>>> You're right, that is a bug, I missed that buf was static and thought
>>> it was just on the stack. That snprintf call should be after the
>>> raw_spin_lock_irqsave. I'll send a patch. Thank you Rasmus. (before my
>>> snprintf series, there was a vsprintf after the raw_spin_lock_irqsave)
> 
> Solved now
> 
>> Can you please also clean up unnecessary ()s you added in at least a
>> few places. Thanks.
> 
> Alexei said he took care of this .:)
> 
>>>> 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
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'.

Rasmus