linux-kernel - Re: [BUG] NULL pointer dereference probably caused by kernel/trace/ring

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Date:   Tue, 1 Nov 2022 21:07:20 +0100
From:   Roland Ruckerbauer <roland.rucky@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        linux-kernel@...r.kernel.org, Takashi Iwai <tiwai@...e.de>,
        regressions@...ts.linux.dev,
        Steven Noonan <steven.noonan@...il.com>
Subject: Re: [BUG] NULL pointer dereference probably caused by kernel/trace/ring_buffer.c

I tried it with one extra modification:

diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
index c3f354cfc5ba..98cae02184e0 100644
--- a/kernel/trace/ring_buffer.c
+++ b/kernel/trace/ring_buffer.c
@@ -937,6 +937,9 @@ void ring_buffer_wake_waiters(struct trace_buffer
*buffer, int cpu)
        struct ring_buffer_per_cpu *cpu_buffer;
        struct rb_irq_work *rbwork;

+       if (!buffer)
+               return;
+
        if (cpu == RING_BUFFER_ALL_CPUS) {

                /* Wake up individual ones too. One level recursion */
@@ -945,9 +948,16 @@ void ring_buffer_wake_waiters(struct trace_buffer
*buffer, int cpu)

                rbwork = &buffer->irq_work;
        } else {
+               if (WARN_ON_ONCE(!buffer->buffers))
+                       return;
+               if (WARN_ON_ONCE(cpu >= nr_cpu_ids))
+                       return;
                cpu_buffer = buffer->buffers[cpu];
                rbwork = &cpu_buffer->irq_work;
        }
+       if (WARN_ON_ONCE(!rbwork))
+               return;
+

        rbwork->wait_index++;
        /* make sure the waiters see the new index */

None of the warnings triggered. I inspected the BUG message and the
disassembly, and figured the faulting code must be:

960        rbwork->wait_index++;
   0xffffffff811b73bd <+77>:    addq   $0x1,0x50(%rbx)

Meaning the rbwork pointer is not null, but also not a valid pointer.
Subtracting offset of the wait_index gives me address 0x178, which
sure seems wrong.

I think I will try a gdb session with this kernel (but I have not done
this for a long time, might take me a while to get it working).

Thanks,
Roland

Am Di., 1. Nov. 2022 um 20:31 Uhr schrieb Steven Rostedt <rostedt@...dmis.org>:
>
> On Mon, 31 Oct 2022 21:20:33 +0100
> Roland Ruckerbauer <roland.rucky@...il.com> wrote:
>
> > Unfortunately the same thing is still happening.
>
> Can you try this?
>
> If it stops the crash, it should trigger the warning, which hopefully
> will shed more light onto the issue.
>
> Thanks!
>
> -- Steve
>
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 199759c73519..8dd793833b51 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -937,6 +937,9 @@ void ring_buffer_wake_waiters(struct trace_buffer *buffer, int cpu)
>         struct ring_buffer_per_cpu *cpu_buffer;
>         struct rb_irq_work *rbwork;
>
> +       if (!buffer)
> +               return;
> +
>         if (cpu == RING_BUFFER_ALL_CPUS) {
>
>                 /* Wake up individual ones too. One level recursion */
> @@ -945,6 +948,10 @@ void ring_buffer_wake_waiters(struct trace_buffer *buffer, int cpu)
>
>                 rbwork = &buffer->irq_work;
>         } else {
> +               if (WARN_ON_ONCE(!buffer->buffers))
> +                       return;
> +               if (WARN_ON_ONCE(cpu >= nr_cpu_ids))
> +                       return;
>                 cpu_buffer = buffer->buffers[cpu];
>                 rbwork = &cpu_buffer->irq_work;
>         }

View attachment "dmesg.log" of type "text/x-log" (111827 bytes)