Re: LTP tracing crashed on arm64 rk3399-rock-pi-4 - pc : ftrace_ops_test

[Naresh Kamboju]

On Thu, 13 Jun 2024 at 02:47, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Wed, 12 Jun 2024 12:51:30 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> > > [  100.600222] Hardware name: Radxa ROCK Pi 4B (DT)
> > > [  100.600229] pstate: 800003c5 (Nzcv DAIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> > > [  100.600239] pc : ftrace_ops_test+0x34/0x138
> >
> > Hmm, could you show the exact line of the above code? Specifically we have:
> >
> >       rcu_assign_pointer(hash.filter_hash, ops->func_hash->filter_hash);
> >       rcu_assign_pointer(hash.notrace_hash, ops->func_hash->notrace_hash);
> >
> > Hmm, it's a NULL pointer dereference at 0x8, so ops is likely not NULL,
> > as func_hash is much farther down. But if func_hash is NULL,
> > filter_hash is at the 0x8 offset.
> >
> > So now the question is, how did func_hash become NULL. It should always
> > be pointing at something. May have to do with the subops. Will investigate.
> >
>
> >
> >
> > > [  100.600258] lr : function_graph_enter+0x144/0x208
>
> I wonder if we need the following patch:
>
> diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
> index 8317d1a7f43a..fc205ad167a9 100644
> --- a/kernel/trace/fgraph.c
> +++ b/kernel/trace/fgraph.c
> @@ -641,7 +641,7 @@ int function_graph_enter(unsigned long ret, unsigned long func,
>         {
>                 for_each_set_bit(i, &fgraph_array_bitmask,
>                                          sizeof(fgraph_array_bitmask) * BITS_PER_BYTE) {
> -                       struct fgraph_ops *gops = fgraph_array[i];
> +                       struct fgraph_ops *gops = READ_ONCE(fgraph_array[i]);
>                         int save_curr_ret_stack;
>
>                         if (gops == &fgraph_stub)
>
>
> Because if the compiler decides to re-read gops from fgraph_array[i] after the
> above check for the following line that does:
>
>                         save_curr_ret_stack = current->curr_ret_stack;
>                         if (ftrace_ops_test(&gops->ops, func, NULL) &&
>                             gops->entryfunc(&trace, gops))
>                                 bitmap |= BIT(i);
>
>
> and gops now points to fgraph_stub, it will trigger this bug.
>
> Can you apply the above change and see if the bug goes away?

I will apply this patch and run the test in a loop.
Since it is only seen once. Not sure I could validate this and confirm.

>
> Thanks,
>
> -- Steve

- Naresh