Re: WARN_ON_ONCE

[Michael Ellerman <mpe@ellerman.id.au>]

Dmitry Vyukov <dvyukov@google.com> writes:
> On Thu, Dec 3, 2020 at 10:19 AM Dmitry Vyukov <dvyukov@google.com> wrote:
>> On Thu, Dec 3, 2020 at 10:10 AM Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
>> >
>> > Hi!
>> >
>> > Syzkaller triggered WARN_ON_ONCE at
>> >
>> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/tracepoint.c?h=v5.10-rc6#n266
>> >
>> >
>> > ===
>> > static int tracepoint_add_func(struct tracepoint *tp,
>> >                                struct tracepoint_func *func, int prio)
>> > {
>> >         struct tracepoint_func *old, *tp_funcs;
>> >         int ret;
>> >
>> >         if (tp->regfunc && !static_key_enabled(&tp->key)) {
>> >                 ret = tp->regfunc();
>> >                 if (ret < 0)
>> >                         return ret;
>> >         }
>> >
>> >         tp_funcs = rcu_dereference_protected(tp->funcs,
>> >                         lockdep_is_held(&tracepoints_mutex));
>> >         old = func_add(&tp_funcs, func, prio);
>> >         if (IS_ERR(old)) {
>> >                 WARN_ON_ONCE(PTR_ERR(old) != -ENOMEM);
>> >                 return PTR_ERR(old);
>> >         }
>> >
>> > ===
>> >
>> > What is the common approach here? Syzkaller reacts on this as if it was
>> > a bug but WARN_ON_ONCE here seems intentional. Do we still push for
>> > removing such warnings?

AFAICS it is a bug if that fires.

See the commit that added it:
  d66a270be331 ("tracepoint: Do not warn on ENOMEM")

Which says:
  Tracepoint should only warn when a kernel API user does not respect the
  required preconditions (e.g. same tracepoint enabled twice, or called
  to remove a tracepoint that does not exist).
  
  Silence warning in out-of-memory conditions, given that the error is
  returned to the caller.


So if you're seeing it then you've someone caused it to return something
other than ENOMEM, and that is a bug.

cheers