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SubjectRe: WARN_ON_ONCE
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On 04/12/2020 12:25, Michael Ellerman wrote:
> 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,


This says that the userspace can trigger the warning if it does not use
the API right.


> 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.


This is an userspace bug which registers the same thing twice, the
kernel returns a correct error. The question is should it warn by
WARN_ON or pr_err(). The comment in bug.h suggests pr_err() is the right
way, is not it?


--
Alexey

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