Re: [PATCH] kcov: properly check for softirq context

[Marco Elver <elver@google.com>]

On Mon, 29 Jul 2024 at 11:42, Marco Elver <elver@google.com> wrote:
>
> On Mon, 29 Jul 2024 at 04:22, <andrey.konovalov@linux.dev> wrote:
> >
> > From: Andrey Konovalov <andreyknvl@gmail.com>
> >
> > When collecting coverage from softirqs, KCOV uses in_serving_softirq() to
> > check whether the code is running in the softirq context. Unfortunately,
> > in_serving_softirq() is > 0 even when the code is running in the hardirq
> > or NMI context for hardirqs and NMIs that happened during a softirq.
> >
> > As a result, if a softirq handler contains a remote coverage collection
> > section and a hardirq with another remote coverage collection section
> > happens during handling the softirq, KCOV incorrectly detects a nested
> > softirq coverate collection section and prints a WARNING, as reported
> > by syzbot.
> >
> > This issue was exposed by commit a7f3813e589f ("usb: gadget: dummy_hcd:
> > Switch to hrtimer transfer scheduler"), which switched dummy_hcd to using
> > hrtimer and made the timer's callback be executed in the hardirq context.
> >
> > Change the related checks in KCOV to account for this behavior of
> > in_serving_softirq() and make KCOV ignore remote coverage collection
> > sections in the hardirq and NMI contexts.
> >
> > This prevents the WARNING printed by syzbot but does not fix the inability
> > of KCOV to collect coverage from the __usb_hcd_giveback_urb when dummy_hcd
> > is in use (caused by a7f3813e589f); a separate patch is required for that.
> >
> > Reported-by: syzbot+2388cdaeb6b10f0c13ac@syzkaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=2388cdaeb6b10f0c13ac
> > Fixes: 5ff3b30ab57d ("kcov: collect coverage from interrupts")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Andrey Konovalov <andreyknvl@gmail.com>
> > ---
> >  kernel/kcov.c | 15 ++++++++++++---
> >  1 file changed, 12 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/kcov.c b/kernel/kcov.c
> > index f0a69d402066e..274b6b7c718de 100644
> > --- a/kernel/kcov.c
> > +++ b/kernel/kcov.c
> > @@ -161,6 +161,15 @@ static void kcov_remote_area_put(struct kcov_remote_area *area,
> >         kmsan_unpoison_memory(&area->list, sizeof(area->list));
> >  }
> >
> > +/*
> > + * Unlike in_serving_softirq(), this function returns false when called during
> > + * a hardirq or an NMI that happened in the softirq context.
> > + */
> > +static inline bool in_softirq_really(void)
> > +{
> > +       return in_serving_softirq() && !in_hardirq() && !in_nmi();
> > +}
>
> Not sure you need this function. Check if just this will give you what you want:
>
>   interrupt_context_level() == 1
>
> I think the below condition could then also just become:
>
>   if (interrupt_context_level() == 1 && t->kcov_softirq)
>
> Although the softirq_count() helper has a special PREEMPT_RT variant,
> and interrupt_context_level() doesn't, so it's not immediately obvious
> to me if that's also ok on PREEMPT_RT kernels.
>
> Maybe some RT folks can help confirm that using
> interrupt_context_level()==1 does what your above function does also
> on RT kernels.

Hmm, so Thomas just told me that softirqs always run in threaded
context on RT and because there's no nesting,
interrupt_context_level() won't work for what I had imagined here.

So your current solution is fine.

Acked-by: Marco Elver <elver@google.com>

> >  static notrace bool check_kcov_mode(enum kcov_mode needed_mode, struct task_struct *t)
> >  {
> >         unsigned int mode;
> > @@ -170,7 +179,7 @@ static notrace bool check_kcov_mode(enum kcov_mode needed_mode, struct task_stru
> >          * so we ignore code executed in interrupts, unless we are in a remote
> >          * coverage collection section in a softirq.
> >          */
> > -       if (!in_task() && !(in_serving_softirq() && t->kcov_softirq))
> > +       if (!in_task() && !(in_softirq_really() && t->kcov_softirq))
> >                 return false;
> >         mode = READ_ONCE(t->kcov_mode);
> >         /*
> > @@ -849,7 +858,7 @@ void kcov_remote_start(u64 handle)
> >
> >         if (WARN_ON(!kcov_check_handle(handle, true, true, true)))
> >                 return;
> > -       if (!in_task() && !in_serving_softirq())
> > +       if (!in_task() && !in_softirq_really())
> >                 return;
> >
> >         local_lock_irqsave(&kcov_percpu_data.lock, flags);
> > @@ -991,7 +1000,7 @@ void kcov_remote_stop(void)
> >         int sequence;
> >         unsigned long flags;
> >
> > -       if (!in_task() && !in_serving_softirq())
> > +       if (!in_task() && !in_softirq_really())
> >                 return;
> >
> >         local_lock_irqsave(&kcov_percpu_data.lock, flags);
> > --
> > 2.25.1
> >