| Message ID | 20240729022158.92059-1-andrey.konovalov@linux.dev (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | kcov: properly check for softirq context | expand |
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. > 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 >
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 > >
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(); +} + 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);