Currently there's a kcov remote coverage collection section in __usb_hcd_giveback_urb(). Initially that section was added based on the assumption that usb_hcd_giveback_urb() can only be called in interrupt context as indicated by a comment before it. This is what happens when syzkaller is fuzzing the USB stack via the dummy_hcd driver. As it turns out, it's actually valid to call usb_hcd_giveback_urb() in task context, provided that the caller turned off the interrupts; USB/IP does exactly that. This can lead to a nested KCOV remote coverage collection sections both trying to collect coverage in task context. This isn't supported by kcov, and leads to a WARNING. Change __usb_hcd_giveback_urb() to only call kcov_remote_*() callbacks when it's being executed in a softirq. To avoid calling in_serving_softirq() directly in the driver code, add a couple of new kcov wrappers. As the result of this change, the coverage from USB/IP related usb_hcd_giveback_urb() calls won't be collected, but the WARNING is fixed. A potential future improvement would be to support nested remote coverage collection sections, but this patch doesn't address that. Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx> Acked-by: Marco Elver <elver@xxxxxxxxxx> --- Changes in v5: - Don't call in_serving_softirq() in USB driver code directly, do that via kcov wrappers. --- drivers/usb/core/hcd.c | 9 +++++++-- include/linux/kcov.h | 21 +++++++++++++++++++++ 2 files changed, 28 insertions(+), 2 deletions(-) diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c index 2c6b9578a7d3..7bafd01e05fb 100644 --- a/drivers/usb/core/hcd.c +++ b/drivers/usb/core/hcd.c @@ -1646,9 +1646,14 @@ static void __usb_hcd_giveback_urb(struct urb *urb) /* pass ownership to the completion handler */ urb->status = status; - kcov_remote_start_usb((u64)urb->dev->bus->busnum); + /* + * This function can be called in task context inside another remote + * coverage collection section, but kcov doesn't support that kind of + * recursion yet. Only collect coverage in softirq context for now. + */ + kcov_remote_start_usb_softirq((u64)urb->dev->bus->busnum); urb->complete(urb); - kcov_remote_stop(); + kcov_remote_stop_softirq(); usb_anchor_resume_wakeups(anchor); atomic_dec(&urb->use_count); diff --git a/include/linux/kcov.h b/include/linux/kcov.h index a10e84707d82..4e3037dc1204 100644 --- a/include/linux/kcov.h +++ b/include/linux/kcov.h @@ -52,6 +52,25 @@ static inline void kcov_remote_start_usb(u64 id) kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_USB, id)); } +/* + * The softirq flavor of kcov_remote_*() functions is introduced as a temporary + * work around for kcov's lack of nested remote coverage sections support in + * task context. Adding suport for nested sections is tracked in: + * https://bugzilla.kernel.org/show_bug.cgi?id=210337 + */ + +static inline void kcov_remote_start_usb_softirq(u64 id) +{ + if (in_serving_softirq()) + kcov_remote_start_usb(id); +} + +static inline void kcov_remote_stop_softirq(void) +{ + if (in_serving_softirq()) + kcov_remote_stop(); +} + #else static inline void kcov_task_init(struct task_struct *t) {} @@ -66,6 +85,8 @@ static inline u64 kcov_common_handle(void) } static inline void kcov_remote_start_common(u64 id) {} static inline void kcov_remote_start_usb(u64 id) {} +static inline void kcov_remote_start_usb_softirq(u64 id) {} +static inline void kcov_remote_stop_softirq(void) {} #endif /* CONFIG_KCOV */ #endif /* _LINUX_KCOV_H */ -- 2.29.2.454.gaff20da3a2-goog
