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. The approach this patch takes is to add another set of kcov_remote_*() callbacks that specify the context they are supposed to be executed in. If the current context doesn't match the mask provided to a callback, that callback is ignored. KCOV currently only supports collecting remote coverage in two contexts: task and softirq. This patch constraints KCOV to only collect coverage from __usb_hcd_giveback_urb() when it's executed in softirq context. As the result, 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 <andreyk...@google.com> Acked-by: Marco Elver <el...@google.com> --- Changes v3->v4: - Drop unnecessary returns from kcov callbacks. --- Documentation/dev-tools/kcov.rst | 6 ++++++ drivers/usb/core/hcd.c | 4 ++-- include/linux/kcov.h | 31 +++++++++++++++++++++++++++++-- kernel/kcov.c | 26 +++++++++++++++++++------- 4 files changed, 56 insertions(+), 11 deletions(-) diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst index 8548b0b04e43..2c0f58988512 100644 --- a/Documentation/dev-tools/kcov.rst +++ b/Documentation/dev-tools/kcov.rst @@ -235,6 +235,12 @@ saved to the kcov_handle field in the current task_struct and needs to be passed to the newly spawned threads via custom annotations. Those threads should in turn be annotated with kcov_remote_start()/kcov_remote_stop(). +Besides the annotations that only accept a handle, there are also +kcov_remote_start_context()/kcov_remote_stop_context() that accept a +context mask. This mask describes the contexts in which these annotations +should be applied. E.g. specifying KCOV_CONTEXT_SOFTIRQ will result in the +corresponding annotations being ignored in any context other than softirq. + Internally kcov stores handles as u64 integers. The top byte of a handle is used to denote the id of a subsystem that this handle belongs to, and the lower 4 bytes are used to denote the id of a thread instance within diff --git a/drivers/usb/core/hcd.c b/drivers/usb/core/hcd.c index a33b849e8beb..ea93d9ebcb2e 100644 --- a/drivers/usb/core/hcd.c +++ b/drivers/usb/core/hcd.c @@ -1646,9 +1646,9 @@ 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); + 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..a9c025c3e1df 100644 --- a/include/linux/kcov.h +++ b/include/linux/kcov.h @@ -22,6 +22,10 @@ enum kcov_mode { KCOV_MODE_TRACE_CMP = 3, }; +#define KCOV_CONTEXT_TASK (1u << 0) +#define KCOV_CONTEXT_SOFTIRQ (1u << 1) +#define KCOV_CONTEXT_MASK (KCOV_CONTEXT_TASK | KCOV_CONTEXT_SOFTIRQ) + #define KCOV_IN_CTXSW (1 << 30) void kcov_task_init(struct task_struct *t); @@ -38,10 +42,21 @@ do { \ } while (0) /* See Documentation/dev-tools/kcov.rst for usage details. */ -void kcov_remote_start(u64 handle); -void kcov_remote_stop(void); + +void kcov_remote_start_context(u64 handle, unsigned int context); +void kcov_remote_stop_context(unsigned int context); u64 kcov_common_handle(void); +static inline void kcov_remote_start(u64 handle) +{ + kcov_remote_start_context(handle, KCOV_CONTEXT_MASK); +} + +static inline void kcov_remote_stop(void) +{ + kcov_remote_stop_context(KCOV_CONTEXT_MASK); +} + static inline void kcov_remote_start_common(u64 id) { kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_COMMON, id)); @@ -52,6 +67,16 @@ static inline void kcov_remote_start_usb(u64 id) kcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_USB, id)); } +static inline void kcov_remote_start_usb_softirq(u64 id) +{ + kcov_remote_start_context(kcov_remote_handle(KCOV_SUBSYSTEM_USB, id), KCOV_CONTEXT_SOFTIRQ); +} + +static inline void kcov_remote_stop_softirq(void) +{ + kcov_remote_stop_context(KCOV_CONTEXT_SOFTIRQ); +} + #else static inline void kcov_task_init(struct task_struct *t) {} @@ -66,6 +91,8 @@ 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 */ diff --git a/kernel/kcov.c b/kernel/kcov.c index 6b8368be89c8..3ccdbe060f47 100644 --- a/kernel/kcov.c +++ b/kernel/kcov.c @@ -808,7 +808,8 @@ static void kcov_remote_softirq_stop(struct task_struct *t) } } -void kcov_remote_start(u64 handle) +/* Also see kcov_remote_start() defined in include/linux/kcov.h. */ +void kcov_remote_start_context(u64 handle, unsigned int context) { struct task_struct *t = current; struct kcov_remote *remote; @@ -821,7 +822,11 @@ void kcov_remote_start(u64 handle) if (WARN_ON(!kcov_check_handle(handle, true, true, true))) return; - if (!in_task() && !in_serving_softirq()) + if (WARN_ON((context & ~KCOV_CONTEXT_MASK) || !context)) + return; + if (in_task() && !(context & KCOV_CONTEXT_TASK)) + return; + if (in_serving_softirq() && !(context & KCOV_CONTEXT_SOFTIRQ)) return; local_irq_save(flags); @@ -894,7 +899,7 @@ void kcov_remote_start(u64 handle) local_irq_restore(flags); } -EXPORT_SYMBOL(kcov_remote_start); +EXPORT_SYMBOL(kcov_remote_start_context); static void kcov_move_area(enum kcov_mode mode, void *dst_area, unsigned int dst_area_size, void *src_area) @@ -951,8 +956,11 @@ static void kcov_move_area(enum kcov_mode mode, void *dst_area, } } -/* See the comment before kcov_remote_start() for usage details. */ -void kcov_remote_stop(void) +/* + * Also see kcov_remote_stop() defined in include/linux/kcov.h. + * See the comment before kcov_remote_start_context() for usage details. + */ +void kcov_remote_stop_context(unsigned int context) { struct task_struct *t = current; struct kcov *kcov; @@ -962,7 +970,11 @@ void kcov_remote_stop(void) int sequence; unsigned long flags; - if (!in_task() && !in_serving_softirq()) + if (WARN_ON((context & ~KCOV_CONTEXT_MASK) || !context)) + return; + if (in_task() && !(context & KCOV_CONTEXT_TASK)) + return; + if (in_serving_softirq() && !(context & KCOV_CONTEXT_SOFTIRQ)) return; local_irq_save(flags); @@ -1018,7 +1030,7 @@ void kcov_remote_stop(void) /* Get in kcov_remote_start(). */ kcov_put(kcov); } -EXPORT_SYMBOL(kcov_remote_stop); +EXPORT_SYMBOL(kcov_remote_stop_context); /* See the comment before kcov_remote_start() for usage details. */ u64 kcov_common_handle(void) -- 2.28.0.1011.ga647a8990f-goog