[PATCH v4] kcov, usb: specify contexts for remote coverage sections

[Andrey Konovalov <andreyknvl@google.com>]

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