[PATCH RFC 1/2] kcov: collect coverage from interrupts

From: Andrey Konovalov
Date: Thu Dec 12 2019 - 12:15:56 EST

This change extends kcov remote coverage support to allow collecting
coverage from interrupts in addition to kernel background threads.

To collect coverage from code that is executed in interrupt context, a
part of that code has to be annotated with kcov_remote_start/stop() in a
similar way as how it is done for global kernel background threads. Then
the handle used for the annotations has to be passed to the
KCOV_REMOTE_ENABLE ioctl.

Internally this patch adjusts the __sanitizer_cov_trace_pc() compiler
inserted callback to not bail out when called from interrupt context.
kcov_remote_start/stop() are updated to save/restore the current per
task kcov state in a per-cpu area (in case the interrupt came when the
kernel was already collecting coverage in task context). Coverage from
interrupts is collected into pre-allocated per-cpu areas, whose size is
controlled by the new CONFIG_KCOV_IRQ_AREA_SIZE.

This patch also cleans up some of kcov debug messages.

Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
---
Documentation/dev-tools/kcov.rst | 16 +--
drivers/usb/gadget/udc/dummy_hcd.c | 1 +
include/linux/sched.h | 3 +
kernel/kcov.c | 196 +++++++++++++++++++----------
lib/Kconfig.debug | 9 ++
5 files changed, 154 insertions(+), 71 deletions(-)

diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
index 1c4e1825d769..fc2bc6883431 100644
--- a/Documentation/dev-tools/kcov.rst
+++ b/Documentation/dev-tools/kcov.rst
@@ -217,14 +217,14 @@ This allows to collect coverage from two types of kernel background
threads: the global ones, that are spawned during kernel boot in a limited
number of instances (e.g. one USB hub_event() worker thread is spawned per
USB HCD); and the local ones, that are spawned when a user interacts with
-some kernel interface (e.g. vhost workers).
+some kernel interface (e.g. vhost workers); as well as from interrupts.

-To enable collecting coverage from a global background thread, a unique
-global handle must be assigned and passed to the corresponding
-kcov_remote_start() call. Then a userspace process can pass a list of such
-handles to the KCOV_REMOTE_ENABLE ioctl in the handles array field of the
-kcov_remote_arg struct. This will attach the used kcov device to the code
-sections, that are referenced by those handles.
+To enable collecting coverage from a global background thread or from an
+interrupt, a unique global handle must be assigned and passed to the
+corresponding kcov_remote_start() call. Then a userspace process can pass
+a list of such handles to the KCOV_REMOTE_ENABLE ioctl in the handles
+array field of the kcov_remote_arg struct. This will attach the used kcov
+device to the code sections, that are referenced by those handles.

Since there might be many local background threads spawned from different
userspace processes, we can't use a single global handle per annotation.
@@ -242,7 +242,7 @@ handles as they don't belong to a particular subsystem. The bytes 4-7 are
currently reserved and must be zero. In the future the number of bytes
used for the subsystem or handle ids might be increased.

-When a particular userspace proccess collects coverage by via a common
+When a particular userspace proccess collects coverage via a common
handle, kcov will collect coverage for each code section that is annotated
to use the common handle obtained as kcov_handle from the current
task_struct. However non common handles allow to collect coverage
diff --git a/drivers/usb/gadget/udc/dummy_hcd.c b/drivers/usb/gadget/udc/dummy_hcd.c
index 4c9d1e49d5ed..faf84ada71a5 100644
--- a/drivers/usb/gadget/udc/dummy_hcd.c
+++ b/drivers/usb/gadget/udc/dummy_hcd.c
@@ -38,6 +38,7 @@
#include <linux/usb/gadget.h>
#include <linux/usb/hcd.h>
#include <linux/scatterlist.h>
+#include <linux/kcov.h>

#include <asm/byteorder.h>
#include <linux/io.h>
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 467d26046416..47d1d556f795 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1229,6 +1229,9 @@ struct task_struct {

/* KCOV sequence number: */
int kcov_sequence;
+
+ /* Collect coverage from interrupt context: */
+ bool kcov_interrupt;
#endif

#ifdef CONFIG_MEMCG
diff --git a/kernel/kcov.c b/kernel/kcov.c
index f50354202dbe..5b48a7a5c465 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -26,6 +26,7 @@
#include <asm/setup.h>

#define kcov_debug(fmt, ...) pr_debug("%s: " fmt, __func__, ##__VA_ARGS__)
+#define kcov_err(fmt, ...) pr_err("%s: " fmt, __func__, ##__VA_ARGS__)

/* Number of 64-bit words written per one comparison: */
#define KCOV_WORDS_PER_CMP 4
@@ -86,6 +87,17 @@ static DEFINE_SPINLOCK(kcov_remote_lock);
static DEFINE_HASHTABLE(kcov_remote_map, 4);
static struct list_head kcov_remote_areas = LIST_HEAD_INIT(kcov_remote_areas);

+struct kcov_remote_irq_data {
+ unsigned int mode;
+ unsigned int size;
+ void *area;
+ struct kcov *kcov;
+ unsigned long flags;
+};
+
+DEFINE_PER_CPU(struct kcov_remote_irq_data, kcov_remote_irq);
+DEFINE_PER_CPU(void *, kcov_irq_area);
+
/* Must be called with kcov_remote_lock locked. */
static struct kcov_remote *kcov_remote_find(u64 handle)
{
@@ -98,6 +110,7 @@ static struct kcov_remote *kcov_remote_find(u64 handle)
return NULL;
}

+/* Must be called with kcov_remote_lock locked. */
static struct kcov_remote *kcov_remote_add(struct kcov *kcov, u64 handle)
{
struct kcov_remote *remote;
@@ -119,16 +132,13 @@ static struct kcov_remote_area *kcov_remote_area_get(unsigned int size)
struct kcov_remote_area *area;
struct list_head *pos;

- kcov_debug("size = %u\n", size);
list_for_each(pos, &kcov_remote_areas) {
area = list_entry(pos, struct kcov_remote_area, list);
if (area->size == size) {
list_del(&area->list);
- kcov_debug("rv = %px\n", area);
return area;
}
}
- kcov_debug("rv = NULL\n");
return NULL;
}

@@ -136,7 +146,6 @@ static struct kcov_remote_area *kcov_remote_area_get(unsigned int size)
static void kcov_remote_area_put(struct kcov_remote_area *area,
unsigned int size)
{
- kcov_debug("area = %px, size = %u\n", area, size);
INIT_LIST_HEAD(&area->list);
area->size = size;
list_add(&area->list, &kcov_remote_areas);
@@ -148,9 +157,11 @@ static notrace bool check_kcov_mode(enum kcov_mode needed_mode, struct task_stru

/*
* We are interested in code coverage as a function of a syscall inputs,
- * so we ignore code executed in interrupts.
+ * so we ignore code executed in interrupts, unless we are in a remote
+ * coverage collection section happening in an interrupt, which is
+ * indicated by the t->kcov_interrupt flag.
*/
- if (!in_task())
+ if (!in_task() && !t->kcov_interrupt)
return false;
mode = READ_ONCE(t->kcov_mode);
/*
@@ -331,12 +342,12 @@ static void kcov_stop(struct task_struct *t)
barrier();
t->kcov_size = 0;
t->kcov_area = NULL;
+ t->kcov = NULL;
}

static void kcov_task_reset(struct task_struct *t)
{
kcov_stop(t);
- t->kcov = NULL;
t->kcov_sequence = 0;
t->kcov_handle = 0;
}
@@ -361,18 +372,18 @@ static void kcov_remote_reset(struct kcov *kcov)
int bkt;
struct kcov_remote *remote;
struct hlist_node *tmp;
+ unsigned long flags;

- spin_lock(&kcov_remote_lock);
+ spin_lock_irqsave(&kcov_remote_lock, flags);
hash_for_each_safe(kcov_remote_map, bkt, tmp, remote, hnode) {
if (remote->kcov != kcov)
continue;
- kcov_debug("removing handle %llx\n", remote->handle);
hash_del(&remote->hnode);
kfree(remote);
}
/* Do reset before unlock to prevent races with kcov_remote_start(). */
kcov_reset(kcov);
- spin_unlock(&kcov_remote_lock);
+ spin_unlock_irqrestore(&kcov_remote_lock, flags);
}

static void kcov_disable(struct task_struct *t, struct kcov *kcov)
@@ -401,12 +412,13 @@ static void kcov_put(struct kcov *kcov)
void kcov_task_exit(struct task_struct *t)
{
struct kcov *kcov;
+ unsigned long flags;

kcov = t->kcov;
if (kcov == NULL)
return;

- spin_lock(&kcov->lock);
+ spin_lock_irqsave(&kcov->lock, flags);
kcov_debug("t = %px, kcov->t = %px\n", t, kcov->t);
/*
* For KCOV_ENABLE devices we want to make sure that t->kcov->t == t,
@@ -430,12 +442,12 @@ void kcov_task_exit(struct task_struct *t)
* By combining all three checks into one we get:
*/
if (WARN_ON(kcov->t != t)) {
- spin_unlock(&kcov->lock);
+ spin_unlock_irqrestore(&kcov->lock, flags);
return;
}
/* Just to not leave dangling references behind. */
kcov_disable(t, kcov);
- spin_unlock(&kcov->lock);
+ spin_unlock_irqrestore(&kcov->lock, flags);
kcov_put(kcov);
}

@@ -446,12 +458,13 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
struct kcov *kcov = vma->vm_file->private_data;
unsigned long size, off;
struct page *page;
+ unsigned long flags;

area = vmalloc_user(vma->vm_end - vma->vm_start);
if (!area)
return -ENOMEM;

- spin_lock(&kcov->lock);
+ spin_lock_irqsave(&kcov->lock, flags);
size = kcov->size * sizeof(unsigned long);
if (kcov->mode != KCOV_MODE_INIT || vma->vm_pgoff != 0 ||
vma->vm_end - vma->vm_start != size) {
@@ -461,7 +474,7 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
if (!kcov->area) {
kcov->area = area;
vma->vm_flags |= VM_DONTEXPAND;
- spin_unlock(&kcov->lock);
+ spin_unlock_irqrestore(&kcov->lock, flags);
for (off = 0; off < size; off += PAGE_SIZE) {
page = vmalloc_to_page(kcov->area + off);
if (vm_insert_page(vma, vma->vm_start + off, page))
@@ -470,7 +483,7 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
return 0;
}
exit:
- spin_unlock(&kcov->lock);
+ spin_unlock_irqrestore(&kcov->lock, flags);
vfree(area);
return res;
}
@@ -550,10 +563,10 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
int mode, i;
struct kcov_remote_arg *remote_arg;
struct kcov_remote *remote;
+ unsigned long flags;

switch (cmd) {
case KCOV_INIT_TRACE:
- kcov_debug("KCOV_INIT_TRACE\n");
/*
* Enable kcov in trace mode and setup buffer size.
* Must happen before anything else.
@@ -572,7 +585,6 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
kcov->mode = KCOV_MODE_INIT;
return 0;
case KCOV_ENABLE:
- kcov_debug("KCOV_ENABLE\n");
/*
* Enable coverage for the current task.
* At this point user must have been enabled trace mode,
@@ -598,7 +610,6 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
kcov_get(kcov);
return 0;
case KCOV_DISABLE:
- kcov_debug("KCOV_DISABLE\n");
/* Disable coverage for the current task. */
unused = arg;
if (unused != 0 || current->kcov != kcov)
@@ -610,7 +621,6 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
kcov_put(kcov);
return 0;
case KCOV_REMOTE_ENABLE:
- kcov_debug("KCOV_REMOTE_ENABLE\n");
if (kcov->mode != KCOV_MODE_INIT || !kcov->area)
return -EINVAL;
t = current;
@@ -627,41 +637,42 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
kcov->t = t;
kcov->remote = true;
kcov->remote_size = remote_arg->area_size;
- spin_lock(&kcov_remote_lock);
+ spin_lock_irqsave(&kcov_remote_lock, flags);
for (i = 0; i < remote_arg->num_handles; i++) {
- kcov_debug("handle %llx\n", remote_arg->handles[i]);
if (!kcov_check_handle(remote_arg->handles[i],
false, true, false)) {
- spin_unlock(&kcov_remote_lock);
+ spin_unlock_irqrestore(&kcov_remote_lock,
+ flags);
kcov_disable(t, kcov);
return -EINVAL;
}
remote = kcov_remote_add(kcov, remote_arg->handles[i]);
if (IS_ERR(remote)) {
- spin_unlock(&kcov_remote_lock);
+ spin_unlock_irqrestore(&kcov_remote_lock,
+ flags);
kcov_disable(t, kcov);
return PTR_ERR(remote);
}
}
if (remote_arg->common_handle) {
- kcov_debug("common handle %llx\n",
- remote_arg->common_handle);
if (!kcov_check_handle(remote_arg->common_handle,
true, false, false)) {
- spin_unlock(&kcov_remote_lock);
+ spin_unlock_irqrestore(&kcov_remote_lock,
+ flags);
kcov_disable(t, kcov);
return -EINVAL;
}
remote = kcov_remote_add(kcov,
remote_arg->common_handle);
if (IS_ERR(remote)) {
- spin_unlock(&kcov_remote_lock);
+ spin_unlock_irqrestore(&kcov_remote_lock,
+ flags);
kcov_disable(t, kcov);
return PTR_ERR(remote);
}
t->kcov_handle = remote_arg->common_handle;
}
- spin_unlock(&kcov_remote_lock);
+ spin_unlock_irqrestore(&kcov_remote_lock, flags);
/* Put either in kcov_task_exit() or in KCOV_DISABLE. */
kcov_get(kcov);
return 0;
@@ -677,6 +688,7 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
struct kcov_remote_arg *remote_arg = NULL;
unsigned int remote_num_handles;
unsigned long remote_arg_size;
+ unsigned long flags;

if (cmd == KCOV_REMOTE_ENABLE) {
if (get_user(remote_num_handles, (unsigned __user *)(arg +
@@ -697,9 +709,9 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg)
}

kcov = filep->private_data;
- spin_lock(&kcov->lock);
+ spin_lock_irqsave(&kcov->lock, flags);
res = kcov_ioctl_locked(kcov, cmd, arg);
- spin_unlock(&kcov->lock);
+ spin_unlock_irqrestore(&kcov->lock, flags);

kfree(remote_arg);

@@ -716,8 +728,8 @@ static const struct file_operations kcov_fops = {

/*
* kcov_remote_start() and kcov_remote_stop() can be used to annotate a section
- * of code in a kernel background thread to allow kcov to be used to collect
- * coverage from that part of code.
+ * of code in a kernel background thread or in an interrupt to allow kcov to be
+ * used to collect coverage from that part of code.
*
* The handle argument of kcov_remote_start() identifies a code section that is
* used for coverage collection. A userspace process passes this handle to
@@ -728,9 +740,9 @@ static const struct file_operations kcov_fops = {
* the type of the kernel thread whose code is being annotated.
*
* For global kernel threads that are spawned in a limited number of instances
- * (e.g. one USB hub_event() worker thread is spawned per USB HCD), each
- * instance must be assigned a unique 4-byte instance id. The instance id is
- * then combined with a 1-byte subsystem id to get a handle via
+ * (e.g. one USB hub_event() worker thread is spawned per USB HCD) and for
+ * interrupts, each instance must be assigned a unique 4-byte instance id. The
+ * instance id is then combined with a 1-byte subsystem id to get a handle via
* kcov_remote_handle(subsystem_id, instance_id).
*
* For local kernel threads that are spawned from system calls handler when a
@@ -749,13 +761,48 @@ static const struct file_operations kcov_fops = {
*
* See Documentation/dev-tools/kcov.rst for more details.
*
- * Internally, this function looks up the kcov device associated with the
+ * Internally, kcov_remote_start() looks up the kcov device associated with the
* provided handle, allocates an area for coverage collection, and saves the
* pointers to kcov and area into the current task_struct to allow coverage to
* be collected via __sanitizer_cov_trace_pc()
* In turns kcov_remote_stop() clears those pointers from task_struct to stop
* collecting coverage and copies all collected coverage into the kcov area.
*/
+
+void kcov_remote_swap_irq(struct task_struct *t,
+ struct kcov_remote_irq_data *irq)
+{
+ t->kcov = xchg(&irq->kcov, t->kcov);
+ t->kcov_size = xchg(&irq->size, t->kcov_size);
+ t->kcov_area = xchg(&irq->area, t->kcov_area);
+ t->kcov_mode = xchg(&irq->mode, t->kcov_mode);
+}
+
+bool kcov_remote_start_irq(struct task_struct *t)
+{
+ struct kcov_remote_irq_data *irq = &get_cpu_var(kcov_remote_irq);
+
+ if (t->kcov_interrupt) {
+ /* Nested interrupt. */
+ put_cpu_var(kcov_remote_irq);
+ return false;
+ }
+ t->kcov_interrupt = true;
+ kcov_remote_swap_irq(t, irq);
+ put_cpu_var(kcov_remote_irq);
+ return true;
+}
+
+void kcov_remote_stop_irq(struct task_struct *t)
+{
+ struct kcov_remote_irq_data *irq = &get_cpu_var(kcov_remote_irq);
+
+ kcov_remote_swap_irq(t, irq);
+ t->kcov_interrupt = false;
+ local_irq_restore(irq->flags);
+ put_cpu_var(kcov_remote_irq);
+}
+
void kcov_remote_start(u64 handle)
{
struct kcov_remote *remote;
@@ -764,28 +811,32 @@ void kcov_remote_start(u64 handle)
unsigned int size;
enum kcov_mode mode;
int sequence;
+ unsigned long flags;

if (WARN_ON(!kcov_check_handle(handle, true, true, true)))
return;
- if (WARN_ON(!in_task()))
+ if (WARN_ON(in_nmi()))
return;
t = current;
/*
- * Check that kcov_remote_start is not called twice
- * nor called by user tasks (with enabled kcov).
+ * Check that kcov_remote_start is not called twice in background
+ * threads nor called by user tasks (with enabled kcov).
*/
- if (WARN_ON(t->kcov))
+ if (WARN_ON(in_task() && t->kcov))
return;

- kcov_debug("handle = %llx\n", handle);
-
- spin_lock(&kcov_remote_lock);
+ spin_lock_irqsave(&kcov_remote_lock, flags);
remote = kcov_remote_find(handle);
if (!remote) {
- kcov_debug("no remote found");
- spin_unlock(&kcov_remote_lock);
+ spin_unlock_irqrestore(&kcov_remote_lock, flags);
return;
}
+ if (!in_task() && !kcov_remote_start_irq(t)) {
+ spin_unlock_irqrestore(&kcov_remote_lock, flags);
+ return;
+ }
+ kcov_debug("handle = %llx, context: %s\n", handle,
+ in_task() ? "task" : "irq");
/* Put in kcov_remote_stop(). */
kcov_get(remote->kcov);
t->kcov = remote->kcov;
@@ -793,11 +844,19 @@ void kcov_remote_start(u64 handle)
* Read kcov fields before unlock to prevent races with
* KCOV_DISABLE / kcov_remote_reset().
*/
- size = remote->kcov->remote_size;
+ size = min_t(unsigned int, remote->kcov->remote_size,
+ CONFIG_KCOV_IRQ_AREA_SIZE);
mode = remote->kcov->mode;
sequence = remote->kcov->sequence;
- area = kcov_remote_area_get(size);
- spin_unlock(&kcov_remote_lock);
+ if (in_task()) {
+ area = kcov_remote_area_get(size);
+ spin_unlock_irqrestore(&kcov_remote_lock, flags);
+ } else {
+ area = *this_cpu_ptr(&kcov_irq_area);
+ /* Flags are restored by kcov_remote_stop_irq(). */
+ this_cpu_ptr(&kcov_remote_irq)->flags = flags;
+ spin_unlock(&kcov_remote_lock);
+ }

if (!area) {
area = vmalloc(size * sizeof(unsigned long));
@@ -810,8 +869,6 @@ void kcov_remote_start(u64 handle)
/* Reset coverage size. */
*(u64 *)area = 0;

- kcov_debug("area = %px, size = %u", area, size);
-
kcov_start(t, size, area, mode, sequence);

}
@@ -880,30 +937,30 @@ void kcov_remote_stop(void)
void *area = t->kcov_area;
unsigned int size = t->kcov_size;
int sequence = t->kcov_sequence;
+ unsigned long flags;

- if (!kcov) {
- kcov_debug("no kcov found\n");
+ if (!kcov)
return;
- }

kcov_stop(t);
- t->kcov = NULL;

- spin_lock(&kcov->lock);
+ spin_lock_irqsave(&kcov->lock, flags);
/*
* KCOV_DISABLE could have been called between kcov_remote_start()
- * and kcov_remote_stop(), hence the check.
+ * and kcov_remote_stop(), hence the sequence check.
*/
- kcov_debug("move if: %d == %d && %d\n",
- sequence, kcov->sequence, (int)kcov->remote);
if (sequence == kcov->sequence && kcov->remote)
kcov_move_area(kcov->mode, kcov->area, kcov->size, area);
- spin_unlock(&kcov->lock);
+ spin_unlock_irqrestore(&kcov->lock, flags);

- spin_lock(&kcov_remote_lock);
- kcov_remote_area_put(area, size);
- spin_unlock(&kcov_remote_lock);
+ if (in_task()) {
+ spin_lock_irqsave(&kcov_remote_lock, flags);
+ kcov_remote_area_put(area, size);
+ spin_unlock_irqrestore(&kcov_remote_lock, flags);
+ } else
+ kcov_remote_stop_irq(t);

+ /* Get in kcov_remote_start(). */
kcov_put(kcov);
}
EXPORT_SYMBOL(kcov_remote_stop);
@@ -917,6 +974,19 @@ EXPORT_SYMBOL(kcov_common_handle);

static int __init kcov_init(void)
{
+ int cpu;
+
+ for_each_possible_cpu(cpu) {
+ void **area = per_cpu_ptr(&kcov_irq_area, cpu);
+
+ *area = vmalloc(CONFIG_KCOV_IRQ_AREA_SIZE *
+ sizeof(unsigned long));
+ if (!*area) {
+ kcov_err("failed to allocate per-cpu area\n");
+ return -ENOMEM;
+ }
+ }
+
/*
* The kcov debugfs file won't ever get removed and thus,
* there is no need to protect it against removal races. The
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index d1842fe756d5..03f98c463a9e 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1695,6 +1695,15 @@ config KCOV_INSTRUMENT_ALL
filesystem fuzzing with AFL) then you will want to enable coverage
for more specific subsets of files, and should say n here.

+config KCOV_IRQ_AREA_SIZE
+ hex "Size of interrupt coverage collection area in words"
+ depends on KCOV
+ default 0x40000
+ help
+ KCOV uses preallocated per-cpu areas to collect coverage from
+ interrupts. This specifies the size of those areas in the number
+ of unsigned long words.
+
menuconfig RUNTIME_TESTING_MENU
bool "Runtime Testing"
def_bool y
--
2.24.1.735.g03f4e72817-goog