On Sat, Aug 02, 2025 at 02:26:49PM +0000, Yunseong Kim wrote:

The KCOV subsystem currently utilizes standard spinlock_t and local_lock_t for synchronization. In PREEMPT_RT configurations, these locks can be implemented via rtmutexes and may therefore sleep. This behavior is problematic as kcov locks are sometimes used in atomic contexts or protect data accessed during critical instrumentation paths where sleeping is not permissible.

Address these issues to make kcov PREEMPT_RT friendly:

1. Convert kcov->lock and kcov_remote_lock from spinlock_t to raw_spinlock_t. This ensures they remain true, non-sleeping spinlocks even on PREEMPT_RT kernels.

2. Refactor the KCOV_REMOTE_ENABLE path to move memory allocations out of the critical section. All necessary struct kcov_remote structures are now pre-allocated individually in kcov_ioctl() using GFP_KERNEL (allowing sleep) before acquiring the raw spinlocks.

3. Modify the ioctl handling logic to utilize these pre-allocated structures within the critical section. kcov_remote_add() is modified to accept a pre-allocated structure instead of allocating one internally.

4. Remove the local_lock_t protection for kcov_percpu_data in kcov_remote_start/stop(). Since local_lock_t can also sleep under RT, and the required protection is against local interrupts when accessing per-CPU data, it is replaced with explicit local_irq_save/restore().

Link: https://lore.kernel.org/all/20250725201400.1078395-2-ysk@kzalloc.com/t/#u Fixes: f85d39dd7ed8 ("kcov, usb: disable interrupts in kcov_remote_start_usb_softirq") Cc: Andrey Konovalov andreyknvl@gmail.com Cc: Tetsuo Handa penguin-kernel@i-love.sakura.ne.jp Cc: Alan Stern stern@rowland.harvard.edu Cc: Dmitry Vyukov dvyukov@google.com Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Sebastian Andrzej Siewior bigeasy@linutronix.de Cc: Byungchul Park byungchul@sk.com Cc: stable@vger.kernel.org Cc: kasan-dev@googlegroups.com Cc: syzkaller@googlegroups.com Cc: linux-usb@vger.kernel.org Cc: linux-rt-devel@lists.linux.dev Signed-off-by: Yunseong Kim ysk@kzalloc.com

---

kernel/kcov.c | 243 +++++++++++++++++++++++++++----------------------- 1 file changed, 130 insertions(+), 113 deletions(-)

diff --git a/kernel/kcov.c b/kernel/kcov.c index 187ba1b80bda..9c8e4325cff8 100644 --- a/kernel/kcov.c +++ b/kernel/kcov.c @@ -54,7 +54,7 @@ struct kcov { */ refcount_t refcount; /* The lock protects mode, size, area and t. */

• spinlock_t lock;

• raw_spinlock_t lock; enum kcov_mode mode; /* Size of arena (in long's). */ unsigned int size;

@@ -84,13 +84,12 @@ struct kcov_remote { struct hlist_node hnode; }; -static DEFINE_SPINLOCK(kcov_remote_lock); +static DEFINE_RAW_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_percpu_data { void *irq_area;

• local_lock_t lock;

unsigned int saved_mode; unsigned int saved_size; @@ -99,9 +98,7 @@ struct kcov_percpu_data { int saved_sequence; }; -static DEFINE_PER_CPU(struct kcov_percpu_data, kcov_percpu_data) = {

• .lock = INIT_LOCAL_LOCK(lock),

-}; +static DEFINE_PER_CPU(struct kcov_percpu_data, kcov_percpu_data); /* Must be called with kcov_remote_lock locked. */ static struct kcov_remote *kcov_remote_find(u64 handle) @@ -116,15 +113,9 @@ static struct kcov_remote *kcov_remote_find(u64 handle) } /* Must be called with kcov_remote_lock locked. */ -static struct kcov_remote *kcov_remote_add(struct kcov *kcov, u64 handle) +static struct kcov_remote *kcov_remote_add(struct kcov *kcov, u64 handle,

• 			   struct kcov_remote *remote)
  

{

• struct kcov_remote *remote;
• if (kcov_remote_find(handle))

• return ERR_PTR(-EEXIST);
  

• remote = kmalloc(sizeof(*remote), GFP_ATOMIC);
• if (!remote)

• return ERR_PTR(-ENOMEM);
  

  remote->handle = handle; remote->kcov = kcov; hash_add(kcov_remote_map, &remote->hnode, handle);

@@ -404,9 +395,8 @@ static void kcov_remote_reset(struct kcov *kcov) int bkt; struct kcov_remote *remote; struct hlist_node *tmp;

• unsigned long flags;

• spin_lock_irqsave(&kcov_remote_lock, flags);

• raw_spin_lock(&kcov_remote_lock); hash_for_each_safe(kcov_remote_map, bkt, tmp, remote, hnode) { if (remote->kcov != kcov) continue;

@@ -415,7 +405,7 @@ static void kcov_remote_reset(struct kcov *kcov) } /* Do reset before unlock to prevent races with kcov_remote_start(). */ kcov_reset(kcov);

• spin_unlock_irqrestore(&kcov_remote_lock, flags);

• raw_spin_unlock(&kcov_remote_lock);

} static void kcov_disable(struct task_struct *t, struct kcov *kcov) @@ -450,7 +440,7 @@ void kcov_task_exit(struct task_struct *t) if (kcov == NULL) return;

• spin_lock_irqsave(&kcov->lock, flags);

• raw_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,

@@ -475,12 +465,12 @@ 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_irqrestore(&kcov->lock, flags);
  

• raw_spin_unlock_irqrestore(&kcov->lock, flags);
  

  return; } /* Just to not leave dangling references behind. */ kcov_disable(t, kcov);

• spin_unlock_irqrestore(&kcov->lock, flags);

• raw_spin_unlock_irqrestore(&kcov->lock, flags); kcov_put(kcov);

} @@ -492,14 +482,14 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma) struct page *page; unsigned long flags;

• spin_lock_irqsave(&kcov->lock, flags);

• raw_spin_lock_irqsave(&kcov->lock, flags); size = kcov->size * sizeof(unsigned long); if (kcov->area == NULL || vma->vm_pgoff != 0 || vma->vm_end - vma->vm_start != size) { res = -EINVAL; goto exit; }

• spin_unlock_irqrestore(&kcov->lock, flags);

• raw_spin_unlock_irqrestore(&kcov->lock, flags); vm_flags_set(vma, VM_DONTEXPAND); for (off = 0; off < size; off += PAGE_SIZE) { page = vmalloc_to_page(kcov->area + off);

@@ -511,7 +501,7 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma) } return 0; exit:

• spin_unlock_irqrestore(&kcov->lock, flags);

• raw_spin_unlock_irqrestore(&kcov->lock, flags); return res;

} @@ -525,7 +515,7 @@ static int kcov_open(struct inode *inode, struct file *filep) kcov->mode = KCOV_MODE_DISABLED; kcov->sequence = 1; refcount_set(&kcov->refcount, 1);

• spin_lock_init(&kcov->lock);

• raw_spin_lock_init(&kcov->lock); filep->private_data = kcov; return nonseekable_open(inode, filep);

} @@ -586,10 +576,8 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd, unsigned long arg) { struct task_struct *t;

• unsigned long flags, unused;
• int mode, i;
• struct kcov_remote_arg *remote_arg;
• struct kcov_remote *remote;

• unsigned long unused;
• int mode;

switch (cmd) { case KCOV_ENABLE: @@ -627,69 +615,80 @@ static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd, kcov_disable(t, kcov); kcov_put(kcov); return 0;

• case KCOV_REMOTE_ENABLE:

• if (kcov->mode != KCOV_MODE_INIT || !kcov->area)
  

• 	return -EINVAL;
  

• t = current;
  

• if (kcov->t != NULL || t->kcov != NULL)
  

• 	return -EBUSY;
  

• remote_arg = (struct kcov_remote_arg *)arg;
  

• mode = kcov_get_mode(remote_arg->trace_mode);
  

• if (mode < 0)
  

• 	return mode;
  

• if ((unsigned long)remote_arg->area_size >
  

•     LONG_MAX / sizeof(unsigned long))
  

• 	return -EINVAL;
  

• kcov->mode = mode;
  

• t->kcov = kcov;
  

•        t->kcov_mode = KCOV_MODE_REMOTE;
  

• kcov->t = t;
  

• kcov->remote = true;
  

• kcov->remote_size = remote_arg->area_size;
  

• spin_lock_irqsave(&kcov_remote_lock, flags);
  

• for (i = 0; i < remote_arg->num_handles; i++) {
  

• 	if (!kcov_check_handle(remote_arg->handles[i],
  

• 				false, true, false)) {
  

• 		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_irqrestore(&kcov_remote_lock,
  

• 					flags);
  

• 		kcov_disable(t, kcov);
  

• 		return PTR_ERR(remote);
  

• 	}
  

• }
  

• if (remote_arg->common_handle) {
  

• 	if (!kcov_check_handle(remote_arg->common_handle,
  

• 				true, false, false)) {
  

• 		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_irqrestore(&kcov_remote_lock,
  

• 					flags);
  

• 		kcov_disable(t, kcov);
  

• 		return PTR_ERR(remote);
  

• 	}
  

• 	t->kcov_handle = remote_arg->common_handle;
  

• }
  

• spin_unlock_irqrestore(&kcov_remote_lock, flags);
  

• /* Put either in kcov_task_exit() or in KCOV_DISABLE. */
  

• kcov_get(kcov);
  

• return 0;
  

  default: return -ENOTTY; }

} +static int kcov_ioctl_locked_remote_enabled(struct kcov *kcov,

• 		 unsigned int cmd, unsigned long arg,
  

• 		 struct kcov_remote *remote_handles,
  

• 		 struct kcov_remote *remote_common_handle)
  

+{

• struct task_struct *t;
• int mode, i, ret;
• struct kcov_remote_arg *remote_arg;
• if (kcov->mode != KCOV_MODE_INIT || !kcov->area)

• return -EINVAL;
  

• t = current;
• if (kcov->t != NULL || t->kcov != NULL)

• return -EBUSY;
  

• remote_arg = (struct kcov_remote_arg *)arg;
• mode = kcov_get_mode(remote_arg->trace_mode);
• if (mode < 0)

• return mode;
  

• if ((unsigned long)remote_arg->area_size >

• LONG_MAX / sizeof(unsigned long))
  

• return -EINVAL;
  

• kcov->mode = mode;
• t->kcov = kcov;
• t->kcov_mode = KCOV_MODE_REMOTE;
• kcov->t = t;
• kcov->remote = true;
• kcov->remote_size = remote_arg->area_size;
• raw_spin_lock(&kcov_remote_lock);
• for (i = 0; i < remote_arg->num_handles; i++) {

• if (!kcov_check_handle(remote_arg->handles[i],
  

• 			false, true, false)) {
  

• 	ret = -EINVAL;
  

• 	goto err;
  

• }
  

• if (kcov_remote_find(remote_arg->handles[i])) {
  

• 	ret = -EEXIST;
  

• 	goto err;
  

• }
  

• kcov_remote_add(kcov, remote_arg->handles[i],
  

• 	&remote_handles[i]);
  

• }
• if (remote_arg->common_handle) {

• if (!kcov_check_handle(remote_arg->common_handle,
  

• 			true, false, false)) {
  

• 	ret = -EINVAL;
  

• 	goto err;
  

• }
  

• if (kcov_remote_find(remote_arg->common_handle)) {
  

• 	ret = -EEXIST;
  

• 	goto err;
  

• }
  

• kcov_remote_add(kcov,
  

• 	remote_arg->common_handle, remote_common_handle);
  

• t->kcov_handle = remote_arg->common_handle;
  

• }
• raw_spin_unlock(&kcov_remote_lock);
• /* Put either in kcov_task_exit() or in KCOV_DISABLE. */
• kcov_get(kcov);
• return 0;

+err:

• raw_spin_unlock(&kcov_remote_lock);
• kcov_disable(t, kcov);
• kfree(remote_common_handle);
• kfree(remote_handles);
• return ret;

+}

static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg) { struct kcov *kcov; @@ -697,6 +696,7 @@ 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;

• struct kcov_remote *remote_handles, *remote_common_handle; unsigned long size, flags; void *area;

@@ -716,16 +716,16 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg) area = vmalloc_user(size * sizeof(unsigned long)); if (area == NULL) return -ENOMEM;

• spin_lock_irqsave(&kcov->lock, flags);
  

• raw_spin_lock_irqsave(&kcov->lock, flags);
  

  if (kcov->mode != KCOV_MODE_DISABLED) {

• 	spin_unlock_irqrestore(&kcov->lock, flags);
  

• 	raw_spin_unlock_irqrestore(&kcov->lock, flags);
  vfree(area);
  return -EBUSY;
  

  } kcov->area = area; kcov->size = size; kcov->mode = KCOV_MODE_INIT;

• spin_unlock_irqrestore(&kcov->lock, flags);
  

• raw_spin_unlock_irqrestore(&kcov->lock, flags);
  

  return 0; case KCOV_REMOTE_ENABLE: if (get_user(remote_num_handles, (unsigned __user *)(arg +

@@ -743,18 +743,35 @@ static long kcov_ioctl(struct file *filep, unsigned int cmd, unsigned long arg) return -EINVAL; } arg = (unsigned long)remote_arg;

• fallthrough;
  

• remote_handles = kmalloc_array(remote_arg->num_handles,
  

• 			sizeof(struct kcov_remote), GFP_KERNEL);
  

• if (!remote_handles)
  

• 	return -ENOMEM;
  

• remote_common_handle = kmalloc(sizeof(struct kcov_remote), GFP_KERNEL);
  

• if (!remote_common_handle) {
  

• 	kfree(remote_handles);
  

• 	return -ENOMEM;
  

• }
  

• raw_spin_lock_irqsave(&kcov->lock, flags);
  

• res = kcov_ioctl_locked_remote_enabled(kcov, cmd, arg,
  

• 		remote_handles, remote_common_handle);
  

• raw_spin_unlock_irqrestore(&kcov->lock, flags);
  

• kfree(remote_arg);
  

• break;
  

  default: /*

•  * KCOV_ENABLE, KCOV_DISABLE:
  

  • All other commands can be normally executed under a spin lock, so we
  • obtain and release it here in order to simplify kcov_ioctl_locked().
  */

• spin_lock_irqsave(&kcov->lock, flags);
  

• raw_spin_lock_irqsave(&kcov->lock, flags);
  

  res = kcov_ioctl_locked(kcov, cmd, arg);

• spin_unlock_irqrestore(&kcov->lock, flags);
  

• kfree(remote_arg);
  

• return res;
  

• raw_spin_unlock_irqrestore(&kcov->lock, flags);
  

• break;
  

  }
• return res;

} static const struct file_operations kcov_fops = { @@ -862,7 +879,7 @@ void kcov_remote_start(u64 handle) if (!in_task() && !in_softirq_really()) return;

• local_lock_irqsave(&kcov_percpu_data.lock, flags);

• local_irq_save(flags);

/* * Check that kcov_remote_start() is not called twice in background @@ -870,7 +887,7 @@ void kcov_remote_start(u64 handle) */ mode = READ_ONCE(t->kcov_mode); if (WARN_ON(in_task() && kcov_mode_enabled(mode))) {

• local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
  

• local_irq_restore(flags);
  

  return; } /*

@@ -879,15 +896,15 @@ void kcov_remote_start(u64 handle) * happened while collecting coverage from a background thread. */ if (WARN_ON(in_serving_softirq() && t->kcov_softirq)) {

• local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
  

• local_irq_restore(flags);
  

  return; }

• spin_lock(&kcov_remote_lock);

• raw_spin_lock(&kcov_remote_lock); remote = kcov_remote_find(handle); if (!remote) {

• spin_unlock(&kcov_remote_lock);
  

• local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
  

• raw_spin_unlock(&kcov_remote_lock);
  

• local_irq_restore(flags);
  

  return; } kcov_debug("handle = %llx, context: %s\n", handle,

@@ -908,17 +925,17 @@ void kcov_remote_start(u64 handle) size = CONFIG_KCOV_IRQ_AREA_SIZE; area = this_cpu_ptr(&kcov_percpu_data)->irq_area; }

• spin_unlock(&kcov_remote_lock);

• raw_spin_unlock(&kcov_remote_lock);

/* Can only happen when in_task(). */ if (!area) {

• local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
  

• local_irq_restore(flags);
  

  area = vmalloc(size * sizeof(unsigned long)); if (!area) { kcov_put(kcov); return; }

• local_lock_irqsave(&kcov_percpu_data.lock, flags);
  

• local_irq_save(flags);
  

  }

/* Reset coverage size. */ @@ -930,7 +947,7 @@ void kcov_remote_start(u64 handle) } kcov_start(t, kcov, size, area, mode, sequence);

• local_unlock_irqrestore(&kcov_percpu_data.lock, flags);

• local_irq_restore(flags);

} EXPORT_SYMBOL(kcov_remote_start); @@ -1004,12 +1021,12 @@ void kcov_remote_stop(void) if (!in_task() && !in_softirq_really()) return;

• local_lock_irqsave(&kcov_percpu_data.lock, flags);

• local_irq_save(flags);

mode = READ_ONCE(t->kcov_mode); barrier(); if (!kcov_mode_enabled(mode)) {

• local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
  

• local_irq_restore(flags);
  

  return; } /*

@@ -1017,12 +1034,12 @@ void kcov_remote_stop(void) * actually found the remote handle and started collecting coverage. */ if (in_serving_softirq() && !t->kcov_softirq) {

• local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
  

• local_irq_restore(flags);
  

  return; } /* Make sure that kcov_softirq is only set when in softirq. */ if (WARN_ON(!in_serving_softirq() && t->kcov_softirq)) {

• local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
  

• local_irq_restore(flags);
  

  return; }

@@ -1037,22 +1054,22 @@ void kcov_remote_stop(void) kcov_remote_softirq_stop(t); }

• spin_lock(&kcov->lock);

• raw_spin_lock(&kcov->lock); /*
  • KCOV_DISABLE could have been called between kcov_remote_start()
  • and kcov_remote_stop(), hence the sequence check.
  */ if (sequence == kcov->sequence && kcov->remote) kcov_move_area(kcov->mode, kcov->area, kcov->size, area);

• spin_unlock(&kcov->lock);

• raw_spin_unlock(&kcov->lock);

if (in_task()) {

• spin_lock(&kcov_remote_lock);
  

• raw_spin_lock(&kcov_remote_lock);
  

  kcov_remote_area_put(area, size);

• spin_unlock(&kcov_remote_lock);
  

• raw_spin_unlock(&kcov_remote_lock);
  

  }

• local_unlock_irqrestore(&kcov_percpu_data.lock, flags);

• local_irq_restore(flags);

/* Get in kcov_remote_start(). */ kcov_put(kcov); -- 2.50.0

Hi,

This is the friendly patch-bot of Greg Kroah-Hartman. You have sent him a patch that has triggered this response. He used to manually respond to these common problems, but in order to save his sanity (he kept writing the same thing over and over, yet to different people), I was created. Hopefully you will not take offence and will fix the problem in your patch and resubmit it so that it can be accepted into the Linux kernel tree.

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