Messages in this thread Patch in this message |  | | Date | Sat, 9 May 2026 17:09:44 +0900 | | Subject | [PATCH v3] kcov: move kcov_remote_data to task_struct for RT and remove local_lock | | From | Tetsuo Handa <> |
| |
In CONFIG_PREEMPT_RT=y kernels, softirqs are executed in a per-CPU ksoftirqd thread or in the context of the task that raised the softirq. This means in_task() can return true even while serving softirqs. This behavior causes KCOV to incorrectly identify the context, leading to state corruption and various kcov-related warnings reported by syzbot.
Furthermore, commit d5d2c51f1e5f ("kcov: replace local_irq_save() with a local_lock_t") introduced local_lock to protect per-CPU kcov_percpu_data. However, the need for this physical CPU-level locking is questionable because:
1. kcov_remote_start/stop() already bail out early if called from in_hardirq() or in_nmi(). 2. The core tracing function check_kcov_mode() also skips hardirq/NMI contexts, meaning no KCOV state is accessed during hardirqs even if they interrupt a KCOV-enabled task/softirq. 3. In non-RT kernels, softirqs do not nest, so no concurrent access to per-CPU data occurs between softirqs. 4. In RT kernels, while softirqs can be preempted, this patch moves the KCOV state from per-CPU variables to task_struct (per-task), eliminating the contention on shared per-CPU resources.
By moving kcov_remote_data to task_struct for RT kernels and replacing the in_task() check with !in_serving_softirq(), we ensure consistent context detection. Since the data is now isolated per-task and not accessed by hardirqs, the local_lock (and the original local_irq_save) is no longer necessary and is removed to reduce overhead.
Changes:
* Move remote coverage state from kcov_percpu_data to task_struct under CONFIG_PREEMPT_RT. * Replace in_task() with !in_serving_softirq() in kcov_remote_start/stop() for accurate context tracking. * Remove local_lock and IRQ disabling from kcov_remote_start/stop() as the state is now task-local and hardirqs are already excluded. * Ensure CONFIG_PREEMPT_RT=y uses kcov_remote_area_get() (the vmalloc-backed pool) instead of the single per-CPU irq_area.
Analyzed-by: AI Mode in Google Search (no mail address) Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> Fixes: 5ff3b30ab57d ("kcov: collect coverage from interrupts") --- Only compile tested. I don't have environment to measure how not preallocating CONFIG_KCOV_IRQ_AREA_SIZE bytes of buffers and add to the global pool in kcov_init() impacts reliability / performance. Please be sure to test this patch using CONFIG_PREEMPT_RT=y and CONFIG_PREEMPT_RT=n kernels in local syzkaller environment before sending upstream.
This patch is expected to address various KCOV related reports. But I don't add Closes: lines because we can't tell whether this patch alone is sufficient for marking as Closes:. We will find the answer some time after being sent to upstream.
https://syzkaller.appspot.com/bug?extid=90984d3713722683112e https://syzkaller.appspot.com/bug?extid=47cf95ca1f9dcca872c8 https://syzkaller.appspot.com/bug?extid=8a173e13208949931dc7 https://syzkaller.appspot.com/bug?extid=3f51ad7ac3ae57a6fdcc https://syzkaller.appspot.com/bug?extid=e6686317bd9fe911591a
include/linux/sched.h | 10 +++++ kernel/kcov.c | 94 +++++++++++++++++++++++++------------------ lib/Kconfig.debug | 1 + 3 files changed, 65 insertions(+), 40 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h index 368c7b4d7cb5..2c963f4271d6 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1522,6 +1522,16 @@ struct task_struct { /* Collect coverage from softirq context: */ unsigned int kcov_softirq; + +#ifdef CONFIG_PREEMPT_RT + /* Temporary storage for preempting remote coverage collection: */ + unsigned int kcov_saved_mode; + unsigned int kcov_saved_size; + void *kcov_saved_area; + struct kcov *kcov_saved_kcov; + int kcov_saved_sequence; +#endif + #endif #ifdef CONFIG_MEMCG_V1 diff --git a/kernel/kcov.c b/kernel/kcov.c index 0b369e88c7c9..965c11a75b36 100644 --- a/kernel/kcov.c +++ b/kernel/kcov.c @@ -88,9 +88,9 @@ 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); +#ifndef CONFIG_PREEMPT_RT struct kcov_percpu_data { void *irq_area; - local_lock_t lock; unsigned int saved_mode; unsigned int saved_size; @@ -100,8 +100,8 @@ struct kcov_percpu_data { }; static DEFINE_PER_CPU(struct kcov_percpu_data, kcov_percpu_data) = { - .lock = INIT_LOCAL_LOCK(lock), }; +#endif /* Must be called with kcov_remote_lock locked. */ static struct kcov_remote *kcov_remote_find(u64 handle) @@ -823,8 +823,38 @@ static inline bool kcov_mode_enabled(unsigned int mode) return (mode & ~KCOV_IN_CTXSW) != KCOV_MODE_DISABLED; } +#ifdef CONFIG_PREEMPT_RT +static void kcov_remote_softirq_start(struct task_struct *t) +{ + unsigned int mode; + + mode = READ_ONCE(t->kcov_mode); + barrier(); + if (kcov_mode_enabled(mode)) { + t->kcov_saved_mode = mode; + t->kcov_saved_size = t->kcov_size; + t->kcov_saved_area = t->kcov_area; + t->kcov_saved_sequence = t->kcov_sequence; + t->kcov_saved_kcov = t->kcov; + kcov_stop(t); + } +} + +static void kcov_remote_softirq_stop(struct task_struct *t) +{ + if (t->kcov_saved_kcov) { + kcov_start(t, t->kcov_saved_kcov, t->kcov_saved_size, + t->kcov_saved_area, t->kcov_saved_mode, + t->kcov_saved_sequence); + t->kcov_saved_mode = 0; + t->kcov_saved_size = 0; + t->kcov_saved_area = NULL; + t->kcov_saved_sequence = 0; + t->kcov_saved_kcov = NULL; + } +} +#else static void kcov_remote_softirq_start(struct task_struct *t) - __must_hold(&kcov_percpu_data.lock) { struct kcov_percpu_data *data = this_cpu_ptr(&kcov_percpu_data); unsigned int mode; @@ -842,7 +872,6 @@ static void kcov_remote_softirq_start(struct task_struct *t) } static void kcov_remote_softirq_stop(struct task_struct *t) - __must_hold(&kcov_percpu_data.lock) { struct kcov_percpu_data *data = this_cpu_ptr(&kcov_percpu_data); @@ -857,6 +886,7 @@ static void kcov_remote_softirq_stop(struct task_struct *t) data->saved_kcov = NULL; } } +#endif void kcov_remote_start(u64 handle) { @@ -867,43 +897,35 @@ void kcov_remote_start(u64 handle) void *area; unsigned int size; int sequence; - unsigned long flags; - if (WARN_ON(!kcov_check_handle(handle, true, true, true))) + /* Don't use in_task() in order to allow consistent checks in RT kernels. */ + if (in_hardirq() || in_nmi()) return; - if (!in_task() && !in_softirq_really()) + if (WARN_ON(!kcov_check_handle(handle, true, true, true))) return; - local_lock_irqsave(&kcov_percpu_data.lock, flags); - /* * Check that kcov_remote_start() is not called twice in background * threads nor called by user tasks (with enabled kcov). */ - mode = READ_ONCE(t->kcov_mode); - if (WARN_ON(in_task() && kcov_mode_enabled(mode))) { - local_unlock_irqrestore(&kcov_percpu_data.lock, flags); + if (WARN_ON(!in_serving_softirq() && kcov_mode_enabled(READ_ONCE(t->kcov_mode)))) return; - } /* * Check that kcov_remote_start() is not called twice in softirqs. * Note, that kcov_remote_start() can be called from a softirq that * 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); + if (WARN_ON(in_serving_softirq() && t->kcov_softirq)) return; - } 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); return; } kcov_debug("handle = %llx, context: %s\n", handle, - in_task() ? "task" : "softirq"); + !in_serving_softirq() ? "task" : "softirq"); kcov = remote->kcov; /* Put in kcov_remote_stop(). */ kcov_get(kcov); @@ -915,6 +937,10 @@ void kcov_remote_start(u64 handle) */ mode = context_unsafe(kcov->mode); sequence = kcov->sequence; +#ifdef CONFIG_PREEMPT_RT + size = kcov->remote_size; + area = kcov_remote_area_get(size); +#else if (in_task()) { size = kcov->remote_size; area = kcov_remote_area_get(size); @@ -922,17 +948,16 @@ void kcov_remote_start(u64 handle) size = CONFIG_KCOV_IRQ_AREA_SIZE; area = this_cpu_ptr(&kcov_percpu_data)->irq_area; } +#endif spin_unlock(&kcov_remote_lock); - /* Can only happen when in_task(). */ + /* Can only happen when CONFIG_PREEMPT_RT=y or in_task(). */ if (!area) { - local_unlock_irqrestore(&kcov_percpu_data.lock, flags); area = vmalloc(size * sizeof(unsigned long)); if (!area) { kcov_put(kcov); return; } - local_lock_irqsave(&kcov_percpu_data.lock, flags); } /* Reset coverage size. */ @@ -943,9 +968,6 @@ void kcov_remote_start(u64 handle) t->kcov_softirq = 1; } kcov_start(t, kcov, size, area, mode, sequence); - - local_unlock_irqrestore(&kcov_percpu_data.lock, flags); - } EXPORT_SYMBOL(kcov_remote_start); @@ -1022,32 +1044,24 @@ void kcov_remote_stop(void) void *area; unsigned int size; int sequence; - unsigned long flags; - if (!in_task() && !in_softirq_really()) + /* Don't use in_task() in order to allow consistent checks in RT kernels. */ + if (in_hardirq() || in_nmi()) return; - local_lock_irqsave(&kcov_percpu_data.lock, flags); - mode = READ_ONCE(t->kcov_mode); barrier(); - if (!kcov_mode_enabled(mode)) { - local_unlock_irqrestore(&kcov_percpu_data.lock, flags); + if (!kcov_mode_enabled(mode)) return; - } /* * When in softirq, check if the corresponding kcov_remote_start() * actually found the remote handle and started collecting coverage. */ - if (in_serving_softirq() && !t->kcov_softirq) { - local_unlock_irqrestore(&kcov_percpu_data.lock, flags); + if (in_serving_softirq() && !t->kcov_softirq) 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); + if (WARN_ON(!in_serving_softirq() && t->kcov_softirq)) return; - } kcov = t->kcov; area = t->kcov_area; @@ -1069,14 +1083,12 @@ void kcov_remote_stop(void) kcov_move_area(kcov->mode, kcov->area, kcov->size, area); spin_unlock(&kcov->lock); - if (in_task()) { + if (IS_ENABLED(CONFIG_PREEMPT_RT) || in_task()) { spin_lock(&kcov_remote_lock); kcov_remote_area_put(area, size); spin_unlock(&kcov_remote_lock); } - local_unlock_irqrestore(&kcov_percpu_data.lock, flags); - /* Get in kcov_remote_start(). */ kcov_put(kcov); } @@ -1119,6 +1131,7 @@ static void __init selftest(void) static int __init kcov_init(void) { +#ifndef CONFIG_PREEMPT_RT int cpu; for_each_possible_cpu(cpu) { @@ -1128,6 +1141,7 @@ static int __init kcov_init(void) return -ENOMEM; per_cpu_ptr(&kcov_percpu_data, cpu)->irq_area = area; } +#endif /* * The kcov debugfs file won't ever get removed and thus, diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index 13f3297aa823..493be2c73c9d 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -2247,6 +2247,7 @@ config KCOV_INSTRUMENT_ALL config KCOV_IRQ_AREA_SIZE hex "Size of interrupt coverage collection area in words" depends on KCOV + depends on !PREEMPT_RT default 0x40000 help KCOV uses preallocated per-cpu areas to collect coverage from -- 2.47.3
|  |