AI Trajectory

null

{
  "EnableConfigs": [
    "PREEMPT_RT"
  ],
  "FocusSymbols": [
    "kcov_remote_softirq_start",
    "kcov_remote_softirq_stop",
    "kcov_remote_area_get",
    "kcov_remote_area_put",
    "kcov_remote_start",
    "kcov_remote_stop"
  ],
  "Reasoning": "The patch modifies the core KCOV coverage collection mechanism, specifically changing how remote coverage is collected in softirq contexts and adding support for softirq preemption (which occurs under PREEMPT_RT). This is functional kernel logic reachable via KCOV ioctls and remote coverage triggers (like USB or vhost).",
  "WorthFuzzing": true
}

null

{
  "PatchDiff": "commit 8da711c35d5639b07f56a11c785502e1a569e3dd\nAuthor: syz-cluster \u003ctriage@syzkaller.com\u003e\nDate:   Mon Jul 20 15:45:12 2026 +0000\n\n    syz-cluster: applied patch under review\n\ndiff --git a/include/linux/sched.h b/include/linux/sched.h\nindex 373bcc0598d10b..7a53c15cecb52d 100644\n--- a/include/linux/sched.h\n+++ b/include/linux/sched.h\n@@ -1543,6 +1543,14 @@ struct task_struct {\n \n \t/* Collect coverage from softirq context: */\n \tunsigned int\t\t\tkcov_softirq;\n+\n+\t/* Temporary storage for preempting remote coverage collection: */\n+\tunsigned int\t\t\tkcov_saved_mode;\n+\tunsigned int\t\t\tkcov_saved_size;\n+\tvoid\t\t\t\t*kcov_saved_area;\n+\tstruct kcov\t\t\t*kcov_saved_kcov;\n+\tint\t\t\t\tkcov_saved_sequence;\n+\n #endif\n \n #ifdef CONFIG_MEMCG_V1\ndiff --git a/kernel/kcov.c b/kernel/kcov.c\nindex 1df373fb562bc8..a7514303eff38a 100644\n--- a/kernel/kcov.c\n+++ b/kernel/kcov.c\n@@ -86,17 +86,12 @@ struct kcov_remote {\n \n static DEFINE_SPINLOCK(kcov_remote_lock);\n static DEFINE_HASHTABLE(kcov_remote_map, 4);\n-static struct list_head kcov_remote_areas = LIST_HEAD_INIT(kcov_remote_areas);\n+static struct list_head kcov_remote_areas[2] = {\n+\tLIST_HEAD_INIT(kcov_remote_areas[0]), LIST_HEAD_INIT(kcov_remote_areas[1])\n+};\n \n struct kcov_percpu_data {\n-\tvoid\t\t\t*irq_area;\n \tlocal_lock_t\t\tlock;\n-\n-\tunsigned int\t\tsaved_mode;\n-\tunsigned int\t\tsaved_size;\n-\tvoid\t\t\t*saved_area;\n-\tstruct kcov\t\t*saved_kcov;\n-\tint\t\t\tsaved_sequence;\n };\n \n static DEFINE_PER_CPU(struct kcov_percpu_data, kcov_percpu_data) = {\n@@ -132,12 +127,13 @@ static struct kcov_remote *kcov_remote_add(struct kcov *kcov, u64 handle)\n }\n \n /* Must be called with kcov_remote_lock locked. */\n-static struct kcov_remote_area *kcov_remote_area_get(unsigned int size)\n+static struct kcov_remote_area *kcov_remote_area_get(unsigned int size, bool irq)\n {\n \tstruct kcov_remote_area *area;\n \tstruct list_head *pos;\n+\tstruct list_head *list = \u0026kcov_remote_areas[irq];\n \n-\tlist_for_each(pos, \u0026kcov_remote_areas) {\n+\tlist_for_each(pos, list) {\n \t\tarea = list_entry(pos, struct kcov_remote_area, list);\n \t\tif (area-\u003esize == size) {\n \t\t\tlist_del(\u0026area-\u003elist);\n@@ -149,11 +145,11 @@ static struct kcov_remote_area *kcov_remote_area_get(unsigned int size)\n \n /* Must be called with kcov_remote_lock locked. */\n static void kcov_remote_area_put(struct kcov_remote_area *area,\n-\t\t\t\t\tunsigned int size)\n+\t\t\t\t unsigned int size, bool irq)\n {\n \tINIT_LIST_HEAD(\u0026area-\u003elist);\n \tarea-\u003esize = size;\n-\tlist_add(\u0026area-\u003elist, \u0026kcov_remote_areas);\n+\tlist_add(\u0026area-\u003elist, \u0026kcov_remote_areas[irq]);\n \t/*\n \t * KMSAN doesn't instrument this file, so it may not know area-\u003elist\n \t * is initialized. Unpoison it explicitly to avoid reports in\n@@ -390,6 +386,12 @@ void kcov_task_init(struct task_struct *t)\n \tkcov_task_reset(t);\n \tt-\u003ekcov_remote = NULL;\n \tt-\u003ekcov_handle = current-\u003ekcov_handle;\n+\tt-\u003ekcov_softirq = 0;\n+\tt-\u003ekcov_saved_mode = 0;\n+\tt-\u003ekcov_saved_size = 0;\n+\tt-\u003ekcov_saved_area = NULL;\n+\tt-\u003ekcov_saved_kcov = NULL;\n+\tt-\u003ekcov_saved_sequence = 0;\n }\n \n static void kcov_reset(struct kcov *kcov)\n@@ -836,17 +838,16 @@ static inline bool kcov_mode_enabled(unsigned int mode)\n static void kcov_remote_softirq_start(struct task_struct *t)\n \t__must_hold(\u0026kcov_percpu_data.lock)\n {\n-\tstruct kcov_percpu_data *data = this_cpu_ptr(\u0026kcov_percpu_data);\n \tunsigned int mode;\n \n \tmode = READ_ONCE(t-\u003ekcov_mode);\n \tbarrier();\n \tif (kcov_mode_enabled(mode)) {\n-\t\tdata-\u003esaved_mode = mode;\n-\t\tdata-\u003esaved_size = t-\u003ekcov_size;\n-\t\tdata-\u003esaved_area = t-\u003ekcov_area;\n-\t\tdata-\u003esaved_sequence = t-\u003ekcov_sequence;\n-\t\tdata-\u003esaved_kcov = t-\u003ekcov;\n+\t\tt-\u003ekcov_saved_mode = mode;\n+\t\tt-\u003ekcov_saved_size = t-\u003ekcov_size;\n+\t\tt-\u003ekcov_saved_area = t-\u003ekcov_area;\n+\t\tt-\u003ekcov_saved_sequence = t-\u003ekcov_sequence;\n+\t\tt-\u003ekcov_saved_kcov = t-\u003ekcov;\n \t\tkcov_stop(t);\n \t}\n }\n@@ -854,17 +855,15 @@ static void kcov_remote_softirq_start(struct task_struct *t)\n static void kcov_remote_softirq_stop(struct task_struct *t)\n \t__must_hold(\u0026kcov_percpu_data.lock)\n {\n-\tstruct kcov_percpu_data *data = this_cpu_ptr(\u0026kcov_percpu_data);\n-\n-\tif (data-\u003esaved_kcov) {\n-\t\tkcov_start(t, data-\u003esaved_kcov, data-\u003esaved_size,\n-\t\t\t\tdata-\u003esaved_area, data-\u003esaved_mode,\n-\t\t\t\tdata-\u003esaved_sequence);\n-\t\tdata-\u003esaved_mode = 0;\n-\t\tdata-\u003esaved_size = 0;\n-\t\tdata-\u003esaved_area = NULL;\n-\t\tdata-\u003esaved_sequence = 0;\n-\t\tdata-\u003esaved_kcov = NULL;\n+\tif (t-\u003ekcov_saved_kcov) {\n+\t\tkcov_start(t, t-\u003ekcov_saved_kcov, t-\u003ekcov_saved_size,\n+\t\t\t   t-\u003ekcov_saved_area, t-\u003ekcov_saved_mode,\n+\t\t\t   t-\u003ekcov_saved_sequence);\n+\t\tt-\u003ekcov_saved_mode = 0;\n+\t\tt-\u003ekcov_saved_size = 0;\n+\t\tt-\u003ekcov_saved_area = NULL;\n+\t\tt-\u003ekcov_saved_sequence = 0;\n+\t\tt-\u003ekcov_saved_kcov = NULL;\n \t}\n }\n \n@@ -927,17 +926,17 @@ void kcov_remote_start(u64 handle)\n \tsequence = kcov-\u003esequence;\n \tif (in_task()) {\n \t\tsize = kcov-\u003eremote_size;\n-\t\tarea = kcov_remote_area_get(size);\n+\t\tarea = kcov_remote_area_get(size, false);\n \t} else {\n \t\tsize = CONFIG_KCOV_IRQ_AREA_SIZE;\n-\t\tarea = this_cpu_ptr(\u0026kcov_percpu_data)-\u003eirq_area;\n+\t\tarea = kcov_remote_area_get(size, true);\n \t}\n \tspin_unlock(\u0026kcov_remote_lock);\n \n-\t/* Can only happen when in_task(). */\n+\t/* Allocate new buffer if we can sleep. */\n \tif (!area) {\n \t\tlocal_unlock_irqrestore(\u0026kcov_percpu_data.lock, flags);\n-\t\tarea = vmalloc(size * sizeof(unsigned long));\n+\t\tarea = in_task() ? vmalloc(size * sizeof(unsigned long)) : NULL;\n \t\tif (!area) {\n \t\t\tkcov_put(kcov);\n \t\t\treturn;\n@@ -1079,11 +1078,9 @@ void kcov_remote_stop(void)\n \t\tkcov_move_area(kcov-\u003emode, kcov-\u003earea, kcov-\u003esize, area);\n \tspin_unlock(\u0026kcov-\u003elock);\n \n-\tif (in_task()) {\n-\t\tspin_lock(\u0026kcov_remote_lock);\n-\t\tkcov_remote_area_put(area, size);\n-\t\tspin_unlock(\u0026kcov_remote_lock);\n-\t}\n+\tspin_lock(\u0026kcov_remote_lock);\n+\tkcov_remote_area_put(area, size, !in_task());\n+\tspin_unlock(\u0026kcov_remote_lock);\n \n \tlocal_unlock_irqrestore(\u0026kcov_percpu_data.lock, flags);\n \n@@ -1129,14 +1126,21 @@ static void __init selftest(void)\n \n static int __init kcov_init(void)\n {\n-\tint cpu;\n+\tint cpu = num_possible_cpus();\n+\n+#ifdef CONFIG_PREEMPT_RT\n+\t/* Allocate some extra buffers in order to prepare for softirq preemption. */\n+\tcpu = cpu \u003e= 4 ? cpu * 2 : cpu + 4;\n+#endif\n+\twhile (cpu--) {\n+\t\tvoid *area = vmalloc(CONFIG_KCOV_IRQ_AREA_SIZE * sizeof(unsigned long));\n+\t\tunsigned long flags;\n \n-\tfor_each_possible_cpu(cpu) {\n-\t\tvoid *area = vmalloc_node(CONFIG_KCOV_IRQ_AREA_SIZE *\n-\t\t\t\tsizeof(unsigned long), cpu_to_node(cpu));\n \t\tif (!area)\n \t\t\treturn -ENOMEM;\n-\t\tper_cpu_ptr(\u0026kcov_percpu_data, cpu)-\u003eirq_area = area;\n+\t\tspin_lock_irqsave(\u0026kcov_remote_lock, flags);\n+\t\tkcov_remote_area_put(area, CONFIG_KCOV_IRQ_AREA_SIZE, true);\n+\t\tspin_unlock_irqrestore(\u0026kcov_remote_lock, flags);\n \t}\n \n \t/*\ndiff --git a/lib/Kconfig.debug b/lib/Kconfig.debug\nindex 1244dcac2294ad..12786379bf1d72 100644\n--- a/lib/Kconfig.debug\n+++ b/lib/Kconfig.debug\n@@ -2247,10 +2247,11 @@ config KCOV_INSTRUMENT_ALL\n config KCOV_IRQ_AREA_SIZE\n \thex \"Size of interrupt coverage collection area in words\"\n \tdepends on KCOV\n+\trange 0x80 0x1000000\n \tdefault 0x40000\n \thelp\n-\t  KCOV uses preallocated per-cpu areas to collect coverage from\n-\t  soft interrupts. This specifies the size of those areas in the\n+\t  KCOV uses preallocated areas to collect coverage from soft\n+\t  interrupts. This specifies the size of those areas in the\n \t  number of unsigned long words.\n \n config KCOV_SELFTEST\n"
}

best-expensive

null

{
  "EnableConfigs": [
    "PREEMPT_RT"
  ],
  "FocusSymbols": [
    "kcov_remote_softirq_start",
    "kcov_remote_softirq_stop",
    "kcov_remote_area_get",
    "kcov_remote_area_put",
    "kcov_remote_start",
    "kcov_remote_stop"
  ],
  "Reasoning": "The patch modifies the core KCOV coverage collection mechanism, specifically changing how remote coverage is collected in softirq contexts and adding support for softirq preemption (which occurs under PREEMPT_RT). This is functional kernel logic reachable via KCOV ioctls and remote coverage triggers (like USB or vhost).",
  "WorthFuzzing": true
}

You are an expert Linux kernel maintainer.
Your job is to review a provided patch series and determine
if it makes functional changes to the kernel that should be fuzzed.

IMPORTANT: The changes have ALREADY been applied and committed as the HEAD commit in
your workspace. Do NOT rely on your internal knowledge of the kernel. You must actively
use your code access tools to examine the actual source code and confirm any assumptions.

Return WorthFuzzing=false if the patch only contains:
- Modifications to Documentation/, Kconfig files, or code comments.
- Purely decorative changes, such as logging (e.g., pr_err, printk) or tracepoints.
- Changes to numeric constants or macros that do not functionally alter execution flow.
- Code paths that are impossible to reach in virtualized environments like GCE or QEMU,
even when utilizing software-emulated hardware (e.g., usb gadget, mac80211_hwsim).

If it modifies reachable core kernel logic, drivers, or architectures, use your code search
tools to verify the code can be executed, then return WorthFuzzing=true.

When returning WorthFuzzing=true, you MUST ALSO:
1. Extract any specific kernel functions that should be heavily fuzzed into FocusSymbols.
   Avoid listing generic hot-path functions to prevent skewed test distributions.
2. Identify any specific CONFIG_ options required to properly test this new/modified feature.
   Go and look into the Kconfig files and check for ifdefs around the code, do not make assumptions.
   Do not list too generic configs (we already have them enabled). Only list those that
   specifically cover the modified code. List them in the EnableConfigs output array,
   and DO NOT add a 'CONFIG_' prefix (e.g., return "NET_IPV4" instead of "CONFIG_NET_IPV4").
Prefer calling several tools at the same time to save round-trips.


Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.

For your convenience, here is the diff of the changes:
commit 8da711c35d5639b07f56a11c785502e1a569e3dd
Author: syz-cluster <triage@syzkaller.com>
Date:   Mon Jul 20 15:45:12 2026 +0000

    syz-cluster: applied patch under review

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 373bcc0598d10b..7a53c15cecb52d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1543,6 +1543,14 @@ struct task_struct {
 
 	/* Collect coverage from softirq context: */
 	unsigned int			kcov_softirq;
+
+	/* 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
 
 #ifdef CONFIG_MEMCG_V1
diff --git a/kernel/kcov.c b/kernel/kcov.c
index 1df373fb562bc8..a7514303eff38a 100644
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -86,17 +86,12 @@ struct kcov_remote {
 
 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);
+static struct list_head kcov_remote_areas[2] = {
+	LIST_HEAD_INIT(kcov_remote_areas[0]), LIST_HEAD_INIT(kcov_remote_areas[1])
+};
 
 struct kcov_percpu_data {
-	void			*irq_area;
 	local_lock_t		lock;
-
-	unsigned int		saved_mode;
-	unsigned int		saved_size;
-	void			*saved_area;
-	struct kcov		*saved_kcov;
-	int			saved_sequence;
 };
 
 static DEFINE_PER_CPU(struct kcov_percpu_data, kcov_percpu_data) = {
@@ -132,12 +127,13 @@ static struct kcov_remote *kcov_remote_add(struct kcov *kcov, u64 handle)
 }
 
 /* Must be called with kcov_remote_lock locked. */
-static struct kcov_remote_area *kcov_remote_area_get(unsigned int size)
+static struct kcov_remote_area *kcov_remote_area_get(unsigned int size, bool irq)
 {
 	struct kcov_remote_area *area;
 	struct list_head *pos;
+	struct list_head *list = &kcov_remote_areas[irq];
 
-	list_for_each(pos, &kcov_remote_areas) {
+	list_for_each(pos, list) {
 		area = list_entry(pos, struct kcov_remote_area, list);
 		if (area->size == size) {
 			list_del(&area->list);
@@ -149,11 +145,11 @@ static struct kcov_remote_area *kcov_remote_area_get(unsigned int size)
 
 /* Must be called with kcov_remote_lock locked. */
 static void kcov_remote_area_put(struct kcov_remote_area *area,
-					unsigned int size)
+				 unsigned int size, bool irq)
 {
 	INIT_LIST_HEAD(&area->list);
 	area->size = size;
-	list_add(&area->list, &kcov_remote_areas);
+	list_add(&area->list, &kcov_remote_areas[irq]);
 	/*
 	 * KMSAN doesn't instrument this file, so it may not know area->list
 	 * is initialized. Unpoison it explicitly to avoid reports in
@@ -390,6 +386,12 @@ void kcov_task_init(struct task_struct *t)
 	kcov_task_reset(t);
 	t->kcov_remote = NULL;
 	t->kcov_handle = current->kcov_handle;
+	t->kcov_softirq = 0;
+	t->kcov_saved_mode = 0;
+	t->kcov_saved_size = 0;
+	t->kcov_saved_area = NULL;
+	t->kcov_saved_kcov = NULL;
+	t->kcov_saved_sequence = 0;
 }
 
 static void kcov_reset(struct kcov *kcov)
@@ -836,17 +838,16 @@ static inline bool kcov_mode_enabled(unsigned int mode)
 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;
 
 	mode = READ_ONCE(t->kcov_mode);
 	barrier();
 	if (kcov_mode_enabled(mode)) {
-		data->saved_mode = mode;
-		data->saved_size = t->kcov_size;
-		data->saved_area = t->kcov_area;
-		data->saved_sequence = t->kcov_sequence;
-		data->saved_kcov = t->kcov;
+		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);
 	}
 }
@@ -854,17 +855,15 @@ 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);
-
-	if (data->saved_kcov) {
-		kcov_start(t, data->saved_kcov, data->saved_size,
-				data->saved_area, data->saved_mode,
-				data->saved_sequence);
-		data->saved_mode = 0;
-		data->saved_size = 0;
-		data->saved_area = NULL;
-		data->saved_sequence = 0;
-		data->saved_kcov = NULL;
+	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;
 	}
 }
 
@@ -927,17 +926,17 @@ void kcov_remote_start(u64 handle)
 	sequence = kcov->sequence;
 	if (in_task()) {
 		size = kcov->remote_size;
-		area = kcov_remote_area_get(size);
+		area = kcov_remote_area_get(size, false);
 	} else {
 		size = CONFIG_KCOV_IRQ_AREA_SIZE;
-		area = this_cpu_ptr(&kcov_percpu_data)->irq_area;
+		area = kcov_remote_area_get(size, true);
 	}
 	spin_unlock(&kcov_remote_lock);
 
-	/* Can only happen when in_task(). */
+	/* Allocate new buffer if we can sleep. */
 	if (!area) {
 		local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
-		area = vmalloc(size * sizeof(unsigned long));
+		area = in_task() ? vmalloc(size * sizeof(unsigned long)) : NULL;
 		if (!area) {
 			kcov_put(kcov);
 			return;
@@ -1079,11 +1078,9 @@ void kcov_remote_stop(void)
 		kcov_move_area(kcov->mode, kcov->area, kcov->size, area);
 	spin_unlock(&kcov->lock);
 
-	if (in_task()) {
-		spin_lock(&kcov_remote_lock);
-		kcov_remote_area_put(area, size);
-		spin_unlock(&kcov_remote_lock);
-	}
+	spin_lock(&kcov_remote_lock);
+	kcov_remote_area_put(area, size, !in_task());
+	spin_unlock(&kcov_remote_lock);
 
 	local_unlock_irqrestore(&kcov_percpu_data.lock, flags);
 
@@ -1129,14 +1126,21 @@ static void __init selftest(void)
 
 static int __init kcov_init(void)
 {
-	int cpu;
+	int cpu = num_possible_cpus();
+
+#ifdef CONFIG_PREEMPT_RT
+	/* Allocate some extra buffers in order to prepare for softirq preemption. */
+	cpu = cpu >= 4 ? cpu * 2 : cpu + 4;
+#endif
+	while (cpu--) {
+		void *area = vmalloc(CONFIG_KCOV_IRQ_AREA_SIZE * sizeof(unsigned long));
+		unsigned long flags;
 
-	for_each_possible_cpu(cpu) {
-		void *area = vmalloc_node(CONFIG_KCOV_IRQ_AREA_SIZE *
-				sizeof(unsigned long), cpu_to_node(cpu));
 		if (!area)
 			return -ENOMEM;
-		per_cpu_ptr(&kcov_percpu_data, cpu)->irq_area = area;
+		spin_lock_irqsave(&kcov_remote_lock, flags);
+		kcov_remote_area_put(area, CONFIG_KCOV_IRQ_AREA_SIZE, true);
+		spin_unlock_irqrestore(&kcov_remote_lock, flags);
 	}
 
 	/*
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 1244dcac2294ad..12786379bf1d72 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2247,10 +2247,11 @@ config KCOV_INSTRUMENT_ALL
 config KCOV_IRQ_AREA_SIZE
 	hex "Size of interrupt coverage collection area in words"
 	depends on KCOV
+	range 0x80 0x1000000
 	default 0x40000
 	help
-	  KCOV uses preallocated per-cpu areas to collect coverage from
-	  soft interrupts. This specifies the size of those areas in the
+	  KCOV uses preallocated areas to collect coverage from soft
+	  interrupts. This specifies the size of those areas in the
 	  number of unsigned long words.
 
 config KCOV_SELFTEST

gemini-3.1-pro-preview

null

null

							input: 4682
							output: 25
							thoughts: 610
						

**KCOV Softirq Coverage Patch Analysis**

Okay, so I've got this patch in front of me that's making changes to the Kernel Coverage (KCOV) mechanism, specifically around softirq coverage. My initial thought is that this is interesting because KCOV is a core component that syzkaller, our primary fuzzing tool, relies on heavily. It's used for coverage collection, so any modifications to it are immediately relevant.

The patch's key change is how softirq coverage is gathered. Instead of using per-CPU areas for IRQ coverage, it introduces a global list, `kcov_remote_areas[2]`, splitting it between task and irq. It also relocates the KCOV state data, previously stored in `kcov_percpu_data`, into `task_struct`, presumably to support softirq preemption, something that's particularly relevant with PREEMPT_RT. The commit message mentions "syz-cluster", so this is likely something that has been reviewed already.

My first impulse is to check if this is functional and reachable, which I quickly confirm; KCOV is definitely reachable and crucial to the syzkaller workflow. A slight hesitation comes up as I consider that we're essentially fuzzing the fuzzer's own coverage mechanism. But it's a core kernel feature, and modifying the coverage collection warrants careful attention. We need to assess if this modification is affecting the accuracy or collection methodology of the KCOV data.

Thankfully, syzkaller does have descriptions for KCOV ioctls, so we *can* fuzz it directly.

Now, let's dive into the actual code changes. I've noted that the patch touches functions like `kcov_task_init`, `kcov_remote_softirq_start`, `kcov_remote_softirq_stop`, `kcov_remote_start`, `kcov_remote_stop`, and `kcov_init`. It seems like the core shift is moving `saved_mode`, `saved_size`, `saved_area`, `saved_kcov`, and `saved_sequence` from `kcov_percpu_data` to `task_struct`. Also, changes to `kcov_remote_area_get` and `kcov_remote_area_put` where a `bool irq` argument is introduced, and `kcov_init` is changed to preallocate buffers.

Crucially, I need to check the configurations. `CONFIG_KCOV` is obviously a must-have. Intriguingly, `CONFIG_PREEMPT_RT` is mentioned in `kcov_init`: It seems like the softirq preemption code will allocate more buffers if `PREEMPT_RT` is enabled. That's a strong indicator that enabling `PREEMPT_RT` could be a good idea to thoroughly test the specific code paths impacted by this patch.

However, `PREEMPT_RT` is a big configuration change, affecting the entire kernel preemption model. Syzkaller generally avoids enabling it unless explicitly specified. So, next step is to ensure that `PREEMPT_RT` is actually available.

{
  "Expression": "config PREEMPT_RT",
  "PathPrefix": ""
}

{
  "Output": "kernel/Kconfig.preempt=90=endchoice\nkernel/Kconfig.preempt-91-\nkernel/Kconfig.preempt:92:config PREEMPT_RT\nkernel/Kconfig.preempt-93-\tbool \"Fully Preemptible Kernel (Real-Time)\"\n--\nkernel/Kconfig.preempt-108-\nkernel/Kconfig.preempt:109:config PREEMPT_RT_NEEDS_BH_LOCK\nkernel/Kconfig.preempt-110-\tbool \"Enforce softirq synchronisation on PREEMPT_RT\"\n"
}

gemini-3.1-pro-preview

null

null

							input: 6143
							output: 30
							thoughts: 274
						

**Analyzing KCOV Remote SoftIRQ Functions with PREEMPT_RT**

Okay, so `PREEMPT_RT` is indeed an option here, that much is clear. The key is in the specifics of these `kcov_remote_softirq_start` and `kcov_remote_softirq_stop` functions. They're explicitly called from the softirq context, which raises a flag. If we want to test this, enabling `PREEMPT_RT` is non-negotiable. The patch text itself drives that point home – it mentions the need for "temporary storage for preempting remote coverage collection" and "extra buffers... for softirq preemption," so `PREEMPT_RT` it is.

Now, let's take a look at the relevant symbols and get a broader picture. We have `kcov_remote_softirq_start`, `kcov_remote_softirq_stop`, along with the familiar `kcov_remote_start`, `kcov_remote_stop`, `kcov_remote_area_get`, and `kcov_remote_area_put`.

The more pressing question now is: are `kcov_remote_softirq_start` and `kcov_remote_softirq_stop` in a hot path? Softirq processing *is* a hot path, generally speaking. However, these KCOV functions are only called if KCOV is both enabled *and* active. The next logical step is to pinpoint exactly where these functions are being invoked within the system. Let's trace their call sites and see what's what.

{
  "Expression": "kcov_remote_softirq_start",
  "PathPrefix": ""
}

{
  "Output": "kernel/kcov.c=833=static inline bool kcov_mode_enabled(unsigned int mode)\n--\nkernel/kcov.c-837-\nkernel/kcov.c:838:static void kcov_remote_softirq_start(struct task_struct *t)\nkernel/kcov.c-839-\t__must_hold(\u0026kcov_percpu_data.lock)\n--\nkernel/kcov.c=870=void kcov_remote_start(u64 handle)\n--\nkernel/kcov.c-950-\tif (in_serving_softirq()) {\nkernel/kcov.c:951:\t\tkcov_remote_softirq_start(t);\nkernel/kcov.c-952-\t\tt-\u003ekcov_softirq = 1;\n"
}

gemini-3.1-pro-preview

null

null

							input: 7011
							output: 26
							thoughts: 104
						

**Analyzing KCOV Remote Start Behavior**

Okay, so I'm focusing on this `kcov_remote_softirq_start` function. I see it's called by `kcov_remote_start`. And that makes sense, as I remember `kcov_remote_start` is triggered when a remote KCOV section gets going – think vhost, mac80211_hwsim, that kind of thing. But wait… it's not a direct trigger on *every* softirq, it's tied specifically to calls to `kcov_remote_start`. To clarify this, I'm going to quickly check who's calling `kcov_remote_start` to get a clearer picture of when `kcov_remote_softirq_start` is actually engaged. That should shed some light on the softirq interaction I'm seeing.

{
  "Expression": "kcov_remote_start",
  "PathPrefix": ""
}

{
  "Output": "Documentation/dev-tools/kcov.rst=227=Using KCOV to collect remote coverage requires:\n--\nDocumentation/dev-tools/kcov.rst-229-1. Modifying kernel code to annotate the code section from where coverage\nDocumentation/dev-tools/kcov.rst:230:   should be collected with ``kcov_remote_start`` and ``kcov_remote_stop``.\nDocumentation/dev-tools/kcov.rst-231-\n--\nDocumentation/dev-tools/kcov.rst-234-\nDocumentation/dev-tools/kcov.rst:235:Both ``kcov_remote_start`` and ``kcov_remote_stop`` annotations and the\nDocumentation/dev-tools/kcov.rst-236-``KCOV_REMOTE_ENABLE`` ioctl accept handles that identify particular coverage\n--\nDocumentation/dev-tools/kcov.rst=255=For #1 and #3, a unique global handle must be chosen and passed to the\nDocumentation/dev-tools/kcov.rst:256:corresponding ``kcov_remote_start`` call. Then a userspace process must pass\nDocumentation/dev-tools/kcov.rst-257-this handle to ``KCOV_REMOTE_ENABLE`` in the ``handles`` array field of the\n--\nDocumentation/dev-tools/kcov.rst=266=modifications. Those tasks should in turn use the passed handle in their\nDocumentation/dev-tools/kcov.rst:267:``kcov_remote_start`` and ``kcov_remote_stop`` annotations.\nDocumentation/dev-tools/kcov.rst-268-In the kernel, common handles are wrapped in a ``kcov_common_handle_id``, which\n--\nDocumentation/translations/zh_CN/dev-tools/kcov.rst=28=KCOV 依赖编译器插桩，要求 GCC 6.1.0 及更高版本或者内核支持的任意版本的 Clang。\n--\nDocumentation/translations/zh_CN/dev-tools/kcov.rst-225-\nDocumentation/translations/zh_CN/dev-tools/kcov.rst:226:1. 修改内核源码并使用 ``kcov_remote_start`` 和 ``kcov_remote_stop`` 来标注要收集\nDocumentation/translations/zh_CN/dev-tools/kcov.rst-227-   覆盖率的代码片段。\n--\nDocumentation/translations/zh_CN/dev-tools/kcov.rst-230-\nDocumentation/translations/zh_CN/dev-tools/kcov.rst:231:``kcov_remote_start`` 和 ``kcov_remote_stop`` 的标注以及 ``KCOV_REMOTE_ENABLE``\nDocumentation/translations/zh_CN/dev-tools/kcov.rst-232-ioctl 都接受可以识别特定覆盖率收集片段的句柄。句柄的使用方式取决于匹配代码片段执\n--\nDocumentation/translations/zh_CN/dev-tools/kcov.rst=235=KCOV 支持在如下上下文中收集远程覆盖率：\n--\nDocumentation/translations/zh_CN/dev-tools/kcov.rst-245-对于 #1 和 #3，必须选择一个独特的全局句柄并将其传递给对应的\nDocumentation/translations/zh_CN/dev-tools/kcov.rst:246:``kcov_remote_start`` 调用。一个用户空间进程必须将该句柄存储在\nDocumentation/translations/zh_CN/dev-tools/kcov.rst-247-``kcov_remote_arg`` 结构体的 ``handle`` 数组字段中并将其传递给\n--\nDocumentation/translations/zh_CN/dev-tools/kcov.rst-253-``kcov_handle`` 字段中并且需要通过自定义内核代码的修改来传递给新创建的本地任务\nDocumentation/translations/zh_CN/dev-tools/kcov.rst:254:。这些任务需要在 ``kcov_remote_start`` 和 ``kcov_remote_stop`` 标注中依次使用传递过来的\nDocumentation/translations/zh_CN/dev-tools/kcov.rst-255-句柄。\n--\ndrivers/usb/core/hcd.c=1630=static void __usb_hcd_giveback_urb(struct urb *urb)\n--\ndrivers/usb/core/hcd.c-1655-\t */\ndrivers/usb/core/hcd.c:1656:\tkcov_remote_start_usb_softirq((u64)urb-\u003edev-\u003ebus-\u003ebusnum);\ndrivers/usb/core/hcd.c-1657-\turb-\u003ecomplete(urb);\n--\ndrivers/usb/core/hub.c=5874=static void hub_event(struct work_struct *work)\n--\ndrivers/usb/core/hub.c-5888-\ndrivers/usb/core/hub.c:5889:\tkcov_remote_start_usb((u64)hdev-\u003ebus-\u003ebusnum);\ndrivers/usb/core/hub.c-5890-\n--\ndrivers/usb/usbip/vhci_rx.c=256=int vhci_rx_loop(void *data)\n--\ndrivers/usb/usbip/vhci_rx.c-263-\ndrivers/usb/usbip/vhci_rx.c:264:\t\tkcov_remote_start_common(ud-\u003ekcov_handle);\ndrivers/usb/usbip/vhci_rx.c-265-\t\tvhci_rx_pdu(ud);\n--\ndrivers/vhost/vhost.c=400=static int vhost_run_work_kthread_list(void *data)\n--\ndrivers/vhost/vhost.c-426-\t\t\t__set_current_state(TASK_RUNNING);\ndrivers/vhost/vhost.c:427:\t\t\tkcov_remote_start_common(worker-\u003ekcov_handle);\ndrivers/vhost/vhost.c-428-\t\t\twork-\u003efn(work);\n--\ndrivers/vhost/vhost.c=438=static bool vhost_run_work_list(void *data)\n--\ndrivers/vhost/vhost.c-452-\t\t\tclear_bit(VHOST_WORK_QUEUED, \u0026work-\u003eflags);\ndrivers/vhost/vhost.c:453:\t\t\tkcov_remote_start_common(worker-\u003ekcov_handle);\ndrivers/vhost/vhost.c-454-\t\t\twork-\u003efn(work);\n--\ninclude/linux/kcov.h=37=do {\t\t\t\t\t\t\\\n--\ninclude/linux/kcov.h-41-/* See Documentation/dev-tools/kcov.rst for usage details. */\ninclude/linux/kcov.h:42:void kcov_remote_start(u64 handle);\ninclude/linux/kcov.h-43-void kcov_remote_stop(void);\ninclude/linux/kcov.h=44=struct kcov_common_handle_id kcov_common_handle(void);\ninclude/linux/kcov.h-45-\ninclude/linux/kcov.h:46:static inline void kcov_remote_start_common(struct kcov_common_handle_id id)\ninclude/linux/kcov.h-47-{\ninclude/linux/kcov.h:48:\tkcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_COMMON, id.val));\ninclude/linux/kcov.h-49-}\ninclude/linux/kcov.h-50-\ninclude/linux/kcov.h:51:static inline void kcov_remote_start_usb(u64 id)\ninclude/linux/kcov.h-52-{\ninclude/linux/kcov.h:53:\tkcov_remote_start(kcov_remote_handle(KCOV_SUBSYSTEM_USB, id));\ninclude/linux/kcov.h-54-}\n--\ninclude/linux/kcov.h-62-\ninclude/linux/kcov.h:63:static inline void kcov_remote_start_usb_softirq(u64 id)\ninclude/linux/kcov.h-64-{\ninclude/linux/kcov.h-65-\tif (in_serving_softirq() \u0026\u0026 !in_hardirq())\ninclude/linux/kcov.h:66:\t\tkcov_remote_start_usb(id);\ninclude/linux/kcov.h-67-}\n--\ninclude/linux/kcov.h=97=static inline void kcov_finish_switch(struct task_struct *t) {}\ninclude/linux/kcov.h:98:static inline void kcov_remote_start(u64 handle) {}\ninclude/linux/kcov.h-99-static inline void kcov_remote_stop(void) {}\ninclude/linux/kcov.h=100=static inline struct kcov_common_handle_id kcov_common_handle(void)\n--\ninclude/linux/kcov.h-103-}\ninclude/linux/kcov.h:104:static inline void kcov_remote_start_common(struct kcov_common_handle_id id) {}\ninclude/linux/kcov.h:105:static inline void kcov_remote_start_usb(u64 id) {}\ninclude/linux/kcov.h:106:static inline void kcov_remote_start_usb_softirq(u64 id) {}\ninclude/linux/kcov.h-107-static inline void kcov_remote_stop_softirq(void) {}\n--\ninclude/uapi/linux/kcov.h-8- * Argument for KCOV_REMOTE_ENABLE ioctl, see Documentation/dev-tools/kcov.rst\ninclude/uapi/linux/kcov.h:9: * and the comment before kcov_remote_start() for usage details.\ninclude/uapi/linux/kcov.h-10- */\n--\nkernel/kcov.c=407=static void kcov_remote_reset(struct kcov *kcov)\n--\nkernel/kcov.c-421-\t}\nkernel/kcov.c:422:\t/* Do reset before unlock to prevent races with kcov_remote_start(). */\nkernel/kcov.c-423-\tkcov_reset(kcov);\n--\nkernel/kcov.c=457=void kcov_task_exit(struct task_struct *t)\n--\nkernel/kcov.c-466-\t\t/*\nkernel/kcov.c:467:\t\t * This could be a remote task between kcov_remote_start() and\nkernel/kcov.c-468-\t\t * kcov_remote_stop().\n--\nkernel/kcov.c=782=static const struct file_operations kcov_fops = {\n--\nkernel/kcov.c-790-/*\nkernel/kcov.c:791: * kcov_remote_start() and kcov_remote_stop() can be used to annotate a section\nkernel/kcov.c-792- * of code in a kernel background thread or in a softirq to allow kcov to be\n--\nkernel/kcov.c-794- *\nkernel/kcov.c:795: * The handle argument of kcov_remote_start() identifies a code section that is\nkernel/kcov.c-796- * used for coverage collection. A userspace process passes this handle to\n--\nkernel/kcov.c-818- * and passed to the spawned threads via custom annotations. Those kernel\nkernel/kcov.c:819: * threads must in turn be annotated with kcov_remote_start(common_handle) and\nkernel/kcov.c-820- * kcov_remote_stop(). All of the threads that are spawned by the same process\n--\nkernel/kcov.c-824- *\nkernel/kcov.c:825: * Internally, kcov_remote_start() looks up the kcov device associated with the\nkernel/kcov.c-826- * provided handle, allocates an area for coverage collection, and saves the\n--\nkernel/kcov.c=855=static void kcov_remote_softirq_stop(struct task_struct *t)\n--\nkernel/kcov.c-869-\nkernel/kcov.c:870:void kcov_remote_start(u64 handle)\nkernel/kcov.c-871-{\n--\nkernel/kcov.c-888-\t/*\nkernel/kcov.c:889:\t * Check that kcov_remote_start() is not called twice in background\nkernel/kcov.c-890-\t * threads nor called by user tasks (with enabled kcov).\n--\nkernel/kcov.c-897-\t/*\nkernel/kcov.c:898:\t * Check that kcov_remote_start() is not called twice in softirqs.\nkernel/kcov.c:899:\t * Note, that kcov_remote_start() can be called from a softirq that\nkernel/kcov.c-900-\t * happened while collecting coverage from a background thread.\n--\nkernel/kcov.c-958-}\nkernel/kcov.c:959:EXPORT_SYMBOL(kcov_remote_start);\nkernel/kcov.c-960-\nkernel/kcov.c=961=static void kcov_move_area(enum kcov_mode mode, void *dst_area,\n--\nkernel/kcov.c-1024-\nkernel/kcov.c:1025:/* See the comment before kcov_remote_start() for usage details. */\nkernel/kcov.c-1026-void kcov_remote_stop(void)\n--\nkernel/kcov.c-1047-\t/*\nkernel/kcov.c:1048:\t * When in softirq, check if the corresponding kcov_remote_start()\nkernel/kcov.c-1049-\t * actually found the remote handle and started collecting coverage.\n--\nkernel/kcov.c-1073-\t/*\nkernel/kcov.c:1074:\t * KCOV_DISABLE could have been called between kcov_remote_start()\nkernel/kcov.c-1075-\t * and kcov_remote_stop(), hence the sequence check.\n--\nkernel/kcov.c-1086-\nkernel/kcov.c:1087:\t/* Get in kcov_remote_start(). */\nkernel/kcov.c-1088-\tkcov_put(kcov);\n--\nkernel/kcov.c=1090=EXPORT_SYMBOL(kcov_remote_stop);\nkernel/kcov.c-1091-\nkernel/kcov.c:1092:/* See the comment before kcov_remote_start() for usage details. */\nkernel/kcov.c-1093-struct kcov_common_handle_id kcov_common_handle(void)\n--\nnet/bluetooth/hci_core.c=3989=static void hci_rx_work(struct work_struct *work)\n--\nnet/bluetooth/hci_core.c-4001-\tfor (; (skb = skb_dequeue(\u0026hdev-\u003erx_q)); kcov_remote_stop()) {\nnet/bluetooth/hci_core.c:4002:\t\tkcov_remote_start_common(skb_get_kcov_handle(skb));\nnet/bluetooth/hci_core.c-4003-\n--\nnet/mac80211/iface.c=1800=static void ieee80211_iface_work(struct wiphy *wiphy, struct wiphy_work *work)\n--\nnet/mac80211/iface.c-1817-\twhile ((skb = skb_dequeue(\u0026sdata-\u003eskb_queue))) {\nnet/mac80211/iface.c:1818:\t\tkcov_remote_start_common(skb_get_kcov_handle(skb));\nnet/mac80211/iface.c-1819-\n--\nnet/mac80211/iface.c-1830-\twhile ((skb = skb_dequeue(\u0026sdata-\u003estatus_queue))) {\nnet/mac80211/iface.c:1831:\t\tkcov_remote_start_common(skb_get_kcov_handle(skb));\nnet/mac80211/iface.c-1832-\n--\nnet/mac80211/rx.c=5557=void ieee80211_rx_list(struct ieee80211_hw *hw, struct ieee80211_sta *pubsta,\n--\nnet/mac80211/rx.c-5699-\nnet/mac80211/rx.c:5700:\tkcov_remote_start_common(skb_get_kcov_handle(skb));\nnet/mac80211/rx.c-5701-\n--\nnet/nfc/nci/core.c=1520=static void nci_tx_work(struct work_struct *work)\n--\nnet/nfc/nci/core.c-1536-\t\t\treturn;\nnet/nfc/nci/core.c:1537:\t\tkcov_remote_start_common(skb_get_kcov_handle(skb));\nnet/nfc/nci/core.c-1538-\n--\nnet/nfc/nci/core.c=1559=static void nci_rx_work(struct work_struct *work)\n--\nnet/nfc/nci/core.c-1564-\tfor (; (skb = skb_dequeue(\u0026ndev-\u003erx_q)); kcov_remote_stop()) {\nnet/nfc/nci/core.c:1565:\t\tkcov_remote_start_common(skb_get_kcov_handle(skb));\nnet/nfc/nci/core.c-1566-\n--\nnet/nfc/nci/core.c=1611=static void nci_cmd_work(struct work_struct *work)\n--\nnet/nfc/nci/core.c-1623-\nnet/nfc/nci/core.c:1624:\t\tkcov_remote_start_common(skb_get_kcov_handle(skb));\nnet/nfc/nci/core.c-1625-\t\tatomic_dec(\u0026ndev-\u003ecmd_cnt);\n--\nnet/nfc/nci/hci.c=405=static void nci_hci_msg_rx_work(struct work_struct *work)\n--\nnet/nfc/nci/hci.c-413-\tfor (; (skb = skb_dequeue(\u0026hdev-\u003emsg_rx_queue)); kcov_remote_stop()) {\nnet/nfc/nci/hci.c:414:\t\tkcov_remote_start_common(skb_get_kcov_handle(skb));\nnet/nfc/nci/hci.c-415-\t\tpipe = NCI_HCP_MSG_GET_PIPE(skb-\u003edata[0]);\n--\nnet/nfc/rawsock.c=188=static void rawsock_tx_work(struct work_struct *work)\n--\nnet/nfc/rawsock.c-203-\tskb = skb_dequeue(\u0026sk-\u003esk_write_queue);\nnet/nfc/rawsock.c:204:\tkcov_remote_start_common(skb_get_kcov_handle(skb));\nnet/nfc/rawsock.c-205-\n"
}

gemini-3.1-pro-preview

null

null

							input: 12238
							output: 30
							thoughts: 87
						

Here's my thought process, as if I were analyzing this code:

**Analyzing the SoftIRQ KCOV Hook**

Okay, so I'm digging into how KCOV is integrated with softirqs. I see that `kcov_remote_softirq_start` is involved, and my first instinct is to trace back its call path. Now, the context says it's called from `__do_softirq`. Hang on a second... is that direct? Or does `kcov_softirq_start` sit in between? Let me just quickly verify. I need to be precise here; misreading the call flow would send me down the wrong path entirely. Time to double-check those function calls. Let's see exactly where `kcov_remote_softirq_start` is called. Gotta confirm that connection... and then I can fully understand how KCOV interacts with the SoftIRQ processing.

{
  "Expression": "kcov_remote_softirq_start",
  "PathPrefix": ""
}

{
  "Output": "kernel/kcov.c=833=static inline bool kcov_mode_enabled(unsigned int mode)\n--\nkernel/kcov.c-837-\nkernel/kcov.c:838:static void kcov_remote_softirq_start(struct task_struct *t)\nkernel/kcov.c-839-\t__must_hold(\u0026kcov_percpu_data.lock)\n--\nkernel/kcov.c=870=void kcov_remote_start(u64 handle)\n--\nkernel/kcov.c-950-\tif (in_serving_softirq()) {\nkernel/kcov.c:951:\t\tkcov_remote_softirq_start(t);\nkernel/kcov.c-952-\t\tt-\u003ekcov_softirq = 1;\n"
}

gemini-3.1-pro-preview

null

null

							input: 12770
							output: 28
							thoughts: 83
						

Okay, let me break this down.

**Diving into the KCOV SoftIRQ Logic**

Right, so I'm trying to trace the execution path and I see `kcov_remote_softirq_start` being called. My initial thought was, "Aha! It's from `kcov_remote_start` *conditionally* when `in_serving_softirq()` is true." But, that doesn't quite seem right. I need to be precise here.

Let me just double-check that. Wait a second, the more likely scenario is that `kcov_remote_softirq_start` is actually called from `kcov_softirq_start`. Okay, let's confirm this by taking a closer look at the `kcov_softirq_start` function itself. I need to examine its implementation to be sure. I'll need to check the call graph there. No assumptions, need the hard data.

no matches

{
  "Expression": "kcov_softirq_start",
  "PathPrefix": ""
}

{
  "error": "no matches"
}

gemini-3.1-pro-preview

null

null

							input: 13104
							output: 34
							thoughts: 158
						

**Investigating the KCOV SoftIRQ Interaction**

Okay, so I'm looking at this `kcov_remote_softirq_start` function, and something's not quite right. The output seemed to indicate that it's called from within `kcov_remote_start`, but the initial glance I took suggested otherwise. Let me double-check this. The output says: if the system is serving a softIRQ, then `kcov_remote_softirq_start` is called. It looks like it. Okay, so now I'm thinking, is `kcov_remote_start` actually *calling* `kcov_remote_softirq_start` directly? I need to verify this relationship. Let's trace back to `kcov_remote_start` definition in `kernel/kcov.c` to see how this all connects.

{
  "File": "kernel/kcov.c",
  "FirstLine": 940,
  "LineCount": 30
}

{
  "Contents": " 940:\t\t\tif (!area) {\n 941:\t\t\t\tkcov_put(kcov);\n 942:\t\t\t\treturn;\n 943:\t\t\t}\n 944:\t\t\tlocal_lock_irqsave(\u0026kcov_percpu_data.lock, flags);\n 945:\t\t}\n 946:\t\n 947:\t\t/* Reset coverage size. */\n 948:\t\t*(u64 *)area = 0;\n 949:\t\n 950:\t\tif (in_serving_softirq()) {\n 951:\t\t\tkcov_remote_softirq_start(t);\n 952:\t\t\tt-\u003ekcov_softirq = 1;\n 953:\t\t}\n 954:\t\tkcov_start(t, kcov, size, area, mode, sequence);\n 955:\t\n 956:\t\tlocal_unlock_irqrestore(\u0026kcov_percpu_data.lock, flags);\n 957:\t\n 958:\t}\n 959:\tEXPORT_SYMBOL(kcov_remote_start);\n 960:\t\n 961:\tstatic void kcov_move_area(enum kcov_mode mode, void *dst_area,\n 962:\t\t\t\t\tunsigned int dst_area_size, void *src_area)\n 963:\t{\n 964:\t\tu64 word_size = sizeof(unsigned long);\n 965:\t\tu64 count_size, entry_size_log;\n 966:\t\tu64 dst_len, src_len;\n 967:\t\tvoid *dst_entries, *src_entries;\n 968:\t\tu64 dst_occupied, dst_free, bytes_to_move, entries_moved;\n 969:\t\n"
}

gemini-3.1-pro-preview

null

null

							input: 13952
							output: 35
							thoughts: 150
						

**Understanding the Remote KCOV Start/Stop Mechanism**

Okay, so I'm following the code flow here. From what I can tell, `kcov_remote_start` gets called, and it looks like it's happening from a softirq context, like the `kcov_remote_start_usb_softirq` call suggests. That makes sense; it's likely dealing with interrupts coming from external devices.

Now, the important part: it checks `in_serving_softirq()`.  That’s the key context check. If we *are* in a softirq, it calls `kcov_remote_softirq_start(t)`.  This is clever. It saves the existing KCOV state, if one exists, and *stops* it. We need to preserve the state of any coverage tracing happening at the time. Then, and only then, does it kick off `kcov_start` for the remote coverage. This means we're switching from local KCOV to remote KCOV. The system is probably switching from local instrumentation of the kernel to using a remote instance for profiling.

And finally, when `kcov_remote_stop` is called, it restores the saved KCOV state.  This seems clean – it returns us to the state we were in before the remote profiling started, ensuring no data is lost or corrupted. This is important to ensure nothing happens with our existing tracing, if any. I'll need to dig into the `kcov_remote_stop` function now to understand the exact restoration mechanism.

{
  "File": "kernel/kcov.c",
  "FirstLine": 1080,
  "LineCount": 30
}

{
  "Contents": "1080:\t\n1081:\t\tspin_lock(\u0026kcov_remote_lock);\n1082:\t\tkcov_remote_area_put(area, size, !in_task());\n1083:\t\tspin_unlock(\u0026kcov_remote_lock);\n1084:\t\n1085:\t\tlocal_unlock_irqrestore(\u0026kcov_percpu_data.lock, flags);\n1086:\t\n1087:\t\t/* Get in kcov_remote_start(). */\n1088:\t\tkcov_put(kcov);\n1089:\t}\n1090:\tEXPORT_SYMBOL(kcov_remote_stop);\n1091:\t\n1092:\t/* See the comment before kcov_remote_start() for usage details. */\n1093:\tstruct kcov_common_handle_id kcov_common_handle(void)\n1094:\t{\n1095:\t\tif (!in_task())\n1096:\t\t\treturn (struct kcov_common_handle_id){ .val = 0 };\n1097:\t\treturn (struct kcov_common_handle_id){ .val = current-\u003ekcov_handle };\n1098:\t}\n1099:\tEXPORT_SYMBOL(kcov_common_handle);\n1100:\t\n1101:\t#ifdef CONFIG_KCOV_SELFTEST\n1102:\tstatic void __init selftest(void)\n1103:\t{\n1104:\t\tunsigned long start;\n1105:\t\n1106:\t\tpr_err(\"running self test\\n\");\n1107:\t\t/*\n1108:\t\t * Test that interrupts don't produce spurious coverage.\n1109:\t\t * The coverage callback filters out interrupt code, but only\n"
}

gemini-3.1-pro-preview

null

null

							input: 14940
							output: 30
							thoughts: 22
						

Okay, here's my thought process laid out, assuming I'm already deep into this code:

**Locating the Callers of `kcov_remote_softirq_stop`**

Right, before I dive too deep into the function itself, I need to understand its context. Where is this `kcov_remote_softirq_stop` function being *used*? I need to see the call sites. Understanding how it's invoked will immediately give me clues about its purpose and how it fits into the broader picture of whatever this kcov-related subsystem is doing. I'll need to use my tools to trace where that function is being called, which will show the different paths and conditions involved in executing it. That's the best first step.

{
  "Expression": "kcov_remote_softirq_stop",
  "PathPrefix": ""
}

{
  "Output": "kernel/kcov.c=838=static void kcov_remote_softirq_start(struct task_struct *t)\n--\nkernel/kcov.c-854-\nkernel/kcov.c:855:static void kcov_remote_softirq_stop(struct task_struct *t)\nkernel/kcov.c-856-\t__must_hold(\u0026kcov_percpu_data.lock)\n--\nkernel/kcov.c=1026=void kcov_remote_stop(void)\n--\nkernel/kcov.c-1068-\t\tt-\u003ekcov_softirq = 0;\nkernel/kcov.c:1069:\t\tkcov_remote_softirq_stop(t);\nkernel/kcov.c-1070-\t}\n"
}

gemini-3.1-pro-preview

null

null

							input: 15368
							output: 35
							thoughts: 434
						

**My Assessment of the KCOV Softirq Coverage Patch**

Okay, so this patch is all about how KCOV handles softirq coverage, specifically addressing the preemption issues that arise with `PREEMPT_RT`. Let's break it down in my mind:

Firstly, I see that `kcov_remote_stop` is now directly calling `kcov_remote_softirq_stop`, so that makes sense. The core of the change lies in how the coverage areas are managed.

The key adjustments are clear: `kcov_remote_areas` has been restructured into an array of two lists; one for tasks and another for interrupts, which is a significant architectural shift. Secondly, functions like `kcov_remote_area_get` and `kcov_remote_area_put` now take a boolean to indicate whether we're dealing with an IRQ or a task, essential for differentiating the two.

Then there's the initialization: `kcov_init` now allocates buffers specifically for interrupts, sized according to `CONFIG_KCOV_IRQ_AREA_SIZE`, and places them in the interrupt list. This is key, particularly in an `PREEMPT_RT` environment, where you want to be able to pre-empt softirqs while collecting coverage.

When KCOV starts, it gets an area from the interrupt list if it's *not* running in a task, and conversely, it puts the area back in the interrupt list when it stops, again, if not in a task. This dynamic allocation is critical for managing coverage in a preemptible softirq context. Finally, I notice the saved KCOV state has been moved from `kcov_percpu_data` to `task_struct`, another important structural change.

Functionally, this allows softirq coverage to be preempted, which is precisely what's needed for `PREEMPT_RT` – the main motivation, it seems. My expert intuition tells me this is a valuable change, particularly for real-time systems.

Is it worth fuzzing? Absolutely, without question. This is a core kernel feature that, if broken, would have significant impacts.

As for configurations, I know that for my testing, I'll need `CONFIG_KCOV=y` and definitely `CONFIG_PREEMPT_RT=y` to test the actual preemption aspect of this patch, which is the whole point. I know it's there. I did my checks. `CONFIG_KCOV_ENABLE_COMPARISONS=y` might be relevant, but is probably not strictly necessary for this patch. I usually work with `CONFIG_KCOV_INSTRUMENT_ALL=y`, and `CONFIG_DEBUG_FS=y` is necessary, since KCOV uses debugfs.

Let me just quickly check `kcov_init` to see if I am missing something.

{
  "File": "kernel/kcov.c",
  "FirstLine": 1120,
  "LineCount": 30
}

{
  "Contents": "1120:\t\twhile ((jiffies - start) * MSEC_PER_SEC / HZ \u003c 300)\n1121:\t\t\t;\n1122:\t\tWRITE_ONCE(current-\u003ekcov_mode, 0);\n1123:\t\tpr_err(\"done running self test\\n\");\n1124:\t}\n1125:\t#endif\n1126:\t\n1127:\tstatic int __init kcov_init(void)\n1128:\t{\n1129:\t\tint cpu = num_possible_cpus();\n1130:\t\n1131:\t#ifdef CONFIG_PREEMPT_RT\n1132:\t\t/* Allocate some extra buffers in order to prepare for softirq preemption. */\n1133:\t\tcpu = cpu \u003e= 4 ? cpu * 2 : cpu + 4;\n1134:\t#endif\n1135:\t\twhile (cpu--) {\n1136:\t\t\tvoid *area = vmalloc(CONFIG_KCOV_IRQ_AREA_SIZE * sizeof(unsigned long));\n1137:\t\t\tunsigned long flags;\n1138:\t\n1139:\t\t\tif (!area)\n1140:\t\t\t\treturn -ENOMEM;\n1141:\t\t\tspin_lock_irqsave(\u0026kcov_remote_lock, flags);\n1142:\t\t\tkcov_remote_area_put(area, CONFIG_KCOV_IRQ_AREA_SIZE, true);\n1143:\t\t\tspin_unlock_irqrestore(\u0026kcov_remote_lock, flags);\n1144:\t\t}\n1145:\t\n1146:\t\t/*\n1147:\t\t * The kcov debugfs file won't ever get removed and thus,\n1148:\t\t * there is no need to protect it against removal races. The\n1149:\t\t * use of debugfs_create_file_unsafe() is actually safe here.\n"
}

gemini-3.1-pro-preview

null

null

							input: 16944
							output: 25
							thoughts: 563
						

**Focusing on Remote KCOV Coverage**

Okay, so I'm looking at these KCOV modifications, specifically with `KCOV` and `PREEMPT_RT` enabled. I've got a set of focus symbols in mind: `kcov_remote_softirq_start`, `kcov_remote_softirq_stop`, `kcov_remote_area_get`, `kcov_remote_area_put`, `kcov_remote_start`, and `kcov_remote_stop`. The `kcov_init` function is irrelevant here, it's just the initialization.

The key entry points appear to be `kcov_remote_start` and `kcov_remote_stop`. The critical question is, are these functions reachable from userspace? The answer is yes, through the `KCOV_REMOTE_ENABLE` ioctl. It sets up the handle for remote coverage. Now, this is crucial: the actual `kcov_remote_start` and `kcov_remote_stop` are invoked by kernel threads or softirqs *using* that handle. I'm thinking about things like USB, vhost, anything that's running in the background and might be exercising this code.

Syzkaller, especially, utilizes `KCOV_REMOTE_ENABLE` quite a bit to collect coverage from these background threads – USB, vhost, mac80211_hwsim, that kind of thing. So, those functions are going to be heavily exercised. My initial instinct was to include `kcov_remote_start` as a focus symbol, and then I realized that it is a direct area of change. It's the function that *we* modified and want to be testing.

It's a relatively small function, so should I look for other relevant functions? I'm also remembering that `kcov_task_init` is modified, but that's called on every task creation. That's a hot path, and probably not a good candidate for focus. `kcov_remote_area_get` and `kcov_remote_area_put` are good possibilities.

Reviewing the focus symbols again: `kcov_remote_softirq_start`, `kcov_remote_softirq_stop`, `kcov_remote_area_get`, `kcov_remote_area_put`, `kcov_remote_start`, `kcov_remote_stop`.

And back to `kcov_remote_start` and `kcov_remote_stop`. They are called very frequently if remote coverage is enabled. So, is it a hot path? Yes, it is for remote coverage. And this is *exactly* the feature we want to test. So, I will include them.

So, in summary, I'm focusing on those remote KCOV functions. The configs are `KCOV` and `PREEMPT_RT`. Now I need to check to be sure `PREEMPT_RT` doesn't have any dependencies and is just a bool.

{
  "Expression": "config PREEMPT_RT",
  "PathPrefix": ""
}

{
  "Output": "kernel/Kconfig.preempt=90=endchoice\nkernel/Kconfig.preempt-91-\nkernel/Kconfig.preempt:92:config PREEMPT_RT\nkernel/Kconfig.preempt-93-\tbool \"Fully Preemptible Kernel (Real-Time)\"\n--\nkernel/Kconfig.preempt-108-\nkernel/Kconfig.preempt:109:config PREEMPT_RT_NEEDS_BH_LOCK\nkernel/Kconfig.preempt-110-\tbool \"Enforce softirq synchronisation on PREEMPT_RT\"\n"
}

gemini-3.1-pro-preview

null

null

							input: 18338
							output: 34
							thoughts: 19
						

Here's my attempt at summarizing the thought process, tailored for an expert audience:

**Diving into Preemption Configuration**

Okay, so I need to understand the preemption configuration options within the kernel. Let's start with `kernel/Kconfig.preempt`, specifically around line 92. I'm expecting to find the core definitions and dependencies for the preemption-related settings there. This is where the real meat of the configuration decisions are likely laid out – the `config PREEMPT`, `config PREEMPT_RT` and related options. I'm thinking about the various trade-offs: the impact on real-time performance, scheduling latency, and overall kernel complexity. I'm interested to see how these options are structured, what dependencies exist on other kernel features, and if there are any subtle nuances in how preemption is enabled or disabled at that level. I want to build a clear mental model of the configurations. Time to dig in and see what the code reveals.

{
  "File": "kernel/Kconfig.preempt",
  "FirstLine": 80,
  "LineCount": 30
}

{
  "Contents": "  80:\t\tdepends on !ARCH_NO_PREEMPT\n  81:\t\tdepends on ARCH_HAS_PREEMPT_LAZY\n  82:\t\tselect PREEMPT_BUILD if !PREEMPT_DYNAMIC\n  83:\t\thelp\n  84:\t\t  This option provides a scheduler driven preemption model that\n  85:\t\t  is fundamentally similar to full preemption, but is less\n  86:\t\t  eager to preempt SCHED_NORMAL tasks in an attempt to\n  87:\t\t  reduce lock holder preemption and recover some of the performance\n  88:\t\t  gains seen from using Voluntary preemption.\n  89:\t\n  90:\tendchoice\n  91:\t\n  92:\tconfig PREEMPT_RT\n  93:\t\tbool \"Fully Preemptible Kernel (Real-Time)\"\n  94:\t\tdepends on EXPERT \u0026\u0026 ARCH_SUPPORTS_RT \u0026\u0026 !COMPILE_TEST\n  95:\t\tselect PREEMPTION\n  96:\t\thelp\n  97:\t\t  This option turns the kernel into a real-time kernel by replacing\n  98:\t\t  various locking primitives (spinlocks, rwlocks, etc.) with\n  99:\t\t  preemptible priority-inheritance aware variants, enforcing\n 100:\t\t  interrupt threading and introducing mechanisms to break up long\n 101:\t\t  non-preemptible sections. This makes the kernel, except for very\n 102:\t\t  low level and critical code paths (entry code, scheduler, low\n 103:\t\t  level interrupt handling) fully preemptible and brings most\n 104:\t\t  execution contexts under scheduler control.\n 105:\t\n 106:\t\t  Select this if you are building a kernel for systems which\n 107:\t\t  require real-time guarantees.\n 108:\t\n 109:\tconfig PREEMPT_RT_NEEDS_BH_LOCK\n"
}

gemini-3.1-pro-preview

null

null

							input: 19059
							output: 27
							thoughts: 79
						

**My Reasoning on Real-Time Preemption**

Okay, so I'm looking at how to configure `PREEMPT_RT`. It seems the dependencies are pretty straightforward: I need `EXPERT`, `ARCH_SUPPORTS_RT`, and *not* `COMPILE_TEST`. This means, to get `PREEMPT_RT` running, I should enable both `PREEMPT_RT` and `EXPERT`. 

Now, a quick reminder: I know from experience that `EXPERT` is generally enabled in most of the syzkaller configurations I work with. That's usually not an issue.

But, I need to confirm `ARCH_SUPPORTS_RT` is correctly set. Let me just double check whether that configuration is enabled for the x86 architecture.

{
  "Expression": "config ARCH_SUPPORTS_RT",
  "PathPrefix": ""
}

{
  "Output": "arch/Kconfig=1493=config ARCH_NO_PREEMPT\n--\narch/Kconfig-1495-\narch/Kconfig:1496:config ARCH_SUPPORTS_RT\narch/Kconfig-1497-\tbool\n"
}

gemini-3.1-pro-preview

null

null

							input: 19403
							output: 32
							thoughts: 17
						

Okay, here's my interpretation of that thought process, tailored for an expert audience, and written in the first person:

**Kernel Configuration Query**

Alright, so I'm diving into this x86 configuration, and the first question that pops into my head is: *Does x86 have the `ARCH_SUPPORTS_RT` flag enabled?* That's the critical first check. It immediately tells me a lot about the architectural capabilities and which real-time features might be available, or at least potentially supported, at this stage of the configuration. Without `ARCH_SUPPORTS_RT`, I likely won't even *consider* many of the real-time options down the line. It's a fundamental gating factor, and a good starting point for understanding how this specific x86 setup is configured in relation to real-time workloads. I need to know if the architecture itself even *permits* me to proceed down that path.

{
  "Expression": "select ARCH_SUPPORTS_RT",
  "PathPrefix": "arch/x86"
}

{
  "Output": "arch/x86/Kconfig=58=config X86\n--\narch/x86/Kconfig-132-\tselect ARCH_SUPPORTS_LTO_CLANG_THIN\narch/x86/Kconfig:133:\tselect ARCH_SUPPORTS_RT\narch/x86/Kconfig-134-\tselect ARCH_USE_BUILTIN_BSWAP\n"
}

gemini-3.1-pro-preview

null

null

							input: 19760
							output: 168
							thoughts: 632
						

**My Analysis of the Patch for Fuzzing Potential**

Okay, so I've examined this patch, and the key takeaway is that it introduces functionality tied to `PREEMPT_RT` and its interaction with KCOV, specifically regarding coverage collection in soft interrupt contexts and potential softirq preemption.

First, I confirmed that `PREEMPT_RT` is indeed available and should be included. I see that the patch includes a modification to `lib/Kconfig.debug`, but it's just a range update and help text change related to `KCOV_IRQ_AREA_SIZE`, not the addition of a new config.

Since the focus is now on the preemption aspect under `PREEMPT_RT` and remote KCOV, and given that `KCOV` is already enabled by default in the syzkaller configuration, I'll just need to enable `PREEMPT_RT` to focus on the new preemption functionality. I'm noting `KCOV` to document it as the prerequisite for this work.

I then did a quick check on the key symbols, and the functions I identified: `kcov_remote_softirq_start`, `kcov_remote_softirq_stop`, `kcov_remote_area_get`, `kcov_remote_area_put`, `kcov_remote_start`, and `kcov_remote_stop` are quite specific to this patch. These are all good candidates to target with fuzzing.

Therefore, the patch is definitely worth fuzzing. It's touching core KCOV coverage collection, specifically how coverage is collected in softirq contexts, which are reachable via KCOV ioctls and remote coverage triggers like USB and vhost. The preemption aspect adds a new layer of complexity.

Here's my summary:

*   **WorthFuzzing**: True
*   **Reasoning**: The patch modifies the core KCOV coverage collection mechanism, specifically how remote coverage is collected in softirq contexts and how it handles softirq preemption (which is possible under PREEMPT_RT). This is functional kernel logic that is reachable via KCOV ioctls and remote coverage triggers (like USB or vhost).
*   **FocusSymbols**:
    *   kcov_remote_softirq_start
    *   kcov_remote_softirq_stop
    *   kcov_remote_area_get
    *   kcov_remote_area_put
    *   kcov_remote_start
    *   kcov_remote_stop
*   **EnableConfigs**:
    *   PREEMPT_RT
*Now to call set-results*.

{
  "EnableConfigs": [
    "PREEMPT_RT"
  ],
  "FocusSymbols": [
    "kcov_remote_softirq_start",
    "kcov_remote_softirq_stop",
    "kcov_remote_area_get",
    "kcov_remote_area_put",
    "kcov_remote_start",
    "kcov_remote_stop"
  ],
  "Reasoning": "The patch modifies the core KCOV coverage collection mechanism, specifically changing how remote coverage is collected in softirq contexts and adding support for softirq preemption (which occurs under PREEMPT_RT). This is functional kernel logic reachable via KCOV ioctls and remote coverage triggers (like USB or vhost).",
  "WorthFuzzing": true
}

{
  "EnableConfigs": [
    "PREEMPT_RT"
  ],
  "FocusSymbols": [
    "kcov_remote_softirq_start",
    "kcov_remote_softirq_stop",
    "kcov_remote_area_get",
    "kcov_remote_area_put",
    "kcov_remote_start",
    "kcov_remote_stop"
  ],
  "Reasoning": "The patch modifies the core KCOV coverage collection mechanism, specifically changing how remote coverage is collected in softirq contexts and adding support for softirq preemption (which occurs under PREEMPT_RT). This is functional kernel logic reachable via KCOV ioctls and remote coverage triggers (like USB or vhost).",
  "WorthFuzzing": true
}