kcov: Enable context analysis

Enable context analysis for the KCOV subsystem.

Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251219154418.3592607-30-elver@google.com

diff --git a/kernel/Makefile b/kernel/Makefile
index e83669841b8cc6c17b704eb12ea31e11fc18cad8..6785982013dced54d7731dd9404bb4bf5899b635 100644 (file)
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -43,6 +43,8 @@ KASAN_SANITIZE_kcov.o := n
 KCSAN_SANITIZE_kcov.o := n
 UBSAN_SANITIZE_kcov.o := n
 KMSAN_SANITIZE_kcov.o := n
+
+CONTEXT_ANALYSIS_kcov.o := y
 CFLAGS_kcov.o := $(call cc-option, -fno-conserve-stack) -fno-stack-protector
 
 obj-y += sched/

diff --git a/kernel/kcov.c b/kernel/kcov.c
index 6563141f5de9e799259aaf117fc57d0b864745c4..6cbc6e2d8aeeb0b89be6a90adb3dfffa41763acb 100644 (file)
--- a/kernel/kcov.c
+++ b/kernel/kcov.c
@@ -55,13 +55,13 @@ struct kcov {
        refcount_t              refcount;
        /* The lock protects mode, size, area and t. */
        spinlock_t              lock;
-       enum kcov_mode          mode;
+       enum kcov_mode          mode __guarded_by(&lock);
        /* Size of arena (in long's). */
-       unsigned int            size;
+       unsigned int            size __guarded_by(&lock);
        /* Coverage buffer shared with user space. */
-       void                    *area;
+       void                    *area __guarded_by(&lock);
        /* Task for which we collect coverage, or NULL. */
-       struct task_struct      *t;
+       struct task_struct      *t __guarded_by(&lock);
        /* Collecting coverage from remote (background) threads. */
        bool                    remote;
        /* Size of remote area (in long's). */
@@ -391,6 +391,7 @@ void kcov_task_init(struct task_struct *t)
 }
 
 static void kcov_reset(struct kcov *kcov)
+       __must_hold(&kcov->lock)
 {
        kcov->t = NULL;
        kcov->mode = KCOV_MODE_INIT;
@@ -400,6 +401,7 @@ static void kcov_reset(struct kcov *kcov)
 }
 
 static void kcov_remote_reset(struct kcov *kcov)
+       __must_hold(&kcov->lock)
 {
        int bkt;
        struct kcov_remote *remote;
@@ -419,6 +421,7 @@ static void kcov_remote_reset(struct kcov *kcov)
 }
 
 static void kcov_disable(struct task_struct *t, struct kcov *kcov)
+       __must_hold(&kcov->lock)
 {
        kcov_task_reset(t);
        if (kcov->remote)
@@ -435,8 +438,11 @@ static void kcov_get(struct kcov *kcov)
 static void kcov_put(struct kcov *kcov)
 {
        if (refcount_dec_and_test(&kcov->refcount)) {
-               kcov_remote_reset(kcov);
-               vfree(kcov->area);
+               /* Context-safety: no references left, object being destroyed. */
+               context_unsafe(
+                       kcov_remote_reset(kcov);
+                       vfree(kcov->area);
+               );
                kfree(kcov);
        }
 }
@@ -491,6 +497,7 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
        unsigned long size, off;
        struct page *page;
        unsigned long flags;
+       void *area;
 
        spin_lock_irqsave(&kcov->lock, flags);
        size = kcov->size * sizeof(unsigned long);
@@ -499,10 +506,11 @@ static int kcov_mmap(struct file *filep, struct vm_area_struct *vma)
                res = -EINVAL;
                goto exit;
        }
+       area = kcov->area;
        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);
+               page = vmalloc_to_page(area + off);
                res = vm_insert_page(vma, vma->vm_start + off, page);
                if (res) {
                        pr_warn_once("kcov: vm_insert_page() failed\n");
@@ -522,10 +530,10 @@ static int kcov_open(struct inode *inode, struct file *filep)
        kcov = kzalloc(sizeof(*kcov), GFP_KERNEL);
        if (!kcov)
                return -ENOMEM;
+       spin_lock_init(&kcov->lock);
        kcov->mode = KCOV_MODE_DISABLED;
        kcov->sequence = 1;
        refcount_set(&kcov->refcount, 1);
-       spin_lock_init(&kcov->lock);
        filep->private_data = kcov;
        return nonseekable_open(inode, filep);
 }
@@ -556,6 +564,7 @@ static int kcov_get_mode(unsigned long arg)
  * vmalloc fault handling path is instrumented.
  */
 static void kcov_fault_in_area(struct kcov *kcov)
+       __must_hold(&kcov->lock)
 {
        unsigned long stride = PAGE_SIZE / sizeof(unsigned long);
        unsigned long *area = kcov->area;
@@ -584,6 +593,7 @@ static inline bool kcov_check_handle(u64 handle, bool common_valid,
 
 static int kcov_ioctl_locked(struct kcov *kcov, unsigned int cmd,
                             unsigned long arg)
+       __must_hold(&kcov->lock)
 {
        struct task_struct *t;
        unsigned long flags, unused;
@@ -814,6 +824,7 @@ 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;
@@ -831,6 +842,7 @@ 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);
 
@@ -896,10 +908,12 @@ void kcov_remote_start(u64 handle)
        /* Put in kcov_remote_stop(). */
        kcov_get(kcov);
        /*
-        * Read kcov fields before unlock to prevent races with
-        * KCOV_DISABLE / kcov_remote_reset().
+        * Read kcov fields before unlocking kcov_remote_lock to prevent races
+        * with KCOV_DISABLE and kcov_remote_reset(); cannot acquire kcov->lock
+        * here, because it might lead to deadlock given kcov_remote_lock is
+        * acquired _after_ kcov->lock elsewhere.
         */
-       mode = kcov->mode;
+       mode = context_unsafe(kcov->mode);
        sequence = kcov->sequence;
        if (in_task()) {
                size = kcov->remote_size;