[PATCH v4 18/22] kasan: remove atomic accesses to stack ring entries

[andrey.konovalov@linux.dev]

From: Andrey Konovalov <andreyknvl@google.com>

Remove the atomic accesses to entry fields in save_stack_info and
kasan_complete_mode_report_info for tag-based KASAN modes.

These atomics are not required, as the read/write lock prevents the
entries from being read (in kasan_complete_mode_report_info) while being
written (in save_stack_info) and the try_cmpxchg prevents the same entry
from being rewritten (in save_stack_info) in the unlikely case of wrapping
during writing.

Reviewed-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

---

Changes v1->v2:
- This is a new patch.
---
 mm/kasan/report_tags.c | 25 +++++++------------------
 mm/kasan/tags.c        | 13 +++++--------
 2 files changed, 12 insertions(+), 26 deletions(-)

diff --git a/mm/kasan/report_tags.c b/mm/kasan/report_tags.c
index 8b8bfdb3cfdb..78abdcde5da9 100644
--- a/mm/kasan/report_tags.c
+++ b/mm/kasan/report_tags.c
@@ -31,10 +31,6 @@ void kasan_complete_mode_report_info(struct kasan_report_info *info)
 	unsigned long flags;
 	u64 pos;
 	struct kasan_stack_ring_entry *entry;
-	void *ptr;
-	u32 pid;
-	depot_stack_handle_t stack;
-	bool is_free;
 	bool alloc_found = false, free_found = false;
 
 	if ((!info->cache || !info->object) && !info->bug_type) {
@@ -61,18 +57,11 @@ void kasan_complete_mode_report_info(struct kasan_report_info *info)
 
 		entry = &stack_ring.entries[i % stack_ring.size];
 
-		/* Paired with smp_store_release() in save_stack_info(). */
-		ptr = (void *)smp_load_acquire(&entry->ptr);
-
-		if (kasan_reset_tag(ptr) != info->object ||
-		    get_tag(ptr) != get_tag(info->access_addr))
+		if (kasan_reset_tag(entry->ptr) != info->object ||
+		    get_tag(entry->ptr) != get_tag(info->access_addr))
 			continue;
 
-		pid = READ_ONCE(entry->pid);
-		stack = READ_ONCE(entry->stack);
-		is_free = READ_ONCE(entry->is_free);
-
-		if (is_free) {
+		if (entry->is_free) {
 			/*
 			 * Second free of the same object.
 			 * Give up on trying to find the alloc entry.
@@ -80,8 +69,8 @@ void kasan_complete_mode_report_info(struct kasan_report_info *info)
 			if (free_found)
 				break;
 
-			info->free_track.pid = pid;
-			info->free_track.stack = stack;
+			info->free_track.pid = entry->pid;
+			info->free_track.stack = entry->stack;
 			free_found = true;
 
 			/*
@@ -95,8 +84,8 @@ void kasan_complete_mode_report_info(struct kasan_report_info *info)
 			if (alloc_found)
 				break;
 
-			info->alloc_track.pid = pid;
-			info->alloc_track.stack = stack;
+			info->alloc_track.pid = entry->pid;
+			info->alloc_track.stack = entry->stack;
 			alloc_found = true;
 
 			/*
diff --git a/mm/kasan/tags.c b/mm/kasan/tags.c
index 4fd32121b0fd..b6c017e670d8 100644
--- a/mm/kasan/tags.c
+++ b/mm/kasan/tags.c
@@ -121,15 +121,12 @@ static void save_stack_info(struct kmem_cache *cache, void *object,
 	if (!try_cmpxchg(&entry->ptr, &old_ptr, STACK_RING_BUSY_PTR))
 		goto next; /* Busy slot. */
 
-	WRITE_ONCE(entry->size, cache->object_size);
-	WRITE_ONCE(entry->pid, current->pid);
-	WRITE_ONCE(entry->stack, stack);
-	WRITE_ONCE(entry->is_free, is_free);
+	entry->size = cache->object_size;
+	entry->pid = current->pid;
+	entry->stack = stack;
+	entry->is_free = is_free;
 
-	/*
-	 * Paired with smp_load_acquire() in kasan_complete_mode_report_info().
-	 */
-	smp_store_release(&entry->ptr, (s64)object);
+	entry->ptr = object;
 
 	read_unlock_irqrestore(&stack_ring.lock, flags);
 }
-- 
2.25.1