[PATCH v2 17/19] kasan: remove atomic accesses to stack ring entries

andrey.konovalov
2023-09-13T17:17:41Z

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.

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