linux-kernel - [PATCH v9 1/7] lib/stackdepot: Fix first entry having a 0-handle

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Date: Wed, 14 Feb 2024 18:01:51 +0100
From: Oscar Salvador <osalvador@...e.de>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	Michal Hocko <mhocko@...e.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Marco Elver <elver@...gle.com>,
	Andrey Konovalov <andreyknvl@...il.com>,
	Alexander Potapenko <glider@...gle.com>,
	Oscar Salvador <osalvador@...e.de>
Subject: [PATCH v9 1/7] lib/stackdepot: Fix first entry having a 0-handle

The very first entry of stack_record gets a handle of 0, but this is wrong
because stackdepot treats a 0-handle as a non-valid one.
E.g: See the check in stack_depot_fetch()

Fix this by adding and offset of 1.

This bug has been lurking since the very beginning of stackdepot,
but no one really cared as it seems.
Because of that I am not adding a Fixes tag.

Co-developed-by: Marco Elver <elver@...gle.com>
Signed-off-by: Marco Elver <elver@...gle.com>
Signed-off-by: Oscar Salvador <osalvador@...e.de>
---
 lib/stackdepot.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index 4a7055a63d9f..c043a4186bc5 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -45,15 +45,16 @@
 #define DEPOT_POOL_INDEX_BITS (DEPOT_HANDLE_BITS - DEPOT_OFFSET_BITS - \
 			       STACK_DEPOT_EXTRA_BITS)
 #define DEPOT_POOLS_CAP 8192
+/* The pool_index is offset by 1 so the first record does not have a 0 handle. */
 #define DEPOT_MAX_POOLS \
-	(((1LL << (DEPOT_POOL_INDEX_BITS)) < DEPOT_POOLS_CAP) ? \
-	 (1LL << (DEPOT_POOL_INDEX_BITS)) : DEPOT_POOLS_CAP)
+	(((1LL << (DEPOT_POOL_INDEX_BITS)) - 1 < DEPOT_POOLS_CAP) ? \
+	 (1LL << (DEPOT_POOL_INDEX_BITS)) - 1 : DEPOT_POOLS_CAP)
 
 /* Compact structure that stores a reference to a stack. */
 union handle_parts {
 	depot_stack_handle_t handle;
 	struct {
-		u32 pool_index	: DEPOT_POOL_INDEX_BITS;
+		u32 pool_index	: DEPOT_POOL_INDEX_BITS; /* pool_index is offset by 1 */
 		u32 offset	: DEPOT_OFFSET_BITS;
 		u32 extra	: STACK_DEPOT_EXTRA_BITS;
 	};
@@ -372,7 +373,7 @@ static struct stack_record *depot_pop_free_pool(void **prealloc, size_t size)
 	stack = current_pool + pool_offset;
 
 	/* Pre-initialize handle once. */
-	stack->handle.pool_index = pool_index;
+	stack->handle.pool_index = pool_index + 1;
 	stack->handle.offset = pool_offset >> DEPOT_STACK_ALIGN;
 	stack->handle.extra = 0;
 	INIT_LIST_HEAD(&stack->hash_list);
@@ -483,18 +484,19 @@ static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
 	const int pools_num_cached = READ_ONCE(pools_num);
 	union handle_parts parts = { .handle = handle };
 	void *pool;
+	u32 pool_index = parts.pool_index - 1;
 	size_t offset = parts.offset << DEPOT_STACK_ALIGN;
 	struct stack_record *stack;
 
 	lockdep_assert_not_held(&pool_lock);
 
-	if (parts.pool_index > pools_num_cached) {
+	if (pool_index > pools_num_cached) {
 		WARN(1, "pool index %d out of bounds (%d) for stack id %08x\n",
-		     parts.pool_index, pools_num_cached, handle);
+		     pool_index, pools_num_cached, handle);
 		return NULL;
 	}
 
-	pool = stack_pools[parts.pool_index];
+	pool = stack_pools[pool_index];
 	if (WARN_ON(!pool))
 		return NULL;
 
-- 
2.43.0