tracing: Fix overflow in get_free_elt()

"tracing_map->next_elt" in get_free_elt() is at risk of overflowing.

Once it overflows, new elements can still be inserted into the tracing_map
even though the maximum number of elements (`max_elts`) has been reached.
Continuing to insert elements after the overflow could result in the
tracing_map containing "tracing_map->max_size" elements, leaving no empty
entries.
If any attempt is made to insert an element into a full tracing_map using
`__tracing_map_insert()`, it will cause an infinite loop with preemption
disabled, leading to a CPU hang problem.

Fix this by preventing any further increments to "tracing_map->next_elt"
once it reaches "tracing_map->max_elt".

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: 08d43a5fa063e ("tracing: Add lock-free tracing_map")
Co-developed-by: Cheng-Jui Wang <cheng-jui.wang@mediatek.com>
Link: https://lore.kernel.org/20240805055922.6277-1-Tze-nan.Wu@mediatek.com
Signed-off-by: Cheng-Jui Wang <cheng-jui.wang@mediatek.com>
Signed-off-by: Tze-nan Wu <Tze-nan.Wu@mediatek.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/trace/tracing_map.c b/kernel/trace/tracing_map.c
index a4dcf0f2435213bc2b2b91d677ec18290aa53859..3a56e7c8aa4f67281e57bb6538dc961c4b65c670 100644 (file)
--- a/kernel/trace/tracing_map.c
+++ b/kernel/trace/tracing_map.c
@@ -454,7 +454,7 @@ static struct tracing_map_elt *get_free_elt(struct tracing_map *map)
        struct tracing_map_elt *elt = NULL;
        int idx;
 
-       idx = atomic_inc_return(&map->next_elt);
+       idx = atomic_fetch_add_unless(&map->next_elt, 1, map->max_elts);
        if (idx < map->max_elts) {
                elt = *(TRACING_MAP_ELT(map->elts, idx));
                if (map->ops && map->ops->elt_init)
@@ -699,7 +699,7 @@ void tracing_map_clear(struct tracing_map *map)
 {
        unsigned int i;
 
-       atomic_set(&map->next_elt, -1);
+       atomic_set(&map->next_elt, 0);
        atomic64_set(&map->hits, 0);
        atomic64_set(&map->drops, 0);
 
@@ -783,7 +783,7 @@ struct tracing_map *tracing_map_create(unsigned int map_bits,
 
        map->map_bits = map_bits;
        map->max_elts = (1 << map_bits);
-       atomic_set(&map->next_elt, -1);
+       atomic_set(&map->next_elt, 0);
 
        map->map_size = (1 << (map_bits + 1));
        map->ops = ops;