[PATCH RESEND] tracing: Fix overflow in get_free_elt()

[Tze-nan Wu]

"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".

Co-developed-by: Cheng-Jui Wang <cheng-jui.w...@mediatek.com>
Signed-off-by: Cheng-Jui Wang <cheng-jui.w...@mediatek.com>
Signed-off-by: Tze-nan Wu <tze-nan...@mediatek.com>
---
We have encountered this issue internally after enabling the
throttle_rss_stat feature provided by Perfetto in background for more than
two days, during which `rss_stat` tracepoint was invoked over 2^32 times.
After tracing_map->next_elt overflow, new elements can continue to be 
inserted to the tracing_map belong to `rss_stat`.
Then the CPU could hang inside the while dead loop in function
`__tracing_map_insert()` by calling it after the tracing_map left no empty
entry.

Call trace during hang:
  __tracing_map_insert()
  tracing_map_insert()
  event_hist_trigger()
  event_triggers_call()
  __event_trigger_test_discard()
  trace_event_buffer_commit()
  trace_event_raw_event_rss_stat()
  __traceiter_rss_stat()
  trace_rss_stat()
  mm_trace_rss_stat()
  inc_mm_counter()
  do_swap_page()

throttle_rss_stat is literally a synthetic event triggered by `rss_stat`
with condition:
 1. $echo "rss_stat_throttled unsigned int mm_id unsigned int curr int
    member long size" >> /sys/kernel/tracing/synthetic_events
 2. $echo
    'hist:keys=mm_id,member:bucket=size/0x80000:onchange($bucket).rss_stat_
    throttled(mm_id,curr,member,size)' >
    /sys/kernel/tracing/events/kmem/rss_stat/trigger

The hang issue could also be reproduced easily by calling a customize
trace event `myevent(u64 mycount)` for more than 2^32+(map_size-max_elts)
times during its histogram enabled with the key set to variable "mycount".
While we call `myevent` with different argument "mycount" everytime.

BTW, I added Cheng-jui to Co-developed because we have a lot of discussions
during debugging this.
---
 kernel/trace/tracing_map.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/tracing_map.c b/kernel/trace/tracing_map.c
index a4dcf0f24352..3a56e7c8aa4f 100644
--- 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;
-- 
2.18.0