[PATCH bpf-next v4 0/4] bpf: Free special fields when update hash and local storage maps

[Leon Hwang <leon.hwang@linux.dev>]

In the discussion thread
"[PATCH bpf-next v9 0/7] bpf: Introduce BPF_F_CPU and BPF_F_ALL_CPUS flags for percpu maps"[1],
it was pointed out that missing calls to bpf_obj_free_fields() could
lead to memory leaks.

A selftest was added to confirm that this is indeed a real issue - the
refcount of BPF_KPTR_REF field is not decremented when
bpf_obj_free_fields() is missing after copy_map_value[,_long]().

Further inspection of copy_map_value[,_long]() call sites revealed two
locations affected by this issue:

1. pcpu_copy_value()
2. htab_map_update_elem() when used with BPF_F_LOCK

Similar case happens when update local storage maps with BPF_F_LOCK.

This series fixes the issues by properly calling bpf_obj_free_fields()
(or check_and_free_fields()) after copy_map_value[,_long]() and adds
selftests to verify the fix.

Changes:
v3 -> v4:
* Target bpf-next tree.
* Address comments from Amery:
  * Drop 'bpf_obj_free_fields()' in the path of updating local storage
    maps without BPF_F_LOCK.
  * Drop the corresponding self test.
  * Respin the other test of local storage maps using syscall BPF
    programs.

v2 -> v3:
* Free special fields when update local storage maps without BPF_F_LOCK.
* Add test to verify decrementing refcount when update cgroup local
  storage maps without BPF_F_LOCK.
* Address review from AI bot:
  * Slow path with BPF_F_LOCK (around line 642-646) in
    'bpf_local_storage.c'.
* https://lore.kernel.org/bpf/20251020164608.20536-1-leon.hwang@linux.dev/

v1 -> v2:
* Add test to verify decrementing refcount when update cgroup local
  storage maps with BPF_F_LOCK.
* Address review from AI bot:
  * Fast path without bucket lock (around line 610) in
    'bpf_local_storage.c'.
* https://lore.kernel.org/bpf/20251016145801.47552-1-leon.hwang@linux.dev/

Leon Hwang (4):
  bpf: Free special fields when update [lru_,]percpu_hash maps
  bpf: Free special fields when update hash maps with BPF_F_LOCK
  bpf: Free special fields when update local storage maps with
    BPF_F_LOCK
  selftests/bpf: Add tests to verify freeing the special fields when
    update hash and local storage maps

 kernel/bpf/bpf_local_storage.c                |   2 +
 kernel/bpf/hashtab.c                          |   4 +
 .../bpf/prog_tests/refcounted_kptr.c          | 134 +++++++++++++++++-
 .../selftests/bpf/progs/refcounted_kptr.c     | 129 +++++++++++++++++
 4 files changed, 268 insertions(+), 1 deletion(-)

--
2.51.1