Re: [PATCH bpf v3 3/5] bpf, sockmap: Fix af_unix iter deadlock

[Kuniyuki Iwashima]

On Thu, Mar 5, 2026 at 3:32 PM Michal Luczaj <mhal@xxxxxxx> wrote:
>
> bpf_iter_unix_seq_show() may deadlock when lock_sock_fast() takes the fast
> path and the iter prog attempts to update a sockmap. Which ends up spinning
> at sock_map_update_elem()'s bh_lock_sock():
>
> WARNING: possible recursive locking detected
> test_progs/1393 is trying to acquire lock:
> ffff88811ec25f58 (slock-AF_UNIX){+...}-{3:3}, at: sock_map_update_elem+0xdb/0x1f0
>
> but task is already holding lock:
> ffff88811ec25f58 (slock-AF_UNIX){+...}-{3:3}, at: __lock_sock_fast+0x37/0xe0
>
> other info that might help us debug this:
>  Possible unsafe locking scenario:
>
>        CPU0
>        ----
>   lock(slock-AF_UNIX);
>   lock(slock-AF_UNIX);
>
>  *** DEADLOCK ***
>
>  May be due to missing lock nesting notation
>
> 4 locks held by test_progs/1393:
>  #0: ffff88814b59c790 (&p->lock){+.+.}-{4:4}, at: bpf_seq_read+0x59/0x10d0
>  #1: ffff88811ec25fd8 (sk_lock-AF_UNIX){+.+.}-{0:0}, at: bpf_seq_read+0x42c/0x10d0
>  #2: ffff88811ec25f58 (slock-AF_UNIX){+...}-{3:3}, at: __lock_sock_fast+0x37/0xe0
>  #3: ffffffff85a6a7c0 (rcu_read_lock){....}-{1:3}, at: bpf_iter_run_prog+0x51d/0xb00
>
> Call Trace:
>  dump_stack_lvl+0x5d/0x80
>  print_deadlock_bug.cold+0xc0/0xce
>  __lock_acquire+0x130f/0x2590
>  lock_acquire+0x14e/0x2b0
>  _raw_spin_lock+0x30/0x40
>  sock_map_update_elem+0xdb/0x1f0
>  bpf_prog_2d0075e5d9b721cd_dump_unix+0x55/0x4f4
>  bpf_iter_run_prog+0x5b9/0xb00
>  bpf_iter_unix_seq_show+0x1f7/0x2e0
>  bpf_seq_read+0x42c/0x10d0
>  vfs_read+0x171/0xb20
>  ksys_read+0xff/0x200
>  do_syscall_64+0x6b/0x3a0
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e
>
> Suggested-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
> Suggested-by: Martin KaFai Lau <martin.lau@xxxxxxxxx>
> Fixes: 2c860a43dd77 ("bpf: af_unix: Implement BPF iterator for UNIX domain socket.")
> Signed-off-by: Michal Luczaj <mhal@xxxxxxx>

Reviewed-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>