On Thu, Mar 5, 2026 at 3:32 PM Michal Luczaj <[email protected]> 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 <[email protected]> > Suggested-by: Martin KaFai Lau <[email protected]> > Fixes: 2c860a43dd77 ("bpf: af_unix: Implement BPF iterator for UNIX domain > socket.") > Signed-off-by: Michal Luczaj <[email protected]>
Reviewed-by: Kuniyuki Iwashima <[email protected]>
