Re: [PATCH bpf v2 2/3] tcp: Skip cond_resched() in inet_csk_listen_stop() under BPF context

[bot+bpf-ci]

> tcp: Skip cond_resched() in inet_csk_listen_stop() under BPF context
>
> bpf_sock_destroy() runs from the tcp iterator, under rcu_read_lock(). If
> the sock is a listener that still has children in its accept queue,
> tcp_abort() ends up in inet_csk_listen_stop() and the cond_resched()
> there trips the debug check:
>
> BUG: sleeping function called from invalid context at net/ipv4/inet_connection_sock.c:1523
> in_atomic(): 0, irqs_disabled(): 0, non_block: 0, pid: 628, name: test_progs
> preempt_count: 0, expected: 0
> RCU nest depth: 1, expected: 0
> locks held by test_progs/628: 3, last CPU#3:
>  #0: ffff8881158cee18 (&p->lock){+.+.}-{4:4}, at: bpf_seq_read+0x56/0x1210
>  #1: ffff8881106bb858 (sk_lock-AF_INET6){+.+.}-{0:0}, at: bpf_iter_tcp_seq_show+0x32b/0x4b0
>  #2: ffffffffb435af20 (rcu_read_lock){....}-{1:3}, at: bpf_iter_run_prog+0x46b/0xde0
> CPU: 3 UID: 0 PID: 628 Comm: test_progs Tainted: G        W           7.2.0+ #65 PREEMPT
> Tainted: [W]=WARN
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0xc1/0xf0
>  dump_stack+0x10/0x20
>  __might_resched+0x3d2/0x610
>  inet_csk_listen_stop+0x7b/0xbf0
>  tcp_abort+0x23b/0x3b0
>  bpf_sock_destroy+0xfc/0x140
>  bpf_prog_448133d24601754f_iter_tcp6_server+0x81/0x8a
>  bpf_iter_run_prog+0x538/0xde0
>  bpf_iter_tcp_seq_show+0x26b/0x4b0
>  bpf_seq_read+0x424/0x1210
>  vfs_read+0x197/0xe40
>  ksys_read+0x119/0x240
>  __x64_sys_read+0x72/0xc0
>  x64_sys_call+0x647/0x27e0
>  do_syscall_64+0xe5/0x610
>  entry_SYSCALL_64_after_hwframe+0x76/0x7e
> RIP: 0033:0x7fad39b28aca
> RSP: 002b:00007ffc381c61c0 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> RAX: ffffffffffffffda RBX: 00007ffc381c6a88 RCX: 00007fad39b28aca
> RDX: 0000000000000032 RSI: 00007ffc381c6250 RDI: 0000000000000014
> RBP: 00007ffc381c61e0 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000003
> R13: 0000000000000000 R14: 000055f077c1bbb0 R15: 00007fad3a0f3000
>  </TASK>
>
> The commit that added the kfunc already guards lock_sock() in tcp_abort()
> and udp_abort() with has_current_bpf_ctx(), but missed the listener path.
> Do the same for the cond_resched(), it can't reschedule there anyway.
>
> Fixes: 4ddbcb886268 ("bpf: Add bpf_sock_destroy kfunc")
> Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>

Is the justification "it can't reschedule there anyway" accurate?

With CONFIG_PREEMPT_DYNAMIC=y booted with preempt=none or preempt=voluntary,
cond_resched() expands to __cond_resched() which can actually reschedule.
The splat in the commit message confirms preempt_count is 0 while RCU nest
depth is 1. With preempt_count==0, should_resched(0) can be true and
__cond_resched() will call preempt_schedule_common() for a real reschedule.

Additionally, in configurations with CONFIG_PREEMPT_RCU=n where
rcu_read_lock() is preempt_disable(), __cond_resched() falls through to
rcu_all_qs() which calls rcu_qs() to report a quiescent state from inside
an RCU read-side critical section. That's a correctness problem beyond just
the debug check.

So the call can either reschedule (PREEMPT_DYNAMIC none/voluntary) or report
a bogus quiescent state (non-preemptible RCU). Could the justification be
reworded to explain that the loop runs inside the iterator's RCU read-side
critical section and must not reschedule or report a quiescent state there?
The code change itself is correct and matches the existing pattern in
tcp_abort() and udp_abort().


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