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Date:   Sat, 29 Aug 2020 15:38:43 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Yonghong Song <yhs@...com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        <davem@...emloft.net>
CC:     <daniel@...earbox.net>, <josef@...icpanda.com>,
        <bpoirier@...e.com>, <akpm@...ux-foundation.org>,
        <hannes@...xchg.org>, <netdev@...r.kernel.org>,
        <bpf@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 5/5] selftests/bpf: Add sleepable tests

On 8/29/20 3:13 PM, Yonghong Song wrote:
> 
> When running selftest, I hit the following kernel warning:
> 
> [  250.871267] ============================================
> [  250.871902] WARNING: possible recursive locking detected
> [  250.872561] 5.9.0-rc1+ #830 Not tainted
> [  250.873166] --------------------------------------------
> [  250.873991] true/2053 is trying to acquire lock:
> [  250.874715] ffff8fc1f9cd2068 (&mm->mmap_lock#2){++++}-{3:3}, at: 
> __might_fault+0x3e/0x90
> [  250.875943]
> [  250.875943] but task is already holding lock:
> [  250.876688] ffff8fc1f9cd2068 (&mm->mmap_lock#2){++++}-{3:3}, at: 
> do_mprotect_pkey+0xb5/0x2f0
> [  250.877978]
> [  250.877978] other info that might help us debug this:
> [  250.878797]  Possible unsafe locking scenario:
> [  250.878797]
> [  250.879708]        CPU0
> [  250.880095]        ----
> [  250.880482]   lock(&mm->mmap_lock#2);
> [  250.881063]   lock(&mm->mmap_lock#2);
> [  250.881645]
> [  250.881645]  *** DEADLOCK ***
> [  250.881645]
> [  250.882559]  May be due to missing lock nesting notation
> [  250.882559]
> [  250.883613] 2 locks held by true/2053:
> [  250.884194]  #0: ffff8fc1f9cd2068 (&mm->mmap_lock#2){++++}-{3:3}, at: 
> do_mprotect_pkey+0xb5/0x2f0
> [  250.885558]  #1: ffffffffbc47b8a0 (rcu_read_lock_trace){....}-{0:0}, 
> at: __bpf_prog_enter_sleepable+0x0/0x40
> [  250.887062]
> [  250.887062] stack backtrace:
> [  250.887583] CPU: 1 PID: 2053 Comm: true Not tainted 5.9.0-rc1+ #830
> [  250.888546] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), 
> BIOS 1.9.3-1.el7.centos 04/01/2014
> [  250.889896] Call Trace:
> [  250.890222]  dump_stack+0x78/0xa0
> [  250.890644]  __lock_acquire.cold.74+0x209/0x2e3
> [  250.891350]  lock_acquire+0xba/0x380
> [  250.891919]  ? __might_fault+0x3e/0x90
> [  250.892510]  ? __lock_acquire+0x639/0x20c0
> [  250.893150]  __might_fault+0x68/0x90
> [  250.893717]  ? __might_fault+0x3e/0x90
> [  250.894325]  _copy_from_user+0x1e/0xa0
> [  250.894946]  bpf_copy_from_user+0x22/0x50
> [  250.895581]  bpf_prog_3717002769f30998_test_int_hook+0x76/0x60c
> [  250.896446]  ? __bpf_prog_enter_sleepable+0x3c/0x40
> [  250.897207]  ? __bpf_prog_exit+0xa0/0xa0
> [  250.897819]  bpf_trampoline_18669+0x29/0x1000
> [  250.898476]  bpf_lsm_file_mprotect+0x5/0x10
> [  250.899133]  security_file_mprotect+0x32/0x50
> [  250.899816]  do_mprotect_pkey+0x18a/0x2f0
> [  250.900472]  __x64_sys_mprotect+0x1b/0x20
> [  250.901107]  do_syscall_64+0x33/0x40
> [  250.901670]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [  250.902450] RIP: 0033:0x7fd95c141ef7
> [  250.903014] Code: ff 66 90 b8 0b 00 00 00 0f 05 48 3d 01 f0 ff ff 73 
> 01 c3 48 8d 0d 21 c2 2
> 0 00 f7 d8 89 01 48 83 c8 ff c3 b8 0a 00 00 00 0f 05 <48> 3d 01 f0 ff ff 
> 73 01 c3 48 8d 0d 01
> c2 20 00 f7 d8 89 01 48 83
> [  250.905732] RSP: 002b:00007ffd4c291fe8 EFLAGS: 00000246 ORIG_RAX: 
> 000000000000000a
> [  250.906773] RAX: ffffffffffffffda RBX: 0000000000000005 RCX: 
> 00007fd95c141ef7
> [  250.907866] RDX: 0000000000000000 RSI: 00000000001ff000 RDI: 
> 00007fd95bf20000
> [  250.908906] RBP: 00007ffd4c292320 R08: 0000000000000000 R09: 
> 0000000000000000
> [  250.909915] R10: 00007ffd4c291ff0 R11: 0000000000000246 R12: 
> 00007fd95c341000
> [  250.910919] R13: 00007ffd4c292408 R14: 0000000000000002 R15: 
> 0000000000000801
> 
> Could this be an real issue here?
> 
> do_mprotect_pkey() gets a lock of current->mm->mmap_lock
> before calling security_file_mprotect(bpf_lsm_file_mprotect).
> Later on, when do _copy_to_user(), page fault may happen
> and current->mm->mmap_lock might be acquired again and may
> have a deadlock here?

Hmm. It does sound like dead_lock.
But I don't understand why I don't see this splat.
I have
LOCKDEP=y
DEBUG_ATOMIC_SLEEP=y
LOCK_DEBUGGING_SUPPORT=y
KASAN=y
in my .config and don't see it :(
Could pls send me your .config?
I'll analyze further.
Thanks for the reporting!

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