linux-kernel - Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917

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Date:   Mon, 20 Jul 2020 08:00:36 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     christian@...uner.io, "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917

On 18. 07. 20, 19:14, Oleg Nesterov wrote:
> On 07/18, Jiri Slaby wrote:
>>
>> On 17. 07. 20, 14:40, Oleg Nesterov wrote:
>>>
>>> please see the updated patch below, lets check ptrace_unfreeze() too.
>>
>> Sure, dmesg attached.
> 
> Thanks a lot!
> 
> But I am totally confused...
> 
>> [   94.513944] ------------[ cut here ]------------
>> [   94.513985] do not call blocking ops when !TASK_RUNNING; state=8 set at [<000000002fe279e9>] ptrace_check_attach+0xbf/0x110
> 
> OK, so the ptracer has already did the TASK_TRACED -> __TASK_TRACED change in
> ptrace_freeze_traced(),
> 
>> [   94.514019] WARNING: CPU: 16 PID: 34171 at kernel/sched/core.c:6881 __might_sleep+0x6c/0x70
>> [   94.514020] Modules linked in: ata_generic(E) pata_acpi(E) crc32_pclmul(E) qemu_fw_cfg(E) ata_piix(E) e1000(E) nls_iso8859_1(E) nls_cp437(E) vfat(E) fat(E) virtio_blk(E) virtio_mmio(E) xfs(E) btrfs(E) blake2b_generic(E) xor(E) raid6_pq(E) libcrc32c(E) crc32c_intel(E) reiserfs(E) squashfs(E) fuse(E) dm_snapshot(E) dm_bufio(E) dm_crypt(E) dm_mod(E) binfmt_misc(E) loop(E) sg(E) virtio_rng(E)
>> [   94.514082] CPU: 16 PID: 34171 Comm: strace Tainted: G            E     5.8.0-rc5-100.g55927f9-default #1 openSUSE Tumbleweed (unreleased)
>> [   94.514084] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-0-ga698c89-rebuilt.suse.com 04/01/2014
>> [   94.514087] RIP: 0010:__might_sleep+0x6c/0x70
>> [   94.514090] Code: 41 5c 41 5d e9 25 ff ff ff 48 8b 90 68 1a 00 00 48 8b 70 10 48 c7 c7 10 45 70 8f c6 05 4f a9 68 01 01 48 89 d1 e8 7a bb fc ff <0f> 0b eb c8 0f 1f 44 00 00 55 48 89 e5 41 57 41 56 41 55 49 89 fd
>> [   94.514092] RSP: 0018:ffff9ffa4ba1be00 EFLAGS: 00010286
>> [   94.514093] RAX: 0000000000000000 RBX: ffff8dc82b503e00 RCX: 0000000000000489
>> [   94.514094] RDX: 0000000000000001 RSI: 0000000000000096 RDI: 0000000000000247
>> [   94.514095] RBP: ffffffff8f6ffa6b R08: 0000000000000004 R09: 0000000000000489
>> [   94.514095] R10: 0000000000000000 R11: 0000000000000001 R12: 0000000000000039
>> [   94.514096] R13: 0000000000000000 R14: 0000000000000001 R15: ffff8dc82b5045e4
>> [   94.514098] FS:  00007fa00f1f9240(0000) GS:ffff8dcb0c000000(0000) knlGS:0000000000000000
>> [   94.514099] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [   94.514100] CR2: 0000557b53d25877 CR3: 00000004ca490005 CR4: 0000000000360ee0
>> [   94.514103] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> [   94.514104] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> [   94.514105] Call Trace:
>> [   94.514821]  ptrace_stop+0x1a9/0x300
> 
> This is already wrong. But
> 
> 	Where does this __might_sleep() come from ??? I ses no blocking calls
> 	in ptrace_stop(). Not to mention it is called with ->siglock held and
> 	right after this lock is dropped we take tasklist_lock.

Decoded stacktrace:

> ptrace_stop (include/linux/freezer.h:57 include/linux/freezer.h:67 include/linux/freezer.h:128 include/linux/freezer.h:173 kernel/signal.c:2217)  
> ptrace_do_notify (kernel/signal.c:2272)  
> ptrace_notify (arch/x86/include/asm/paravirt.h:656 arch/x86/include/asm/qspinlock.h:55 include/linux/spinlock.h:211 include/linux/spinlock_api_smp.h:167 include/linux/spinlock.h:403 kernel/signal.c:2282)  
> syscall_trace_enter (include/linux/tracehook.h:73 include/linux/tracehook.h:104 arch/x86/entry/common.c:159)
> do_syscall_64 (arch/x86/entry/common.c:380)  
> entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:124)  

It is try_to_freeze_unsafe in try_to_freeze in freezable_schedule in
ptrace_stop.

> 
> 	How this connects to the debugging patch I sent? Did you see this warning
> 	without that patch?

I suppose this made it appear:
+CONFIG_PREEMPT_COUNT=y
-# CONFIG_DEBUG_ATOMIC_SLEEP is not set
+CONFIG_DEBUG_ATOMIC_SLEEP=y

>> [   94.514888] WARNING: CPU: 16 PID: 34171 at kernel/signal.c:2219 ptrace_stop+0x1d8/0x300
> ...
>> [   94.514899] RIP: 0010:ptrace_stop+0x1d8/0x300
> 
> This is WARN_ON(current->state) added to ptrace_stop(), this can explain
> BUG_ON() in do_notify_parent() you reported.
> 
> So, the tracee returns from schedule() with ->state != TASK_RUNNING ???
> This must not be possible.
> 
> OK, perhaps task->state was changed by ptrace_unfreeze_traced()? this can
> only happen if it races with ttwu(__TASK_TRACED) without ->siglock held,
> nobody should do this.
> 
> Strange.

Please see my other e-mail, all this is with dbfb089d360b applied. Maybe
it makes more sense now?

thanks,
-- 
js