On 08/21/2014 01:17 PM, Oleg Nesterov wrote:
>> Is there a race between kill() and exit() brought on by the kill path only
>> > using the RCU read lock? This doesn't prevent ->real_cred from being
>> > modified, but it looks like this should, in combination with
>> > delayed_put_task_struct(), prevent it from being cleared.
> Yes, rcu should protect us from both delayed_put_pid() and delayed_put_task().
> Everything looks correct... And there are a lot of other similar users of
> find_vpid/find_task_by_vpid/pid_task/etc under rcu, I can't recall any bug
> in this area.
>
> I am puzzled. Note also that ->signal == NULL. Will try to think more,
> but so far I have no any idea.
I've hit something similar earlier today, and it might be related:
[ 973.452840] BUG: unable to handle kernel NULL pointer dereference at
00000000000002b0
[ 973.455347] IP: flush_sigqueue_mask (include/linux/signal.h:118
kernel/signal.c:715)
[ 973.457526] PGD 4dfdc7067 PUD 5f77d9067 PMD 0
[ 973.459216] Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[ 973.460086] Dumping ftrace buffer:
[ 973.460086] (ftrace buffer empty)
[ 973.460086] Modules linked in:
[ 973.460086] CPU: 4 PID: 13145 Comm: trinity-c767 Not tainted
3.17.0-rc2-next-20140826-sasha-00031-gc48c9ac-dirty #1079
[ 973.460086] task: ffff880604800000 ti: ffff880586648000 task.ti:
ffff880586648000
[ 973.460086] RIP: flush_sigqueue_mask (include/linux/signal.h:118
kernel/signal.c:715)
[ 973.460086] RSP: 0018:ffff88058664bec8 EFLAGS: 00010046
[ 973.460086] RAX: 0000000000000000 RBX: fffffffffffff730 RCX: 0000000000010000
[ 973.460086] RDX: 0000000000000000 RSI: 00000000000002a0 RDI: ffff88058664bed8
[ 973.460086] RBP: ffff88058664bf10 R08: 0000000000000001 R09: 0000000000000001
[ 973.460086] R10: 000000000002d201 R11: 0000000000000254 R12: 0000000000000000
[ 973.460086] R13: ffff88058664bf40 R14: ffff880604800000 R15: 0000000000000010
[ 973.460086] FS: 00007fe3a3045700(0000) GS:ffff880277c00000(0000)
knlGS:0000000000000000
[ 973.460086] CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 973.460086] CR2: 00000000000002b0 CR3: 00000004e23d5000 CR4: 00000000000006a0
[ 973.460086] Stack:
[ 973.460086] ffffffffac183690 01017fffb3247180 0000000000010000
0000000000000000
[ 973.460086] 00007fffb3247180 00007fffb3247220 0000000000000011
0000000000000000
[ 973.460086] 0000000000000000 ffff88058664bf78 ffffffffac183ef5
0000000000000000
[ 973.460086] Call Trace:
[ 973.460086] ? do_sigaction (kernel/signal.c:3124 (discriminator 17))
[ 973.460086] SyS_rt_sigaction (kernel/signal.c:3360 kernel/signal.c:3341)
[ 973.460086] tracesys (arch/x86/kernel/entry_64.S:542)
[ 973.460086] Code: b7 49 09 d5 4d 89 6e 10 48 83 c4 08 5b 41 5c 41 5d 41 5e 41
5f 5d c3 66 2e 0f 1f 84 00 00 00 00 00 66 66 66 66 90 48 8b 0f 31 c0 <48> 8b 56
10 48 85 ca 74 7b 55 48 f7 d1 48 89 e5 41 56 48 21 ca
All code
========
0: b7 49 mov $0x49,%bh
2: 09 d5 or %edx,%ebp
4: 4d 89 6e 10 mov %r13,0x10(%r14)
8: 48 83 c4 08 add $0x8,%rsp
c: 5b pop %rbx
d: 41 5c pop %r12
f: 41 5d pop %r13
11: 41 5e pop %r14
13: 41 5f pop %r15
15: 5d pop %rbp
16: c3 retq
17: 66 2e 0f 1f 84 00 00 nopw %cs:0x0(%rax,%rax,1)
1e: 00 00 00
21: 66 66 66 66 90 data32 data32 data32 xchg %ax,%ax
26: 48 8b 0f mov (%rdi),%rcx
29: 31 c0 xor %eax,%eax
2b:* 48 8b 56 10 mov 0x10(%rsi),%rdx <-- trapping
instruction
2f: 48 85 ca test %rcx,%rdx
32: 74 7b je 0xaf
34: 55 push %rbp
35: 48 f7 d1 not %rcx
38: 48 89 e5 mov %rsp,%rbp
3b: 41 56 push %r14
3d: 48 21 ca and %rcx,%rdx
...
Code starting with the faulting instruction
===========================================
0: 48 8b 56 10 mov 0x10(%rsi),%rdx
4: 48 85 ca test %rcx,%rdx
7: 74 7b je 0x84
9: 55 push %rbp
a: 48 f7 d1 not %rcx
d: 48 89 e5 mov %rsp,%rbp
10: 41 56 push %r14
12: 48 21 ca and %rcx,%rdx
...
[ 973.460086] RIP flush_sigqueue_mask (include/linux/signal.h:118
kernel/signal.c:715)
[ 973.460086] RSP <ffff88058664bec8>
[ 973.460086] CR2: 00000000000002b0
Thanks,
Sasha
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