RE: [PATCH] ARM: check __ex_table in do_bad()

["黄吕强 (Lvqiang Huang)" <lvqiang.huang@unisoc.com>]

Sorry for not having described it clearly, please let me add some more information. 

The kernel log for the scenario
[20461.271374] sysrq: SysRq : Show Blocked State
[20461.271405]   task                PC stack   pid father
[20461.271436] mbox-send-threa D c08cfad8     0    38      2 0x00000000
/*and ignore some logs abort the backtrace dump of some TASK_UNINTERRUPTIBLE tasks */
[20461.273387] fsck.exfat      D c08cfad8     0  6221   2276 0x00000000
[20461.273408] Backtrace:
[20461.273430] [<c08cf5d0>] (__schedule) from [<c08cff84>] (schedule+0x90/0xa8)
[20461.273442]  r10:ce009ef0 r9:ce009df4 r8:c0d0790c r7:00000082 r6:7fffffff r5:00000000
[20461.273477]  r4:ce008000
[20461.273497] [<c08cfef4>] (schedule) from [<c08d2b90>] (schedule_timeout+0x2c/0x26c)
[20461.273509]  r4:7fffffff r3:dc8ba693
[20461.273561] Unhandled fault: page domain fault (0x01b) at 0x32848c02
[20461.273576] pgd = d1854000
[20461.273587] [32848c02] *pgd=bb21e835
[20461.273607] Internal error: : 1b [#1] PREEMPT SMP ARM
[20461.278903] CPU: 2 PID: 5917 Comm: watchdog Tainted: G        W  O    4.4.147+ #1
[20461.278929] task: e9beecc0 task.stack: e30a4000
[20461.278949] PC is at for_each_frame+0x18/0x88
[20461.278965] LR is at vprintk_emit+0x470/0x4ec

The Task A: finally crashed task, PID: 5917 Comm: watchdog, running on CPU 2, dumping backtrace of all UN tasks.
The Task B: TASK_UNINTERRUPTIBLE to TASK_RUNNING when Task A is trying to dump its backtrace. 

The first 2 frames dump for task B are ok, see 
[20461.273430] [<c08cf5d0>] (__schedule) from [<c08cff84>] (schedule+0x90/0xa8)
[20461.273497] [<c08cfef4>] (schedule) from [<c08d2b90>] (schedule_timeout+0x2c/0x26c)

Then task A crashed:
[20461.273561] Unhandled fault: page domain fault (0x01b) at 0x32848c02

From the RAM dump after kernel crash, we can see Task B had been scheduled to running on CPU 0. 
crash_arm> ps 6221
   PID    PPID  CPU   TASK    ST  %MEM     VSZ    RSS  COMM
>  6221   2276   0  cde04880  RU   0.4   17784  13596  fsck.exfat

And the backtrace should changed, which cause the crash of Task A.
crash_arm> bt 6221
PID: 6221   TASK: cde04880  CPU: 0   COMMAND: "fsck.exfat"
 #0 [<c0117a5c>] (__kunmap_atomic) from [<c0413ae8>]
 #1 [<c0413894>] (copy_page_to_iter) from [<c01f4788>]
 #2 [<c01f439c>] (generic_file_read_iter) from [<c02725e8>]
 #3 [<c027257c>] (blkdev_read_iter) from [<c023b5b0>]
 #4 [<c023b4f8>] (__vfs_read) from [<c023bd04>]
 #5 [<c023bc78>] (vfs_read) from [<c023c7e0>]
 #6 [<c023c76c>] (sys_pread64) from [<c01079a0>]

This is the race condition, try to backtrace another task is not safe. We can't assume the task won't be scheduled to execution during the backtrace dump. The stack frame should totally change once execute again. 

The __ex_table entry in @for_each_frame should adding for this scenario. But with CONFIG_CPU_SW_DOMAIN_PAN=y, page domain fault may hit and go the do_bad() instead of do_page_fault().

The path may not an optimal solution, I just want to point out the problem, and is there any concern if we check __ex_table in do_bad()? 

Now, our project had enabled CONFIG_ARM_UNWIND=y, it will fail to get an unwind_idx when get a wrong sv_pc, then the unwind abort without kernel crash.

-----Original Message-----
From: 黄吕强 (Lvqiang Huang) 
Sent: Friday, November 08, 2019 1:23 AM
To: Russell King - ARM Linux admin
Cc: ebiederm@xmission.com; dave.hansen@linux.intel.com; anshuman.khandual@arm.com; akpm@linux-foundation.org; f.fainelli@gmail.com; will@kernel.org; tglx@linutronix.de; linux-arm-kernel@lists.infradead.org; linux-kernel@vger.kernel.org
Subject: Re: [PATCH] ARM: check __ex_table in do_bad()


> 在 2019年11月7日，17:24，Russell King - ARM Linux admin 
> <linux@armlinux.org.uk> 写道：
> 
>> On Thu, Nov 07, 2019 at 03:45:13PM +0800, Lvqiang wrote:
>> 
>> We got many crashs in for_each_frame+0x18 arch/arm/lib/backtrace.S
>>    1003: ldr r2, [sv_pc, #-4]
>> 
>> The backtrace is
>>    dump_backtrace
>>    show_stack
>>    sched_show_task
>>    show_state_filter
>>    sysrq_handle_showstate_blocked
>>    __handle_sysrq
>>    write_sysrq_trigger
>>    proc_reg_write
>>    __vfs_write
>>    vfs_write
>>    sys_write
>> 
>> Related Kernel config
>>    CONFIG_CPU_SW_DOMAIN_PAN=y
>>    # CONFIG_ARM_UNWIND is not set
>>    CONFIG_FRAME_POINTER=y
>> 
>> The task A was dumping the stack of an UN task B. However, the task B
> 
> What is "an UN task B"?

UN means TASK_UNINTERRUPTIBLE. 
(Sorry for the typo in the last reply)

>> scheduled to run on another CPU, which cause it stack content changed.
>> Then, task A may hit a page domain fault and die().
>>    [520.661314] Unhandled fault: page domain fault (0x01b) at 
>> 0x32848c02
> 
> So, the backtrace code is trying to access userspace.  It isn't 
> supposed to be accessing userspace - there are no guarantees that 
> userspace will be using frame pointers.  That is the bug.
> 

There is a race condition when try to get the backtrace of another task，whose frames may totally changed during the execution. 

> --
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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