linux-kernel - Re: [PATCH] ARM: check __ex_table in do

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Date:   Thu, 7 Nov 2019 17:22:45 +0000
From:   黄吕强 (Lvqiang Huang) 
        <lvqiang.huang@...soc.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
CC:     "ebiederm@...ssion.com" <ebiederm@...ssion.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "anshuman.khandual@....com" <anshuman.khandual@....com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "will@...nel.org" <will@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.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@...linux.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/
> FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
> According to speedtest.net: 11.9Mbps down 500kbps up

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