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

From: "黄吕强 (Lvqiang Huang)" <lvqiang.huang@unisoc.com>
To: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: "ebiederm@xmission.com" <ebiederm@xmission.com>,
	"dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
	"anshuman.khandual@arm.com" <anshuman.khandual@arm.com>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"f.fainelli@gmail.com" <f.fainelli@gmail.com>,
	"will@kernel.org" <will@kernel.org>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] ARM: check __ex_table in do_bad()
Date: Thu, 7 Nov 2019 17:22:45 +0000	[thread overview]
Message-ID: <CE4726AD-630F-44A0-862B-23BC8E6A7E4C@unisoc.com> (raw)
In-Reply-To: <20191107092404.GV25745@shell.armlinux.org.uk>

> 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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