Re: [PATCH v12 7/7] ARM: kprobes: enable OPTPROBES for ARM 32

[Wang Nan <wangnan0@huawei.com>]

On 2014/12/5 0:21, Jon Medhurst (Tixy) wrote:
> On Thu, 2014-12-04 at 13:36 +0800, Wang Nan wrote:
> 

[trim some text]

> 
> I have retested this patch and on one of the arm test cases I get an
> undefined instruction exception in kprobe_arm_test_cases. When this
> happens PC points to the second nop below. 
> 
> 
> 80028a38:	e320f000 	nop	{0}
> 80028a3c:	e11000b2 	ldrh	r0, [r0, -r2]
> 80028a40:	e320f000 	nop	{0}
> 
> As all three instructions will have probes on them during testing, and
> un-optimised probes are implemented by using an undefined instruction to
> act as a breakpoint, my first thought was that we have a race condition
> somewhere with adding, removing or optimizing probes. Though a reboot a
> retest failed in the same way on the same instruction, so I'm not 100%
> convinced about strictly timing related bugs.
>  

Does the problem appear in your platform in each time? Currently I have only
QEMU machine for testing and haven't seen problem like this before. Could
you please provide a detail steps for me to reproduce it? Or do you just
enable kprobe test code when booting and this exception simply appear twice?

> Meanwhile, I have some review comments of the code below...
> 

[trim some code]

>> +	/*
>> +	 * Add more 4 byte for potential AEABI requirement. If probing is triggered
>> +	 * when SP % 8 == 4, we sub SP by another 4 bytes.
>> +	 */
>> +	stack_protect += orig->ainsn.stack_space + 4;
> 
> The above comment and code don't match up any more with the code in
> optprobe_template_entry, it should be '+ 7' here. Alternatively, change
> the code in optprobe_template_entry back to use 4 as I suggested.
> 
> 

Looks like we don't really need this 4 bytes. ASM code should operate SP correctly in
each case.