Re: [PATCH] kprobes/x86: opt: free optinsn cache when range check fails

[Wang Nan <wangnan0@huawei.com>]

On 2014/7/29 9:43, Masami Hiramatsu wrote:
> (2014/07/28 21:20), Wang Nan wrote:
>> This patch frees optinsn slot when range check error to prevent memory
>> leaks. Before this patch, cache entry in kprobe_insn_cache won't be
>> freed if kprobe optimizing fails due to range check failure.
>>
>> Signed-off-by: Wang Nan <wangnan0@huawei.com>
> 
> Oops, thank you for finding it!
> 
> Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
> 
> BTW, would you really have hit this error?
> I'd like to know the case if this really happens.

I'm not really hit it on x86_64. I found this problem when trying to implement kprobe opt on arm.

On arm, relative jump can only branch on/backward 64MB, which makes it a realistic problem.

> 
>> ---
>>  arch/x86/kernel/kprobes/opt.c | 4 +++-
>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
>> index f304773..f1314d0 100644
>> --- a/arch/x86/kernel/kprobes/opt.c
>> +++ b/arch/x86/kernel/kprobes/opt.c
>> @@ -338,8 +338,10 @@ int arch_prepare_optimized_kprobe(struct optimized_kprobe *op)
>>  	 * a relative jump.
>>  	 */
>>  	rel = (long)op->optinsn.insn - (long)op->kp.addr + RELATIVEJUMP_SIZE;
>> -	if (abs(rel) > 0x7fffffff)
>> +	if (abs(rel) > 0x7fffffff) {
>> +		__arch_remove_optimized_kprobe(op, 0);
>>  		return -ERANGE;
>> +	}
>>  
>>  	buf = (u8 *)op->optinsn.insn;
>>  
>>
> 
>