Re: [PATCH][v3] uprobes/x86: emulate push insns for uprobe on x86

[Yonghong Song <yhs@fb.com>]

On 11/14/17 8:03 AM, Oleg Nesterov wrote:
> On 11/14, Oleg Nesterov wrote:
>>
>>> +#ifdef CONFIG_X86_64
>>> +	if (test_thread_flag(TIF_ADDR32))
>>> +		return -ENOSYS;
>>> +#endif
>>
>> No, this doesn't look right, see my previous email. You should do this
>> check in the "if (insn->length == 2)" branch below, "push bp" should be
>> emulated correctly.
>>
>> And test_thread_flag(TIF_ADDR32) is not right too. The caller is not
>> necessarily the probed task. See is_64bit_mm(mm) in arch_uprobe_analyze_insn().
>>
>> And again... please check if uprobe_init_insn() fails or not in this case
>> (32bit task does, say, "push r8"). If it fails, your V2 should be fine.
>>
>>
>> To remind, uprobes && 32-bit is broken, let me quote my another email:
>>
>> 	The 3rd bug means that you simply can't uprobe a 32bit task on a 64bit
>> 	system, the in_compat_syscall() logic in get_unmapped_area() looks very
>> 	wrong although I need to re-check.
> 
> Yes,
> 
>> I didn't have time for this problem so far. But emulation should work, so
>> you can hopefully test your patch.
> 
> Ah, no, sizeof_long() is broken by the same reason, so you can't test it...

Right. I hacked the emulate_push_stack (original name: push_ret_address) 
with sizeof_long = 4, and 32bit binary uprobe works fine on x86_64 
platform then... But that will involve a bigger change to propogate
the is_64bit_mm() along the call graph.

> 
> OK, I'll try to do something tomorrow, then we will see what can we do
> with your patch...

Thanks for reviewing! I will wait for your further comments/direction
before next step.

> 
> But it would be nice if you can check what uprobe_init_insn() does in this
> case, see above.

As mentioned in my previous email, for 32bit application,
compiler won't generate "push %r8" as "%r8" is only available on
x86_64 platform. For 32bit app, I see "push %bp" etc which does not
have rex_prefix. They cannot be emulated right now due to 
emulate_push_stack needs change.

> 
> Oleg.
>