Re: [PATCH][v2] uprobes/x86: emulate push insns for uprobe on x86

[Yonghong Song <yhs@fb.com>]

On 11/13/17 4:59 AM, Oleg Nesterov wrote:
> The patch looks good to me, but I have a question because I know nothing
> about insn encoding,
> 
> On 11/10, Yonghong Song wrote:
>>
>> +static int push_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
>> +{
>> +	u8 opc1 = OPCODE1(insn), reg_offset = 0;
>> +
>> +	if (opc1 < 0x50 || opc1 > 0x57)
>> +		return -ENOSYS;
>> +
>> +	if (insn->length > 2)
>> +		return -ENOSYS;
>> +	if (insn->length == 2) {
>> +		/* only support rex_prefix 0x41 (x64 only) */
>> +#ifdef CONFIG_X86_64
>> +		if (insn->rex_prefix.nbytes != 1 ||
>> +		    insn->rex_prefix.bytes[0] != 0x41)
>> +			return -ENOSYS;
>> +
>> +		auprobe->push.ilen = 2;
> 
> and the "else" branch does
> 
> 		auprobe->push.ilen = 1;
> 
> you could add
> 		auprobe->push.ilen = insn->length;
> 
> at the end of push_setup_xol_ops() instead, but this is minor/cosmetic,

Will make this change in the next revision.

> 
> 
>> +		switch (opc1) {
>> +		case 0x50:
>> +			reg_offset = offsetof(struct pt_regs, r8);
>> +			break;
>> +		case 0x51:
>> +			reg_offset = offsetof(struct pt_regs, r9);
>> +			break;
>> +		case 0x52:
>> +			reg_offset = offsetof(struct pt_regs, r10);
>> +			break;
>> +		case 0x53:
>> +			reg_offset = offsetof(struct pt_regs, r11);
>> +			break;
>> +		case 0x54:
>> +			reg_offset = offsetof(struct pt_regs, r12);
>> +			break;
>> +		case 0x55:
>> +			reg_offset = offsetof(struct pt_regs, r13);
>> +			break;
>> +		case 0x56:
>> +			reg_offset = offsetof(struct pt_regs, r14);
>> +			break;
>> +		case 0x57:
>> +			reg_offset = offsetof(struct pt_regs, r15);
>> +			break;
>> +		}
>> +#else
>> +		return -ENOSYS;
>> +#endif
> 
> OK, but shouldn't we also return ENOSYS if CONFIG_X86_64=y but the probed task is 32bit?

Just tested with a 32bit app on x86 box and segfaults. Yes, we would 
need to return ENOSYS if the app is 32bit on 64bit system. I may not
be worthwhile to emulate this uncommon case.

I will use mmap_is_ia32 or a variant to test whether the app is
32bit or not. Please let me know whether this is correct approach or not.

> 
> Or in this case uprobe_init_insn(x86_64 => false) should fail and push_setup_xol_ops()
> won't be called?
> 
> Oleg.
>