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

[Yonghong Song]

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.