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SubjectRe: [PATCH][v2] uprobes/x86: emulate push insns for uprobe on x86
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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.
>

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