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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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