Re: [PATCH v9 3.2 1/9] uprobes: Install and remove breakpoints.

[Masami Hiramatsu]

(2012/01/26 0:32), Denys Vlasenko wrote:
> On Wed, Jan 25, 2012 at 4:13 PM, Denys Vlasenko
> <vda.linux@xxxxxxxxxxxxxx> wrote:
>>> +       /*
>>> +        * Convert from rip-relative addressing to indirect addressing
>>> +        * via a scratch register.  Change the r/m field from 0x5 (%rip)
>>> +        * to 0x0 (%rax) or 0x1 (%rcx), and squeeze out the offset field.
>>> +        */
>>> +       reg = MODRM_REG(insn);
>>> +       if (reg == 0) {
>>> +               /*
>>> +                * The register operand (if any) is either the A register
>>> +                * (%rax, %eax, etc.) or (if the 0x4 bit is set in the
>>> +                * REX prefix) %r8.  In any case, we know the C register
>>> +                * is NOT the register operand, so we use %rcx (register
>>> +                * #1) for the scratch register.
>>> +                */
>>> +               uprobe->arch_info.fixups = UPROBES_FIX_RIP_CX;
>>> +               /* Change modrm from 00 000 101 to 00 000 001. */
>>> +               *cursor = 0x1;
> 
> Hmm. I think we have a bug here.
> 
> What if this instruction has REX.B = 1? Granted, REX.B = 1 has no effect on
> rip-relative addressing and therefore normally won't be generated by gcc/as,
> but still. If you replace md and r/m fields as above, you are trying to convert
> 0x12345678(%rip) reference to (%rcx), but if REX.B = 1, then you in fact
> converted it to (%r9)!

Right, thanks for finding :)
And %rax register reference encoding has same problem, doesn't it?

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@xxxxxxxxxxx
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