(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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