On Thu, Jan 26, 2012 at 3:14 PM, Masami Hiramatsu
<masami.hiramatsu.pt@xxxxxxxxxxx> wrote:
> (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?
Yes.
The solution is trivial: "if (REX pfx exists) REX.B = 0;"
Also, I don't remember whether (%rip) addressing is
affected by 0x67 (address size) prefix. If it is, then
nothing needs to be done.
But if there is an exception and CPU ignores 0x67 pfx
for (%rip) addressing, we will need to check for and
remove this pfx.
Otherwise, it'll instruct to use 32-bit registers
for addressing, thus it will turn our (%rax) operand
into (%eax). Not good.
--
vda
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