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)!
--
vda
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