Messages in this thread | | | From | Denys Vlasenko <> | Date | Wed, 25 Jan 2012 16:32:08 +0100 | Subject | Re: [PATCH v9 3.2 1/9] uprobes: Install and remove breakpoints. |
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On Wed, Jan 25, 2012 at 4:13 PM, Denys Vlasenko <vda.linux@googlemail.com> 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)!
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