Messages in this thread | | | From | Denys Vlasenko <> | Date | Thu, 26 Jan 2012 19:28:28 +0100 | Subject | Re: [PATCH v9 3.2 1/9] uprobes: Install and remove breakpoints. |
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On Thu, Jan 26, 2012 at 3:14 PM, Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> wrote: > (2012/01/26 0:32), Denys Vlasenko wrote: >> 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)! > > 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.
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