On Thu, Jan 26, 2012 at 3:14 PM, Masami Hiramatsu
[off-list ref] wrote:

(2012/01/26 0:32), Denys Vlasenko wrote:

quoted

On Wed, Jan 25, 2012 at 4:13 PM, Denys Vlasenko
[off-list ref] wrote:

quoted

quoted

+       /*
+        * 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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