On Tue, Jan 10, 2012 at 12:48 PM, Srikar Dronamraju
[off-list ref] wrote:

+/*
+ * If uprobe->insn doesn't use rip-relative addressing, return
+ * immediately.  Otherwise, rewrite the instruction so that it accesses
+ * its memory operand indirectly through a scratch register.  Set
+ * uprobe->arch_info.fixups and uprobe->arch_info.rip_rela_target_address
+ * accordingly.  (The contents of the scratch register will be saved
+ * before we single-step the modified instruction, and restored
+ * afterward.)
+ *
+ * We do this because a rip-relative instruction can access only a
+ * relatively small area (+/- 2 GB from the instruction), and the XOL
+ * area typically lies beyond that area.  At least for instructions
+ * that store to memory, we can't execute the original instruction
+ * and "fix things up" later, because the misdirected store could be
+ * disastrous.
+ *
+ * Some useful facts about rip-relative instructions:
+ * - There's always a modrm byte.
+ * - There's never a SIB byte.
+ * - The displacement is always 4 bytes.
+ */
+static void handle_riprel_insn(struct mm_struct *mm, struct uprobe *uprobe,
+                                                       struct insn *insn)
+{
+       u8 *cursor;
+       u8 reg;
+
+       if (mm->context.ia32_compat)
+               return;
+
+       uprobe->arch_info.rip_rela_target_address = 0x0;
+       if (!insn_rip_relative(insn))
+               return;
+
+       /*
+        * Point cursor at the modrm byte.  The next 4 bytes are the
+        * displacement.  Beyond the displacement, for some instructions,
+        * is the immediate operand.
+        */
+       cursor = uprobe->insn + insn->prefixes.nbytes
+                       + insn->rex_prefix.nbytes + insn->opcode.nbytes;
+       insn_get_length(insn);
+
+       /*
+        * 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;
+       } else {
+               /* Use %rax (register #0) for the scratch register. */
+               uprobe->arch_info.fixups = UPROBES_FIX_RIP_AX;
+               /* Change modrm from 00 xxx 101 to 00 xxx 000 */
+               *cursor = (reg << 3);
+       }
+
+       /* Target address = address of next instruction + (signed) offset */
+       uprobe->arch_info.rip_rela_target_address = (long)insn->length
+                                       + insn->displacement.value;
+       /* Displacement field is gone; slide immediate field (if any) over. */
+       if (insn->immediate.nbytes) {
+               cursor++;
+               memmove(cursor, cursor + insn->displacement.nbytes,
+                                               insn->immediate.nbytes);
+       }
+       return;
+}

It seems to be possible to store RIP value *without displacement*
into AX/CX and convert rip-relative instruction into AX/CX *relative* one.
Example:
c7 05 78 56 34 12 2a 00 00 00 	movl   $0x2a,0x12345678(%rip)
converts to:
c7 81 78 56 34 12 2a 00 00 00 	movl   $0x2a,0x12345678(%rcx)

This way instruction size stays the same and you don't need
to memmove immediate value.

-- 
vda

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