On Tue, Jan 10, 2012 at 12:48 PM, Srikar Dronamraju
<srikar@xxxxxxxxxxxxxxxxxx> 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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