>
> No, that is a meaningless operation.
> As I originally said,
>
> > insn_get_length(insn);
> > if (insn->rex_prefix.nbytes) {
> > cursor = uprobe->insn + insn_offset_rex_prefix(insn);
> > *cursor &= 0xfe; /* Clearing REX.B bit */
> > }
>
I am confused by why we need to call insn_get_length(insn) before
checking insn->rex_prefix.nbytes? Is it needed.
Denys and Masami, can you please confirm if below is fine.
#ifdef CONFIG_X86_64
/*
* 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;
/* Clear REX.b bit (extension of MODRM.rm field):
* we want to encode rax/rcx, not r8/r9.
*/
if (insn->rex_prefix.nbytes) {
cursor = uprobe->insn + insn_offset_rex_prefix(insn);
*cursor &= 0xfe;
}
/*
* 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_offset_modrm(insn);
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;
}
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