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objtool,x86: Support %riz encodings

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When there's a SIB byte, the register otherwise denoted by r/m will
then be denoted by SIB.base REX.b will now extend this. SIB.index == SP
is magic and notes an index value zero.

This means that there's a bunch of alternative (longer) encodings for
the same thing. Eg. 'ModRM.mod != 3, ModRM.r/m = AX' can be encoded as
'ModRM.mod != 3, ModRM.r/m = SP, SIB.base = AX, SIB.index = SP' which is actually 4
different encodings because the value of SIB.scale is irrelevant,
giving rise to 5 different but equal encodings.

Support these encodings and clean up the SIB handling in general.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lkml.kernel.org/r/20210211173627.472967498@infradead.org
This commit is contained in:
Peter Zijlstra 2021-02-10 11:47:35 +01:00 committed by Ingo Molnar
parent 16ef7f159c
commit 78df6245c3
1 changed files with 48 additions and 19 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

67
tools/objtool/arch/x86/decode.c
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@ -72,6 +72,25 @@ unsigned long arch_jump_destination(struct instruction *insn)
return -1; \
else for (list_add_tail(&op->list, ops_list); op; op = NULL)
/*
* Helpers to decode ModRM/SIB:
*
* r/m| AX CX DX BX | SP | BP | SI DI |
* | R8 R9 R10 R11 | R12 | R13 | R14 R15 |
* Mod+----------------+-----+-----+---------+
* 00 | [r/m] |[SIB]|[IP+]| [r/m] |
* 01 | [r/m + d8] |[S+d]| [r/m + d8] |
* 10 | [r/m + d32] |[S+D]| [r/m + d32] |
* 11 | r/ m |
*
*/
#define is_RIP() ((modrm_rm & 7) == CFI_BP && modrm_mod == 0)
#define have_SIB() ((modrm_rm & 7) == CFI_SP && modrm_mod != 3)
#define rm_is(reg) (have_SIB() ? \
sib_base == (reg) && sib_index == CFI_SP : \
modrm_rm == (reg))
int arch_decode_instruction(const struct elf *elf, const struct section *sec,
unsigned long offset, unsigned int maxlen,
unsigned int *len, enum insn_type *type,
@ -83,7 +102,7 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
unsigned char op1, op2,
rex = 0, rex_b = 0, rex_r = 0, rex_w = 0, rex_x = 0,
modrm = 0, modrm_mod = 0, modrm_rm = 0, modrm_reg = 0,
sib = 0 /* , sib_scale = 0, sib_index = 0, sib_base = 0 */;
sib = 0, /* sib_scale = 0, */ sib_index = 0, sib_base = 0;
struct stack_op *op = NULL;
struct symbol *sym;
@ -125,11 +144,9 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
if (insn.sib.nbytes) {
sib = insn.sib.bytes[0];
/*
sib_scale = X86_SIB_SCALE(sib);
/* sib_scale = X86_SIB_SCALE(sib); */
sib_index = X86_SIB_INDEX(sib) + 8*rex_x;
sib_base = X86_SIB_BASE(sib) + 8*rex_b;
*/
}
switch (op1) {
@ -218,7 +235,10 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
break;
case 0x89:
if (rex_w && modrm_reg == CFI_SP) {
if (!rex_w)
break;
if (modrm_reg == CFI_SP) {
if (modrm_mod == 3) {
/* mov %rsp, reg */
@ -231,13 +251,16 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
break;
} else {
/* skip nontrivial SIB */
if ((modrm_rm & 7) == 4 && !(sib == 0x24 && rex_b == rex_x))
/* skip RIP relative displacement */
if (is_RIP())
break;
/* skip RIP relative displacement */
if ((modrm_rm & 7) == 5 && modrm_mod == 0)
break;
/* skip nontrivial SIB */
if (have_SIB()) {
modrm_rm = sib_base;
if (sib_index != CFI_SP)
break;
}
/* mov %rsp, disp(%reg) */
ADD_OP(op) {
@ -253,7 +276,7 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
break;
}
if (rex_w && modrm_mod == 3 && modrm_rm == CFI_SP) {
if (modrm_mod == 3 && modrm_rm == CFI_SP) {
/* mov reg, %rsp */
ADD_OP(op) {
@ -267,6 +290,9 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
/* fallthrough */
case 0x88:
if (!rex_w)
break;
if ((modrm_mod == 1 || modrm_mod == 2) && modrm_rm == CFI_BP) {
/* mov reg, disp(%rbp) */
@ -280,7 +306,7 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
break;
}
if (rex_w && modrm_rm == CFI_SP && sib == 0x24) {
if (modrm_mod != 3 && rm_is(CFI_SP)) {
/* mov reg, disp(%rsp) */
ADD_OP(op) {
@ -299,7 +325,7 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
if (!rex_w)
break;
if (modrm_mod == 1 && modrm_rm == CFI_BP) {
if ((modrm_mod == 1 || modrm_mod == 2) && modrm_rm == CFI_BP) {
/* mov disp(%rbp), reg */
ADD_OP(op) {
@ -312,7 +338,7 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
break;
}
if (modrm_mod != 3 && modrm_rm == CFI_SP && sib == 0x24) {
if (modrm_mod != 3 && rm_is(CFI_SP)) {
/* mov disp(%rsp), reg */
ADD_OP(op) {
@ -337,13 +363,16 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
if (!rex_w)
break;
/* skip nontrivial SIB */
if ((modrm_rm & 7) == 4 && !(sib == 0x24 && rex_b == rex_x))
/* skip RIP relative displacement */
if (is_RIP())
break;
/* skip RIP relative displacement */
if ((modrm_rm & 7) == 5 && modrm_mod == 0)
break;
/* skip nontrivial SIB */
if (have_SIB()) {
modrm_rm = sib_base;
if (sib_index != CFI_SP)
break;
}
/* lea disp(%src), %dst */
ADD_OP(op) {