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Subject[PATCH v2 10/14] x86/alternative: Handle Jcc __x86_indirect_thunk_\reg
Handle the rare cases where the compiler (clang) does an indirect
conditional tail-call using:

Jcc __x86_indirect_thunk_\reg

For the !RETPOLINE case this can be rewritten to fit the original (6
byte) instruction like:

Jncc.d8 1f
JMP *%\reg
NOP
1:

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/kernel/alternative.c | 38 ++++++++++++++++++++++++++++++++++----
1 file changed, 34 insertions(+), 4 deletions(-)

--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -393,7 +393,8 @@ static int emit_indirect(int op, int reg
static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
{
void (*target)(void);
- int reg, i = 0;
+ int reg, ret, i = 0;
+ u8 op, cc;

target = addr + insn->length + insn->immediate.value;
reg = (target - &__x86_indirect_thunk_rax) /
@@ -408,9 +409,34 @@ static int patch_retpoline(void *addr, s
if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
return -1;

- i = emit_indirect(insn->opcode.bytes[0], reg, bytes);
- if (i < 0)
- return i;
+ op = insn->opcode.bytes[0];
+
+ /*
+ * Convert:
+ *
+ * Jcc.d32 __x86_indirect_thunk_\reg
+ *
+ * into:
+ *
+ * Jncc.d8 1f
+ * jmp *%\reg
+ * nop
+ * 1:
+ */
+ if (op == 0x0f && (insn->opcode.bytes[1] & 0xf0) == 0x80) {
+ cc = insn->opcode.bytes[1] & 0xf;
+ cc ^= 1; /* invert condition */
+
+ bytes[i++] = 0x70 + cc; /* Jcc.d8 */
+ bytes[i++] = insn->length - 2;
+
+ op = JMP32_INSN_OPCODE;
+ }
+
+ ret = emit_indirect(op, reg, bytes + i);
+ if (ret < 0)
+ return ret;
+ i += ret;

for (; i < insn->length;)
bytes[i++] = BYTES_NOP1;
@@ -444,6 +470,10 @@ void __init_or_module noinline apply_ret
case JMP32_INSN_OPCODE:
break;

+ case 0x0f: /* escape */
+ if (op2 >= 0x80 && op2 <= 0x8f)
+ break;
+ fallthrough;
default:
WARN_ON_ONCE(1);
continue;

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