x86/insn,uprobes,alternative: Unify insn_is_nop()

Both uprobes and alternatives have insn_is_nop() variants, unify them
and make sure insn_is_nop() works for both x86_64 and i386.

Specifically, uprobe must not compare userspace instructions to kernel
nops as that does not work right in the compat case.

For the uprobe case we therefore must recognise common 32bit and 64bit
nops. Because uprobe will consume the instruction as a nop, it must
not mistakenly claim a non-nop instruction to be a nop. Eg. 'REX.b3
NOP' is 'xchg %r8,%rax' - not a nop.

For the kernel case similar constraints apply, is it used to optimize
NOPs by replacing strings of short(er) nops with longer nops. Must not
claim an instruction is a nop if it really isn't. Not recognising a
nop is non-fatal.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

2 files changed, 4 insertions, 48 deletions

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 8ee5ff547357..58ce2bdfb3f6 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -9,6 +9,7 @@
 
 #include <asm/text-patching.h>
 #include <asm/insn.h>
+#include <asm/insn-eval.h>
 #include <asm/ibt.h>
 #include <asm/set_memory.h>
 #include <asm/nmi.h>
@@ -346,25 +347,6 @@ static void add_nop(u8 *buf, unsigned int len)
 }
 
 /*
- * Matches NOP and NOPL, not any of the other possible NOPs.
- */
-static bool insn_is_nop(struct insn *insn)
-{
-	/* Anything NOP, but no REP NOP */
-	if (insn->opcode.bytes[0] == 0x90 &&
-	    (!insn->prefixes.nbytes || insn->prefixes.bytes[0] != 0xF3))
-		return true;
-
-	/* NOPL */
-	if (insn->opcode.bytes[0] == 0x0F && insn->opcode.bytes[1] == 0x1F)
-		return true;
-
-	/* TODO: more nops */
-
-	return false;
-}
-
-/*
  * Find the offset of the first non-NOP instruction starting at @offset
  * but no further than @len.
  */
diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 845aeaf36b8d..6318898eb9d0 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -17,6 +17,7 @@
 #include <linux/kdebug.h>
 #include <asm/processor.h>
 #include <asm/insn.h>
+#include <asm/insn-eval.h>
 #include <asm/mmu_context.h>
 #include <asm/nops.h>
 
@@ -1158,35 +1159,12 @@ unlock:
 	mmap_write_unlock(mm);
 }
 
-static bool insn_is_nop(struct insn *insn)
-{
-	return insn->opcode.nbytes == 1 && insn->opcode.bytes[0] == 0x90;
-}
-
-static bool insn_is_nopl(struct insn *insn)
-{
-	if (insn->opcode.nbytes != 2)
-		return false;
-
-	if (insn->opcode.bytes[0] != 0x0f || insn->opcode.bytes[1] != 0x1f)
-		return false;
-
-	if (!insn->modrm.nbytes)
-		return false;
-
-	if (X86_MODRM_REG(insn->modrm.bytes[0]) != 0)
-		return false;
-
-	/* 0f 1f /0 - NOPL */
-	return true;
-}
-
 static bool can_optimize(struct insn *insn, unsigned long vaddr)
 {
 	if (!insn->x86_64 || insn->length != 5)
 		return false;
 
-	if (!insn_is_nop(insn) && !insn_is_nopl(insn))
+	if (!insn_is_nop(insn))
 		return false;
 
 	/* We can't do cross page atomic writes yet. */
@@ -1428,17 +1406,13 @@ static int branch_setup_xol_ops(struct arch_uprobe *auprobe, struct insn *insn)
 	insn_byte_t p;
 	int i;
 
-	/* x86_nops[insn->length]; same as jmp with .offs = 0 */
-	if (insn->length <= ASM_NOP_MAX &&
-	    !memcmp(insn->kaddr, x86_nops[insn->length], insn->length))
+	if (insn_is_nop(insn))
 		goto setup;
 
 	switch (opc1) {
 	case 0xeb:	/* jmp 8 */
 	case 0xe9:	/* jmp 32 */
 		break;
-	case 0x90:	/* prefix* + nop; same as jmp with .offs = 0 */
-		goto setup;
 
 	case 0xe8:	/* call relative */
 		branch_clear_offset(auprobe, insn);