[RFC 01/31] x86/alternative: Refactor INT3 call emulation selftest

[Josh Poimboeuf <jpoimboe@xxxxxxxxxx>]

The INT3 call emulation selftest is a bit fragile as it relies on the
compiler not inserting any extra instructions before the
int3_selftest_ip() definition.

Also, the int3_selftest_ip() symbol overlaps with the int3_selftest
symbol(), which can confuse tooling like objtool.

Fix those issues by slightly reworking the functionality and moving
int3_selftest_ip() to a separate asm function.  While at it, improve the
naming.

Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 arch/x86/kernel/alternative.c | 51 +++++++++++++++++++----------------
 1 file changed, 28 insertions(+), 23 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 89de61243272..56fca5e6ba23 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1546,21 +1546,34 @@ int alternatives_text_reserved(void *start, void *end)
  * See entry_{32,64}.S for more details.
  */
 
-/*
- * We define the int3_magic() function in assembly to control the calling
- * convention such that we can 'call' it from assembly.
- */
-
-extern void int3_magic(unsigned int *ptr); /* defined in asm */
+extern void int3_selftest_asm(unsigned int *ptr);
 
 asm (
 "	.pushsection	.init.text, \"ax\", @progbits\n"
-"	.type		int3_magic, @function\n"
-"int3_magic:\n"
+"	.type		int3_selftest_asm, @function\n"
+"int3_selftest_asm:\n"
 	ANNOTATE_NOENDBR
-"	movl	$1, (%" _ASM_ARG1 ")\n"
+	/*
+	 * INT3 padded with NOP to CALL_INSN_SIZE. The INT3 triggers an
+	 * exception, then the int3_exception_nb notifier emulates a call to
+	 * int3_selftest_callee().
+	 */
+"	int3; nop; nop; nop; nop\n"
 	ASM_RET
-"	.size		int3_magic, .-int3_magic\n"
+"	.size		int3_selftest_asm, . - int3_selftest_asm\n"
+"	.popsection\n"
+);
+
+extern void int3_selftest_callee(unsigned int *ptr);
+
+asm (
+"	.pushsection	.init.text, \"ax\", @progbits\n"
+"	.type		int3_selftest_callee, @function\n"
+"int3_selftest_callee:\n"
+	ANNOTATE_NOENDBR
+"	movl	$0x1234, (%" _ASM_ARG1 ")\n"
+	ASM_RET
+"	.size		int3_selftest_callee, . - int3_selftest_callee\n"
 "	.popsection\n"
 );
 
@@ -1569,7 +1582,7 @@ extern void int3_selftest_ip(void); /* defined in asm below */
 static int __init
 int3_exception_notify(struct notifier_block *self, unsigned long val, void *data)
 {
-	unsigned long selftest = (unsigned long)&int3_selftest_ip;
+	unsigned long selftest = (unsigned long)&int3_selftest_asm;
 	struct die_args *args = data;
 	struct pt_regs *regs = args->regs;
 
@@ -1584,7 +1597,7 @@ int3_exception_notify(struct notifier_block *self, unsigned long val, void *data
 	if (regs->ip - INT3_INSN_SIZE != selftest)
 		return NOTIFY_DONE;
 
-	int3_emulate_call(regs, (unsigned long)&int3_magic);
+	int3_emulate_call(regs, (unsigned long)&int3_selftest_callee);
 	return NOTIFY_STOP;
 }
 
@@ -1600,19 +1613,11 @@ static noinline void __init int3_selftest(void)
 	BUG_ON(register_die_notifier(&int3_exception_nb));
 
 	/*
-	 * Basically: int3_magic(&val); but really complicated :-)
-	 *
-	 * INT3 padded with NOP to CALL_INSN_SIZE. The int3_exception_nb
-	 * notifier above will emulate CALL for us.
+	 * Basically: int3_selftest_callee(&val); but really complicated :-)
 	 */
-	asm volatile ("int3_selftest_ip:\n\t"
-		      ANNOTATE_NOENDBR
-		      "    int3; nop; nop; nop; nop\n\t"
-		      : ASM_CALL_CONSTRAINT
-		      : __ASM_SEL_RAW(a, D) (&val)
-		      : "memory");
+	int3_selftest_asm(&val);
 
-	BUG_ON(val != 1);
+	BUG_ON(val != 0x1234);
 
 	unregister_die_notifier(&int3_exception_nb);
 }
-- 
2.45.2