objtool: Fix ANNOTATE_REACHABLE to be a normal annotation

Currently REACHABLE is weird for being on the instruction after the
instruction it modifies.

Since all REACHABLE annotations have an explicit instruction, flip
them around.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/20241128094312.494176035@infradead.org

Author: Peter Zijlstra

arch/loongarch/include/asm/bug.h

@@ -45,7 +45,7 @@
 #define __WARN_FLAGS(flags)					\
 do {								\
 	instrumentation_begin();				\
-	__BUG_FLAGS(BUGFLAG_WARNING|(flags), ANNOTATE_REACHABLE);\
+	__BUG_FLAGS(BUGFLAG_WARNING|(flags), ANNOTATE_REACHABLE(10001b));\
 	instrumentation_end();					\
 } while (0)
 

arch/x86/entry/entry_64.S

@@ -308,10 +308,9 @@ SYM_CODE_END(xen_error_entry)
 		movq	$-1, ORIG_RAX(%rsp)	/* no syscall to restart */
 	.endif
 
-	call	\cfunc
-
 	/* For some configurations \cfunc ends up being a noreturn. */
 	ANNOTATE_REACHABLE
+	call	\cfunc
 
 	jmp	error_return
 .endm
@@ -529,10 +528,10 @@ SYM_CODE_START(\asmsym)
 	movq	%rsp, %rdi		/* pt_regs pointer into first argument */
 	movq	ORIG_RAX(%rsp), %rsi	/* get error code into 2nd argument*/
 	movq	$-1, ORIG_RAX(%rsp)	/* no syscall to restart */
-	call	\cfunc
 
 	/* For some configurations \cfunc ends up being a noreturn. */
 	ANNOTATE_REACHABLE
+	call	\cfunc
 
 	jmp	paranoid_exit
 

arch/x86/include/asm/bug.h

@@ -92,7 +92,7 @@ do {								\
 do {								\
 	__auto_type __flags = BUGFLAG_WARNING|(flags);		\
 	instrumentation_begin();				\
-	_BUG_FLAGS(ASM_UD2, __flags, ANNOTATE_REACHABLE);	\
+	_BUG_FLAGS(ASM_UD2, __flags, ANNOTATE_REACHABLE(1b));	\
 	instrumentation_end();					\
 } while (0)
 

arch/x86/include/asm/irq_stack.h

@@ -100,8 +100,8 @@
 }
 
 #define ASM_CALL_ARG0							\
-	"call %c[__func]				\n"		\
-	ANNOTATE_REACHABLE
+	"1: call %c[__func]				\n"		\
+	ANNOTATE_REACHABLE(1b)
 
 #define ASM_CALL_ARG1							\
 	"movq	%[arg1], %%rdi				\n"		\

include/linux/objtool.h

@@ -177,11 +177,11 @@
  */
 #define ANNOTATE_UNRET_BEGIN		ASM_ANNOTATE(ANNOTYPE_UNRET_BEGIN)
 /*
- * This should be used directly after an instruction that is considered
+ * This should be used to refer to an instruction that is considered
  * terminating, like a noreturn CALL or UD2 when we know they are not -- eg
  * WARN using UD2.
  */
-#define ANNOTATE_REACHABLE		ASM_ANNOTATE(ANNOTYPE_REACHABLE)
+#define ANNOTATE_REACHABLE(label)	__ASM_ANNOTATE(label, ANNOTYPE_REACHABLE)
 
 #else
 #define ANNOTATE_NOENDBR		ANNOTATE type=ANNOTYPE_NOENDBR

tools/objtool/check.c

@@ -614,19 +614,6 @@ static int init_pv_ops(struct objtool_file *file)
 	return 0;
 }
 
-static struct instruction *find_last_insn(struct objtool_file *file,
-					  struct section *sec)
-{
-	struct instruction *insn = NULL;
-	unsigned int offset;
-	unsigned int end = (sec->sh.sh_size > 10) ? sec->sh.sh_size - 10 : 0;
-
-	for (offset = sec->sh.sh_size - 1; offset >= end && !insn; offset--)
-		insn = find_insn(file, sec, offset);
-
-	return insn;
-}
-
 static int create_static_call_sections(struct objtool_file *file)
 {
 	struct static_call_site *site;
@@ -2281,16 +2268,6 @@ static int read_annotate(struct objtool_file *file,
 		offset = reloc->sym->offset + reloc_addend(reloc);
 		insn = find_insn(file, reloc->sym->sec, offset);
 
-		/*
-		 * Reachable annotations are 'funneh' and act on the previous instruction :/
-		 */
-		if (type == ANNOTYPE_REACHABLE) {
-			if (insn)
-				insn = prev_insn_same_sec(file, insn);
-			else if (offset == reloc->sym->sec->sh.sh_size)
-				insn = find_last_insn(file, reloc->sym->sec);
-		}
-
 		if (!insn) {
 			WARN("bad .discard.annotate_insn entry: %d of type %d", reloc_idx(reloc), type);
 			return -1;