objtool: Remove annotate_{,un}reachable()

There are no users of annotate_reachable() left.

And the annotate_unreachable() usage in unreachable() is plain wrong;
it will hide dangerous fall-through code-gen.

Remove both.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>
Link: https://lore.kernel.org/r/20241128094312.235637588@infradead.org

Author: Peter Zijlstra

include/linux/compiler.h

@@ -109,35 +109,9 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 
 /* Unreachable code */
 #ifdef CONFIG_OBJTOOL
-/*
- * These macros help objtool understand GCC code flow for unreachable code.
- * The __COUNTER__ based labels are a hack to make each instance of the macros
- * unique, to convince GCC not to merge duplicate inline asm statements.
- */
-#define __stringify_label(n) #n
-
-#define __annotate_reachable(c) ({					\
-	asm volatile(__stringify_label(c) ":\n\t"			\
-			".pushsection .discard.reachable\n\t"		\
-			".long " __stringify_label(c) "b - .\n\t"	\
-			".popsection\n\t");				\
-})
-#define annotate_reachable() __annotate_reachable(__COUNTER__)
-
-#define __annotate_unreachable(c) ({					\
-	asm volatile(__stringify_label(c) ":\n\t"			\
-		     ".pushsection .discard.unreachable\n\t"		\
-		     ".long " __stringify_label(c) "b - .\n\t"		\
-		     ".popsection\n\t" : : "i" (c));			\
-})
-#define annotate_unreachable() __annotate_unreachable(__COUNTER__)
-
 /* Annotate a C jump table to allow objtool to follow the code flow */
 #define __annotate_jump_table __section(".rodata..c_jump_table,\"a\",@progbits #")
-
 #else /* !CONFIG_OBJTOOL */
-#define annotate_reachable()
-#define annotate_unreachable()
 #define __annotate_jump_table
 #endif /* CONFIG_OBJTOOL */
 
@@ -147,7 +121,6 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
  * control elsewhere.
  */
 #define unreachable() do {		\
-	annotate_unreachable();		\
 	barrier_before_unreachable();	\
 	__builtin_unreachable();	\
 } while (0)

tools/objtool/check.c

@@ -638,47 +638,8 @@ static int add_dead_ends(struct objtool_file *file)
 	uint64_t offset;
 
 	/*
-	 * Check for manually annotated dead ends.
-	 */
-	rsec = find_section_by_name(file->elf, ".rela.discard.unreachable");
-	if (!rsec)
-		goto reachable;
-
-	for_each_reloc(rsec, reloc) {
-		if (reloc->sym->type == STT_SECTION) {
-			offset = reloc_addend(reloc);
-		} else if (reloc->sym->local_label) {
-			offset = reloc->sym->offset;
-		} else {
-			WARN("unexpected relocation symbol type in %s", rsec->name);
-			return -1;
-		}
-
-		insn = find_insn(file, reloc->sym->sec, offset);
-		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("can't find unreachable insn at %s+0x%" PRIx64,
-				     reloc->sym->sec->name, offset);
-				return -1;
-			}
-		} else {
-			WARN("can't find unreachable insn at %s+0x%" PRIx64,
-			     reloc->sym->sec->name, offset);
-			return -1;
-		}
-
-		insn->dead_end = true;
-	}
-
-reachable:
-	/*
-	 * These manually annotated reachable checks are needed for GCC 4.4,
-	 * where the Linux unreachable() macro isn't supported.  In that case
-	 * GCC doesn't know the "ud2" is fatal, so it generates code as if it's
-	 * not a dead end.
+	 * UD2 defaults to being a dead-end, allow them to be annotated for
+	 * non-fatal, eg WARN.
 	 */
 	rsec = find_section_by_name(file->elf, ".rela.discard.reachable");
 	if (!rsec)