objtool: Detect missing __noreturn annotations

Most "unreachable instruction" warnings these days seem to actually be
the result of a missing __noreturn annotation. Add an explicit check
for that.

Reviewed-by: Miroslav Benes <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Josh Poimboeuf <[e-mail]>

tools/objtool/Documentation/objtool.txt

···
    If it's not actually in a callable function (e.g. kernel entry code),
    change ENDPROC to END.
 
+3. file.o: warning: objtool: foo+0x48c: bar() is missing a __noreturn annotation
+
+   The call from foo() to bar() doesn't return, but bar() is missing the
+   __noreturn annotation.  NOTE: In addition to adding the __noreturn
+   annotation, the function name also needs to be added to
+   'global_noreturns' in tools/objtool/check.c.
 
 4. file.o: warning: objtool: func(): can't find starting instruction
    or

tools/objtool/check.c

···
 
 static int validate_reachable_instructions(struct objtool_file *file)
 {
-	struct instruction *insn;
+	struct instruction *insn, *prev_insn;
+	struct symbol *call_dest;
 	int warnings = 0;
 
 	if (file->ignore_unreachables)
···
 	for_each_insn(file, insn) {
 		if (insn->visited || ignore_unreachable_insn(file, insn))
 			continue;
+
+		prev_insn = prev_insn_same_sec(file, insn);
+		if (prev_insn && prev_insn->dead_end) {
+			call_dest = insn_call_dest(prev_insn);
+			if (call_dest) {
+				WARN_INSN(insn, "%s() is missing a __noreturn annotation",
+					  call_dest->name);
+				warnings++;
+				continue;
+			}
+		}
 
 		WARN_INSN(insn, "unreachable instruction");
 		warnings++;