objtool: Assume only ELF functions do sibling calls

There's an inconsistency in how sibling calls are detected in
non-function asm code, depending on the scope of the object.  If the
target code is external to the object, objtool considers it a sibling
call.  If the target code is internal but not a function, objtool
*doesn't* consider it a sibling call.

This can cause some inconsistencies between per-object and vmlinux.o
validation.

Instead, assume only ELF functions can do sibling calls.  This generally
matches existing reality, and makes sibling call validation consistent
between vmlinux.o and per-object.
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/0e9ab6f3628cc7bf3bde7aa6762d54d7df19ad78.1611263461.git.jpoimboe@redhat.com

tools/objtool/check.c

@@ -110,15 +110,20 @@ static struct instruction *prev_insn_same_sym(struct objtool_file *file,
 
 static bool is_sibling_call(struct instruction *insn)
 {
+	/*
+	 * Assume only ELF functions can make sibling calls.  This ensures
+	 * sibling call detection consistency between vmlinux.o and individual
+	 * objects.
+	 */
+	if (!insn->func)
+		return false;
+
 	/* An indirect jump is either a sibling call or a jump to a table. */
 	if (insn->type == INSN_JUMP_DYNAMIC)
 		return list_empty(&insn->alts);
 
-	if (!is_static_jump(insn))
-		return false;
-
 	/* add_jump_destinations() sets insn->call_dest for sibling calls. */
-	return !!insn->call_dest;
+	return (is_static_jump(insn) && insn->call_dest);
 }
 
 /*
@@ -774,7 +779,7 @@ static int add_jump_destinations(struct objtool_file *file)
 			continue;
 
 		reloc = find_reloc_by_dest_range(file->elf, insn->sec,
-					       insn->offset, insn->len);
+						 insn->offset, insn->len);
 		if (!reloc) {
 			dest_sec = insn->sec;
 			dest_off = arch_jump_destination(insn);
@@ -794,18 +799,21 @@ static int add_jump_destinations(struct objtool_file *file)
 
 			insn->retpoline_safe = true;
 			continue;
-		} else if (reloc->sym->sec->idx) {
-			dest_sec = reloc->sym->sec;
-			dest_off = reloc->sym->sym.st_value +
-				   arch_dest_reloc_offset(reloc->addend);
-		} else {
-			/* external sibling call */
+		} else if (insn->func) {
+			/* internal or external sibling call (with reloc) */
 			insn->call_dest = reloc->sym;
 			if (insn->call_dest->static_call_tramp) {
 				list_add_tail(&insn->static_call_node,
 					      &file->static_call_list);
 			}
 			continue;
+		} else if (reloc->sym->sec->idx) {
+			dest_sec = reloc->sym->sec;
+			dest_off = reloc->sym->sym.st_value +
+				   arch_dest_reloc_offset(reloc->addend);
+		} else {
+			/* non-func asm code jumping to another file */
+			continue;
 		}
 
 		insn->jump_dest = find_insn(file, dest_sec, dest_off);
@@ -854,7 +862,7 @@ static int add_jump_destinations(struct objtool_file *file)
 			} else if (insn->jump_dest->func->pfunc != insn->func->pfunc &&
 				   insn->jump_dest->offset == insn->jump_dest->func->offset) {
 
-				/* internal sibling call */
+				/* internal sibling call (without reloc) */
 				insn->call_dest = insn->jump_dest->func;
 				if (insn->call_dest->static_call_tramp) {
 					list_add_tail(&insn->static_call_node,
@@ -2587,7 +2595,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 
 		case INSN_JUMP_CONDITIONAL:
 		case INSN_JUMP_UNCONDITIONAL:
-			if (func && is_sibling_call(insn)) {
+			if (is_sibling_call(insn)) {
 				ret = validate_sibling_call(insn, &state);
 				if (ret)
 					return ret;
@@ -2609,7 +2617,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 
 		case INSN_JUMP_DYNAMIC:
 		case INSN_JUMP_DYNAMIC_CONDITIONAL:
-			if (func && is_sibling_call(insn)) {
+			if (is_sibling_call(insn)) {
 				ret = validate_sibling_call(insn, &state);
 				if (ret)
 					return ret;