objtool: Support asm jump tables

Objtool detection of asm jump tables would normally just work, except
for the fact that asm retpolines use alternatives.  Objtool thinks the
alternative code path (a jump to the retpoline) is a sibling call.

Don't treat alternative indirect branches as sibling calls when the
original instruction has a jump table.
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Tested-by: Sami Tolvanen <samitolvanen@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Link: https://lore.kernel.org/r/460cf4dc675d64e1124146562cabd2c05aa322e8.1614182415.git.jpoimboe@redhat.com

tools/objtool/check.c

@@ -108,6 +108,18 @@ static struct instruction *prev_insn_same_sym(struct objtool_file *file,
 	for (insn = next_insn_same_sec(file, insn); insn;		\
 	     insn = next_insn_same_sec(file, insn))
 
+static bool is_jump_table_jump(struct instruction *insn)
+{
+	struct alt_group *alt_group = insn->alt_group;
+
+	if (insn->jump_table)
+		return true;
+
+	/* Retpoline alternative for a jump table? */
+	return alt_group && alt_group->orig_group &&
+	       alt_group->orig_group->first_insn->jump_table;
+}
+
 static bool is_sibling_call(struct instruction *insn)
 {
 	/*
@@ -120,7 +132,7 @@ static bool is_sibling_call(struct instruction *insn)
 
 	/* 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);
+		return !is_jump_table_jump(insn);
 
 	/* add_jump_destinations() sets insn->call_dest for sibling calls. */
 	return (is_static_jump(insn) && insn->call_dest);