objtool: Add support for alternatives at the end of a section

Now that the previous patch gave objtool the ability to read retpoline
alternatives, it shows a new warning:

  arch/x86/entry/entry_64.o: warning: objtool: .entry_trampoline: don't know how to handle alternatives at end of section

This is due to the JMP_NOSPEC in entry_SYSCALL_64_trampoline().

Previously, objtool ignored this situation because it wasn't needed, and
it would have required a bit of extra code.  Now that this case exists,
add proper support for it.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Guenter Roeck <linux@roeck-us.net>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/2a30a3c2158af47d891a76e69bb1ef347e0443fd.1517284349.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/objtool/check.c

@@ -594,7 +594,7 @@ static int handle_group_alt(struct objtool_file *file,
 			    struct instruction *orig_insn,
 			    struct instruction **new_insn)
 {
-	struct instruction *last_orig_insn, *last_new_insn, *insn, *fake_jump;
+	struct instruction *last_orig_insn, *last_new_insn, *insn, *fake_jump = NULL;
 	unsigned long dest_off;
 
 	last_orig_insn = NULL;
@@ -610,28 +610,30 @@ static int handle_group_alt(struct objtool_file *file,
 		last_orig_insn = insn;
 	}
 
-	if (!next_insn_same_sec(file, last_orig_insn)) {
-		WARN("%s: don't know how to handle alternatives at end of section",
-		     special_alt->orig_sec->name);
-		return -1;
-	}
-
-	fake_jump = malloc(sizeof(*fake_jump));
-	if (!fake_jump) {
-		WARN("malloc failed");
-		return -1;
+	if (next_insn_same_sec(file, last_orig_insn)) {
+		fake_jump = malloc(sizeof(*fake_jump));
+		if (!fake_jump) {
+			WARN("malloc failed");
+			return -1;
+		}
+		memset(fake_jump, 0, sizeof(*fake_jump));
+		INIT_LIST_HEAD(&fake_jump->alts);
+		clear_insn_state(&fake_jump->state);
+
+		fake_jump->sec = special_alt->new_sec;
+		fake_jump->offset = -1;
+		fake_jump->type = INSN_JUMP_UNCONDITIONAL;
+		fake_jump->jump_dest = list_next_entry(last_orig_insn, list);
+		fake_jump->ignore = true;
 	}
-	memset(fake_jump, 0, sizeof(*fake_jump));
-	INIT_LIST_HEAD(&fake_jump->alts);
-	clear_insn_state(&fake_jump->state);
-
-	fake_jump->sec = special_alt->new_sec;
-	fake_jump->offset = -1;
-	fake_jump->type = INSN_JUMP_UNCONDITIONAL;
-	fake_jump->jump_dest = list_next_entry(last_orig_insn, list);
-	fake_jump->ignore = true;
 
 	if (!special_alt->new_len) {
+		if (!fake_jump) {
+			WARN("%s: empty alternative at end of section",
+			     special_alt->orig_sec->name);
+			return -1;
+		}
+
 		*new_insn = fake_jump;
 		return 0;
 	}
@@ -654,8 +656,14 @@ static int handle_group_alt(struct objtool_file *file,
 			continue;
 
 		dest_off = insn->offset + insn->len + insn->immediate;
-		if (dest_off == special_alt->new_off + special_alt->new_len)
+		if (dest_off == special_alt->new_off + special_alt->new_len) {
+			if (!fake_jump) {
+				WARN("%s: alternative jump to end of section",
+				     special_alt->orig_sec->name);
+				return -1;
+			}
 			insn->jump_dest = fake_jump;
+		}
 
 		if (!insn->jump_dest) {
 			WARN_FUNC("can't find alternative jump destination",
@@ -670,7 +678,8 @@ static int handle_group_alt(struct objtool_file *file,
 		return -1;
 	}
 
-	list_add(&fake_jump->list, &last_new_insn->list);
+	if (fake_jump)
+		list_add(&fake_jump->list, &last_new_insn->list);
 
 	return 0;
 }