objtool: Prevent infinite recursion in noreturn detection

Ingo reported an infinite loop in objtool with a certain randconfig [1].
With the given config, two functions in crypto/ablkcipher.o contained
sibling calls to each other, which threw the recursive call in
dead_end_function() for a loop (literally!).

Split the noreturn detection into two passes.  In the first pass, check
for return instructions.  In the second pass, do the potentially
recursive sibling call check.  In most cases, the first pass will be
good enough.  In the rare case where a second pass is needed, recursion
should hopefully no longer be possible.

[1] https://lkml.kernel.org/r/20160308154909.GA20956@gmail.com

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@infradead.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Bernd Petrovitsch <bernd@petrovitsch.priv.at>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Chris J Arges <chris.j.arges@canonical.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Pedro Alves <palves@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: live-patching@vger.kernel.org
Link: http://lkml.kernel.org/r/16afb602640ef43b7782087d6cca17bf6fc13603.1457502970.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/objtool/builtin-check.c

@@ -125,7 +125,7 @@ static bool ignore_func(struct objtool_file *file, struct symbol *func)
 static bool dead_end_function(struct objtool_file *file, struct symbol *func)
 {
 	int i;
-	struct instruction *insn;
+	struct instruction *insn, *func_insn;
 	bool empty = true;
 
 	/*
@@ -154,10 +154,11 @@ static bool dead_end_function(struct objtool_file *file, struct symbol *func)
 	if (!func->sec)
 		return false;
 
-	insn = find_instruction(file, func->sec, func->offset);
-	if (!insn)
+	func_insn = find_instruction(file, func->sec, func->offset);
+	if (!func_insn)
 		return false;
 
+	insn = func_insn;
 	list_for_each_entry_from(insn, &file->insns, list) {
 		if (insn->sec != func->sec ||
 		    insn->offset >= func->offset + func->len)
@@ -167,6 +168,21 @@ static bool dead_end_function(struct objtool_file *file, struct symbol *func)
 
 		if (insn->type == INSN_RETURN)
 			return false;
+	}
+
+	if (empty)
+		return false;
+
+	/*
+	 * A function can have a sibling call instead of a return.  In that
+	 * case, the function's dead-end status depends on whether the target
+	 * of the sibling call returns.
+	 */
+	insn = func_insn;
+	list_for_each_entry_from(insn, &file->insns, list) {
+		if (insn->sec != func->sec ||
+		    insn->offset >= func->offset + func->len)
+			break;
 
 		if (insn->type == INSN_JUMP_UNCONDITIONAL) {
 			struct instruction *dest = insn->jump_dest;
@@ -194,7 +210,7 @@ static bool dead_end_function(struct objtool_file *file, struct symbol *func)
 			return false;
 	}
 
-	return !empty;
+	return true;
 }
 
 /*