objtool: Track original function across branches

If 'insn->func' is NULL, objtool skips some important checks, including
sibling call validation.  So if some .fixup code does an invalid sibling
call, objtool ignores it.

Treat all code branches (including alts) as part of the original
function by keeping track of the original func value from
validate_functions().

This improves the usefulness of some clang function fallthrough
warnings, and exposes some additional kernel bugs in the process.
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/505df630f33c9717e1ccde6e4b64c5303135c25f.1563413318.git.jpoimboe@redhat.com

tools/objtool/check.c

@@ -1934,13 +1934,12 @@ static int validate_sibling_call(struct instruction *insn, struct insn_state *st
  * each instruction and validate all the rules described in
  * tools/objtool/Documentation/stack-validation.txt.
  */
-static int validate_branch(struct objtool_file *file, struct instruction *first,
-			   struct insn_state state)
+static int validate_branch(struct objtool_file *file, struct symbol *func,
+			   struct instruction *first, struct insn_state state)
 {
 	struct alternative *alt;
 	struct instruction *insn, *next_insn;
 	struct section *sec;
-	struct symbol *func = NULL;
 	int ret;
 
 	insn = first;
@@ -1961,9 +1960,6 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 			return 1;
 		}
 
-		if (insn->func)
-			func = insn->func->pfunc;
-
 		if (func && insn->ignore) {
 			WARN_FUNC("BUG: why am I validating an ignored function?",
 				  sec, insn->offset);
@@ -1985,7 +1981,7 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 
 				i = insn;
 				save_insn = NULL;
-				func_for_each_insn_continue_reverse(file, insn->func, i) {
+				func_for_each_insn_continue_reverse(file, func, i) {
 					if (i->save) {
 						save_insn = i;
 						break;
@@ -2031,7 +2027,7 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 				if (alt->skip_orig)
 					skip_orig = true;
 
-				ret = validate_branch(file, alt->insn, state);
+				ret = validate_branch(file, func, alt->insn, state);
 				if (ret) {
 					if (backtrace)
 						BT_FUNC("(alt)", insn);
@@ -2069,7 +2065,7 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 
 			if (state.bp_scratch) {
 				WARN("%s uses BP as a scratch register",
-				     insn->func->name);
+				     func->name);
 				return 1;
 			}
 
@@ -2109,8 +2105,8 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 			} else if (insn->jump_dest &&
 				   (!func || !insn->jump_dest->func ||
 				    insn->jump_dest->func->pfunc == func)) {
-				ret = validate_branch(file, insn->jump_dest,
-						      state);
+				ret = validate_branch(file, func,
+						      insn->jump_dest, state);
 				if (ret) {
 					if (backtrace)
 						BT_FUNC("(branch)", insn);
@@ -2176,7 +2172,7 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 			break;
 
 		case INSN_CLAC:
-			if (!state.uaccess && insn->func) {
+			if (!state.uaccess && func) {
 				WARN_FUNC("redundant UACCESS disable", sec, insn->offset);
 				return 1;
 			}
@@ -2197,7 +2193,7 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 			break;
 
 		case INSN_CLD:
-			if (!state.df && insn->func)
+			if (!state.df && func)
 				WARN_FUNC("redundant CLD", sec, insn->offset);
 
 			state.df = false;
@@ -2236,7 +2232,7 @@ static int validate_unwind_hints(struct objtool_file *file)
 
 	for_each_insn(file, insn) {
 		if (insn->hint && !insn->visited) {
-			ret = validate_branch(file, insn, state);
+			ret = validate_branch(file, insn->func, insn, state);
 			if (ret && backtrace)
 				BT_FUNC("<=== (hint)", insn);
 			warnings += ret;
@@ -2363,12 +2359,12 @@ static int validate_functions(struct objtool_file *file)
 				continue;
 
 			insn = find_insn(file, sec, func->offset);
-			if (!insn || insn->ignore)
+			if (!insn || insn->ignore || insn->visited)
 				continue;
 
 			state.uaccess = func->alias->uaccess_safe;
 
-			ret = validate_branch(file, insn, state);
+			ret = validate_branch(file, func, insn, state);
 			if (ret && backtrace)
 				BT_FUNC("<=== (func)", insn);
 			warnings += ret;