objtool: Fix gcov check for older versions of GCC

Objtool tries to silence 'unreachable instruction' warnings when it
detects gcov is enabled, because gcov produces a lot of unreachable
instructions and they don't really matter.

However, the 0-day bot is still reporting some unreachable instruction
warnings with CONFIG_GCOV_KERNEL=y on GCC 4.6.4.

As it turns out, objtool's gcov detection doesn't work with older
versions of GCC because they don't create a bunch of symbols with the
'gcov.' prefix like newer versions of GCC do.

Move the gcov check out of objtool and instead just create a new
'--no-unreachable' flag which can be passed in by the kernel Makefile
when CONFIG_GCOV_KERNEL is defined.

Also rename the 'nofp' variable to 'no_fp' for consistency with the new
'no_unreachable' variable.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 9cfffb116887 ("objtool: Skip all "unreachable instruction" warnings for gcov kernels")
Link: http://lkml.kernel.org/r/c243dc78eb2ffdabb6e927844dea39b6033cd395.1500939244.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

scripts/Makefile.build

@@ -262,6 +262,9 @@ objtool_args = check
 ifndef CONFIG_FRAME_POINTER
 objtool_args += --no-fp
 endif
+ifdef CONFIG_GCOV_KERNEL
+objtool_args += --no-unreachable
+endif
 
 # 'OBJECT_FILES_NON_STANDARD := y': skip objtool checking for a directory
 # 'OBJECT_FILES_NON_STANDARD_foo.o := 'y': skip objtool checking for a file

tools/objtool/builtin-check.c

@@ -29,7 +29,7 @@
 #include "builtin.h"
 #include "check.h"
 
-bool nofp;
+bool no_fp, no_unreachable;
 
 static const char * const check_usage[] = {
 	"objtool check [<options>] file.o",
@@ -37,7 +37,8 @@ static const char * const check_usage[] = {
 };
 
 const struct option check_options[] = {
-	OPT_BOOLEAN('f', "no-fp", &nofp, "Skip frame pointer validation"),
+	OPT_BOOLEAN('f', "no-fp", &no_fp, "Skip frame pointer validation"),
+	OPT_BOOLEAN('u', "no-unreachable", &no_unreachable, "Skip 'unreachable instruction' warnings"),
 	OPT_END(),
 };
 
@@ -52,5 +53,5 @@ int cmd_check(int argc, const char **argv)
 
 	objname = argv[0];
 
-	return check(objname, nofp, false);
+	return check(objname, no_fp, no_unreachable, false);
 }

tools/objtool/builtin-orc.c

@@ -37,7 +37,7 @@ static const char *orc_usage[] = {
 };
 
 extern const struct option check_options[];
-extern bool nofp;
+extern bool no_fp, no_unreachable;
 
 int cmd_orc(int argc, const char **argv)
 {
@@ -51,7 +51,7 @@ int cmd_orc(int argc, const char **argv)
 
 		objname = argv[0];
 
-		return check(objname, nofp, true);
+		return check(objname, no_fp, no_unreachable, true);
 
 	}
 

tools/objtool/check.c

@@ -33,7 +33,7 @@ struct alternative {
 };
 
 const char *objname;
-static bool nofp;
+static bool no_fp;
 struct cfi_state initial_func_cfi;
 
 struct instruction *find_insn(struct objtool_file *file,
@@ -59,19 +59,6 @@ static struct instruction *next_insn_same_sec(struct objtool_file *file,
 	return next;
 }
 
-static bool gcov_enabled(struct objtool_file *file)
-{
-	struct section *sec;
-	struct symbol *sym;
-
-	for_each_sec(file, sec)
-		list_for_each_entry(sym, &sec->symbol_list, list)
-			if (!strncmp(sym->name, "__gcov_.", 8))
-				return true;
-
-	return false;
-}
-
 #define func_for_each_insn(file, func, insn)				\
 	for (insn = find_insn(file, func->sec, func->offset);		\
 	     insn && &insn->list != &file->insn_list &&			\
@@ -1174,7 +1161,7 @@ static int update_insn_state(struct instruction *insn, struct insn_state *state)
 				regs[CFI_BP].base = CFI_BP;
 				regs[CFI_BP].offset = -state->stack_size;
 				state->bp_scratch = false;
-			} else if (!nofp) {
+			} else if (!no_fp) {
 
 				WARN_FUNC("unknown stack-related register move",
 					  insn->sec, insn->offset);
@@ -1345,7 +1332,7 @@ static int update_insn_state(struct instruction *insn, struct insn_state *state)
 		}
 
 		/* detect when asm code uses rbp as a scratch register */
-		if (!nofp && insn->func && op->src.reg == CFI_BP &&
+		if (!no_fp && insn->func && op->src.reg == CFI_BP &&
 		    cfa->base != CFI_BP)
 			state->bp_scratch = true;
 		break;
@@ -1593,7 +1580,7 @@ static int validate_branch(struct objtool_file *file, struct instruction *first,
 
 			/* fallthrough */
 		case INSN_CALL_DYNAMIC:
-			if (!nofp && func && !has_valid_stack_frame(&state)) {
+			if (!no_fp && func && !has_valid_stack_frame(&state)) {
 				WARN_FUNC("call without frame pointer save/setup",
 					  sec, insn->offset);
 				return 1;
@@ -1779,15 +1766,6 @@ static int validate_reachable_instructions(struct objtool_file *file)
 		if (insn->visited || ignore_unreachable_insn(insn))
 			continue;
 
-		/*
-		 * gcov produces a lot of unreachable instructions.  If we get
-		 * an unreachable warning and the file has gcov enabled, just
-		 * ignore it, and all other such warnings for the file.  Do
-		 * this here because this is an expensive function.
-		 */
-		if (gcov_enabled(file))
-			return 0;
-
 		WARN_FUNC("unreachable instruction", insn->sec, insn->offset);
 		return 1;
 	}
@@ -1812,13 +1790,13 @@ static void cleanup(struct objtool_file *file)
 	elf_close(file->elf);
 }
 
-int check(const char *_objname, bool _nofp, bool orc)
+int check(const char *_objname, bool _no_fp, bool no_unreachable, bool orc)
 {
 	struct objtool_file file;
 	int ret, warnings = 0;
 
 	objname = _objname;
-	nofp = _nofp;
+	no_fp = _no_fp;
 
 	file.elf = elf_open(objname, orc ? O_RDWR : O_RDONLY);
 	if (!file.elf)
@@ -1829,7 +1807,7 @@ int check(const char *_objname, bool _nofp, bool orc)
 	file.whitelist = find_section_by_name(file.elf, ".discard.func_stack_frame_non_standard");
 	file.rodata = find_section_by_name(file.elf, ".rodata");
 	file.c_file = find_section_by_name(file.elf, ".comment");
-	file.ignore_unreachables = false;
+	file.ignore_unreachables = no_unreachable;
 	file.hints = false;
 
 	arch_initial_func_cfi_state(&initial_func_cfi);

tools/objtool/check.h

@@ -61,7 +61,7 @@ struct objtool_file {
 	bool ignore_unreachables, c_file, hints;
 };
 
-int check(const char *objname, bool nofp, bool orc);
+int check(const char *objname, bool no_fp, bool no_unreachable, bool orc);
 
 struct instruction *find_insn(struct objtool_file *file,
 			      struct section *sec, unsigned long offset);