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>

5 files changed, 17 insertions, 35 deletions

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 4a9a2cec0a1b..854608d42e85 100644
--- a/scripts/Makefile.build
+++ b/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
diff --git a/tools/objtool/builtin-check.c b/tools/objtool/builtin-check.c
index eedf089b1495..57254f5b2779 100644
--- a/tools/objtool/builtin-check.c
+++ b/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);
 }
diff --git a/tools/objtool/builtin-orc.c b/tools/objtool/builtin-orc.c
index 5ca41ab0df48..4c6b5c9ef073 100644
--- a/tools/objtool/builtin-orc.c
+++ b/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);
 
         }
 
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 368275de5f23..3436a942b606 100644
--- a/tools/objtool/check.c
+++ b/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);
diff --git a/tools/objtool/check.h b/tools/objtool/check.h
index ac3d4b13f17b..c9af11f0c8af 100644
--- a/tools/objtool/check.h
+++ b/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);