kconfig: do not write .config if the content is the same

Kconfig updates the .config when it exits even if its content is
exactly the same as before. Since its timestamp becomes newer than
that of other build artifacts, additional processing is invoked,
which is annoying.

- syncconfig is invoked to update include/config/auto.conf, etc.

- kernel/configs.o is recompiled if CONFIG_IKCONFIG is enabled,
  then vmlinux is relinked as well.

If the .config is not changed at all, we do not have to even
touch it. Just bail out showing "No change to .config".

  $ make allmodconfig
  config/conf  --allmodconfig Kconfig
  #
  # configuration written to .config
  #
  $ make allmodconfig
  config/conf  --allmodconfig Kconfig
  #
  # No change to .config
  #

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>

diff --git a/config/confdata.c b/config/confdata.c
index da98566ccc6a45e68f1be3b03e7383831a7eb1a0..da208c47fcb844e3101382fdf38dcc9f773d24b8 100644 (file)
--- a/config/confdata.c
+++ b/config/confdata.c
@@ -3,6 +3,7 @@
  * Copyright (C) 2002 Roman Zippel <zippel@linux-m68k.org>
  */
 
+#include <sys/mman.h>
 #include <sys/stat.h>
 #include <ctype.h>
 #include <errno.h>
@@ -36,6 +37,52 @@ static bool is_dir(const char *path)
        return S_ISDIR(st.st_mode);
 }
 
+/* return true if the given two files are the same, false otherwise */
+static bool is_same(const char *file1, const char *file2)
+{
+       int fd1, fd2;
+       struct stat st1, st2;
+       void *map1, *map2;
+       bool ret = false;
+
+       fd1 = open(file1, O_RDONLY);
+       if (fd1 < 0)
+               return ret;
+
+       fd2 = open(file2, O_RDONLY);
+       if (fd2 < 0)
+               goto close1;
+
+       ret = fstat(fd1, &st1);
+       if (ret)
+               goto close2;
+       ret = fstat(fd2, &st2);
+       if (ret)
+               goto close2;
+
+       if (st1.st_size != st2.st_size)
+               goto close2;
+
+       map1 = mmap(NULL, st1.st_size, PROT_READ, MAP_PRIVATE, fd1, 0);
+       if (map1 == MAP_FAILED)
+               goto close2;
+
+       map2 = mmap(NULL, st2.st_size, PROT_READ, MAP_PRIVATE, fd2, 0);
+       if (map2 == MAP_FAILED)
+               goto close2;
+
+       if (bcmp(map1, map2, st1.st_size))
+               goto close2;
+
+       ret = true;
+close2:
+       close(fd2);
+close1:
+       close(fd1);
+
+       return ret;
+}
+
 /*
  * Create the parent directory of the given path.
  *
@@ -934,6 +981,13 @@ next:
        fclose(out);
 
        if (*tmpname) {
+               if (is_same(name, tmpname)) {
+                       conf_message("No change to %s", name);
+                       unlink(tmpname);
+                       sym_set_change_count(0);
+                       return 0;
+               }
+
                snprintf(oldname, sizeof(oldname), "%s.old", name);
                rename(name, oldname);
                if (rename(tmpname, name))