pstore: Move kmsg_bytes default into Kconfig

While kmsg_bytes can be set for pstore via mount, if a crash occurs
before the mount only partial console log will be stored as kmsg_bytes
defaults to a potentially different hardcoded value in the kernel
(PSTORE_DEFAULT_KMSG_BYTES). This makes it impossible to analyze valuable
post-mortem data especially on the embedded development or in the process
of bringing up new boards. Change this value to be a Kconfig option
with the default of old PSTORE_DEFAULT_KMSG_BYTES

Signed-off-by: Vasile-Laurentiu Stanimir <vasile-laurentiu.stanimir@windriver.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

fs/pstore/Kconfig

@@ -14,6 +14,14 @@ config PSTORE
 	   If you don't have a platform persistent store driver,
 	   say N.
 
+config PSTORE_DEFAULT_KMSG_BYTES
+	int "Default kernel log storage space" if EXPERT
+	depends on PSTORE
+	default "10240"
+	help
+	  Defines default size of pstore kernel log storage.
+	  Can be enlarged if needed, not recommended to shrink it.
+
 config PSTORE_DEFLATE_COMPRESS
 	tristate "DEFLATE (ZLIB) compression"
 	default y

fs/pstore/inode.c

@@ -266,7 +266,7 @@ static void parse_options(char *options)
  */
 static int pstore_show_options(struct seq_file *m, struct dentry *root)
 {
-	if (kmsg_bytes != PSTORE_DEFAULT_KMSG_BYTES)
+	if (kmsg_bytes != CONFIG_PSTORE_DEFAULT_KMSG_BYTES)
 		seq_printf(m, ",kmsg_bytes=%lu", kmsg_bytes);
 	return 0;
 }

fs/pstore/internal.h

@@ -6,7 +6,6 @@
 #include <linux/time.h>
 #include <linux/pstore.h>
 
-#define PSTORE_DEFAULT_KMSG_BYTES 10240
 extern unsigned long kmsg_bytes;
 
 #ifdef CONFIG_PSTORE_FTRACE

fs/pstore/platform.c

@@ -101,7 +101,7 @@ static char *big_oops_buf;
 static size_t big_oops_buf_sz;
 
 /* How much of the console log to snapshot */
-unsigned long kmsg_bytes = PSTORE_DEFAULT_KMSG_BYTES;
+unsigned long kmsg_bytes = CONFIG_PSTORE_DEFAULT_KMSG_BYTES;
 
 void pstore_set_kmsg_bytes(int bytes)
 {