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 <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>

fs/pstore/Kconfig

···
 	   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

···
  */
 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

···
 #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

···
 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)
 {