kexec: add dmesg log symbols to /proc/vmcoreinfo lists

It would be nice to be able to extract the dmesg log from a vmcore file
without needing to keep the debug symbols for the running kernel handy all
the time.  We have a facility to do this in /proc/vmcore.  This patch adds
the log_buf and log_end symbols to the vmcoreinfo area so that tools (like
makedumpfile) can easily extract the dmesg logs from a vmcore image.

[akpm@linux-foundation.org: several fixes and cleanups]
[akpm@linux-foundation.org: fix unused log_buf_kexec_setup()]
[akpm@linux-foundation.org: build fix]
Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Cc: Simon Horman <horms@verge.net.au>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Simon Horman <horms@verge.net.au>
Cc: Vivek Goyal <vgoyal@redhat.com>
Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kernel.h

@@ -242,6 +242,7 @@ extern struct ratelimit_state printk_ratelimit_state;
 extern int printk_ratelimit(void);
 extern bool printk_timed_ratelimit(unsigned long *caller_jiffies,
 				   unsigned int interval_msec);
+void log_buf_kexec_setup(void);
 #else
 static inline int vprintk(const char *s, va_list args)
 	__attribute__ ((format (printf, 1, 0)));
@@ -253,6 +254,9 @@ static inline int printk_ratelimit(void) { return 0; }
 static inline bool printk_timed_ratelimit(unsigned long *caller_jiffies, \
 					  unsigned int interval_msec)	\
 		{ return false; }
+static inline void log_buf_kexec_setup(void)
+{
+}
 #endif
 
 extern int printk_needs_cpu(int cpu);

kernel/kexec.c

@@ -1409,6 +1409,7 @@ static int __init crash_save_vmcoreinfo_init(void)
 	VMCOREINFO_OFFSET(list_head, prev);
 	VMCOREINFO_OFFSET(vm_struct, addr);
 	VMCOREINFO_LENGTH(zone.free_area, MAX_ORDER);
+	log_buf_kexec_setup();
 	VMCOREINFO_LENGTH(free_area.free_list, MIGRATE_TYPES);
 	VMCOREINFO_NUMBER(NR_FREE_PAGES);
 	VMCOREINFO_NUMBER(PG_lru);

kernel/printk.c

@@ -32,6 +32,7 @@
 #include <linux/security.h>
 #include <linux/bootmem.h>
 #include <linux/syscalls.h>
+#include <linux/kexec.h>
 
 #include <asm/uaccess.h>
 
@@ -135,6 +136,24 @@ static char *log_buf = __log_buf;
 static int log_buf_len = __LOG_BUF_LEN;
 static unsigned logged_chars; /* Number of chars produced since last read+clear operation */
 
+#ifdef CONFIG_KEXEC
+/*
+ * This appends the listed symbols to /proc/vmcoreinfo
+ *
+ * /proc/vmcoreinfo is used by various utiilties, like crash and makedumpfile to
+ * obtain access to symbols that are otherwise very difficult to locate.  These
+ * symbols are specifically used so that utilities can access and extract the
+ * dmesg log from a vmcore file after a crash.
+ */
+void log_buf_kexec_setup(void)
+{
+	VMCOREINFO_SYMBOL(log_buf);
+	VMCOREINFO_SYMBOL(log_end);
+	VMCOREINFO_SYMBOL(log_buf_len);
+	VMCOREINFO_SYMBOL(logged_chars);
+}
+#endif
+
 static int __init log_buf_len_setup(char *str)
 {
 	unsigned size = memparse(str, &str);