Add two new functions for reading the kernel log buffer.  Their intention
is to be used by recovery/dump/debug code so the kernel log can be easily
retrieved/parsed in a crash scenario, but they are generic enough for other
people to dream up other fun uses.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
---
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 4300bb4..8d5b6c7 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -156,6 +156,9 @@ asmlinkage int vprintk(const char *fmt, va_list args)
 	__attribute__ ((format (printf, 1, 0)));
 asmlinkage int printk(const char * fmt, ...)
 	__attribute__ ((format (printf, 1, 2))) __cold;
+extern int log_buf_get_len(void);
+extern int log_buf_read(int idx);
+extern int log_buf_copy(char *dest, int idx, int len);
 #else
 static inline int vprintk(const char *s, va_list args)
 	__attribute__ ((format (printf, 1, 0)));
@@ -163,6 +166,9 @@ static inline int vprintk(const char *s, va_list args) { return 0; }
 static inline int printk(const char *s, ...)
 	__attribute__ ((format (printf, 1, 2)));
 static inline int __cold printk(const char *s, ...) { return 0; }
+static inline int log_buf_get_len(void) { return 0; }
+static inline int log_buf_read(int idx); { return 0; }
+static inline int log_buf_copy(char *dest, int idx, int len) { return 0; }
 #endif
 
 unsigned long int_sqrt(unsigned long);
diff --git a/kernel/printk.c b/kernel/printk.c
index 051d27e..35b231a 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -163,6 +163,55 @@ out:
 __setup("log_buf_len=", log_buf_len_setup);
 
 /*
+ * Return the number of unread characters in the log buffer.
+ */
+int log_buf_get_len(void)
+{
+	return log_end - log_start;
+}
+
+/*
+ * Copy a range of characters from the log buffer.
+ */
+int log_buf_copy(char *dest, int idx, int len)
+{
+	int ret, max;
+
+	if (!oops_in_progress)
+		spin_lock_irq(&logbuf_lock);
+
+	max = log_buf_get_len();
+	if (idx < 0 || idx >= max)
+		ret = -1;
+	else {
+		if (len > max)
+			len = max;
+		ret = len;
+		while (len-- > 0) {
+			*dest = LOG_BUF(idx++);
+			++dest;
+		}
+	}
+
+	if (!oops_in_progress)
+		spin_unlock_irq(&logbuf_lock);
+
+	return ret;
+}
+
+/*
+ * Extract a single character from the log buffer.
+ */
+int log_buf_read(int idx)
+{
+	char ret;
+	if (log_buf_copy(&ret, idx, 1) == 1)
+		return ret;
+	else
+		return -1;
+}
+
+/*
  * Commands to do_syslog:
  *
  * 	0 -- Close the log.  Currently a NOP.