printk: add interfaces for external access to the log buffer

Add two new functions for reading the kernel log buffer.  The intention is for
them 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.

[akpm@linux-foundation.org: buncha fixes]
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Robin Getz <rgetz@blackfin.uclinux.org>
Cc: Greg Ungerer <gerg@snapgear.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Paul Mundt <lethal@linux-sh.org>
Acked-by: Tim Bird <tim.bird@am.sony.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/printk.c

@@ -219,6 +219,58 @@ static inline void boot_delay_msec(void)
 }
 #endif
 
+/*
+ * Return the number of unread characters in the log buffer.
+ */
+int log_buf_get_len(void)
+{
+	return logged_chars;
+}
+
+/*
+ * Copy a range of characters from the log buffer.
+ */
+int log_buf_copy(char *dest, int idx, int len)
+{
+	int ret, max;
+	bool took_lock = false;
+
+	if (!oops_in_progress) {
+		spin_lock_irq(&logbuf_lock);
+		took_lock = true;
+	}
+
+	max = log_buf_get_len();
+	if (idx < 0 || idx >= max) {
+		ret = -1;
+	} else {
+		if (len > max)
+			len = max;
+		ret = len;
+		idx += (log_end - max);
+		while (len-- > 0)
+			dest[len] = LOG_BUF(idx + len);
+	}
+
+	if (took_lock)
+		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:
  *