printk/NMI: fix up handling of the full nmi log buffer

vsnprintf() adds the trailing '\0' but it does not count it into the
number of printed characters.  The result is that there is one byte less
space for the real characters in the buffer.

The broken check for the free space might cause that we will repeatedly
try to print 1 character into the buffer, never reach the full buffer,
and do not count the messages as missed.

Also vsnprintf() returns the number of characters that would be printed
if the buffer was big enough.  As a result, s->len might be bigger than
the size of the buffer[*].  And the printk() function might return
bigger len than it really printed.  Both problems are fixed by using
vscnprintf() instead.

Note that I though about increasing the number of missed messages even
when the message was shrunken.  But it made the code even more
complicated.  I think that it is not worth it.  Shrunken messages are
usually easy to recognize.  And it should be a corner case.

[*] The overflown s->len value is crazy and unexpected.  I "made a
mistake" and reported this situation as an internal error when fixed
handling of PR_CONT headers in some other patch.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Petr Mladek <[email protected]>
CcL Sergey Senozhatsky <[email protected]>
Cc: Chris Mason <[email protected]>
Cc: David Sterba <[email protected]>
Cc: Jason Wessel <[email protected]>
Cc: Josef Bacik <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Jaroslav Kysela <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Takashi Iwai <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 3 insertions, 2 deletions

diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
index 16bab471c7e2..152533edc56f 100644
--- a/kernel/printk/nmi.c
+++ b/kernel/printk/nmi.c
@@ -67,7 +67,8 @@ static int vprintk_nmi(const char *fmt, va_list args)
 again:
 	len = atomic_read(&s->len);
 
-	if (len >= sizeof(s->buffer)) {
+	/* The trailing '\0' is not counted into len. */
+	if (len >= sizeof(s->buffer) - 1) {
 		atomic_inc(&nmi_message_lost);
 		return 0;
 	}
@@ -79,7 +80,7 @@ again:
 	if (!len)
 		smp_rmb();
 
-	add = vsnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);
+	add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, args);
 
 	/*
 	 * Do it once again if the buffer has been flushed in the meantime.