From 4a998e322abc935e95efc1a8108e6102be636a43 Mon Sep 17 00:00:00 2001 From: Petr Mladek Date: Mon, 12 Dec 2016 16:45:40 -0800 Subject: [PATCH] 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/20161208174912.GA17042@linux.suse Signed-off-by: Petr Mladek CcL Sergey Senozhatsky Cc: Chris Mason Cc: David Sterba Cc: Jason Wessel Cc: Josef Bacik Cc: Joe Perches Cc: Jaroslav Kysela Cc: Steven Rostedt Cc: Takashi Iwai Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/printk/nmi.c | 5 +++-- 1 file 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 @@ static int vprintk_nmi(const char *fmt, va_list args) 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.