printk: add comment about tricky check for text buffer size

There is no check for potential "text_len" overflow.  It is not needed
because only valid level is detected.  It took me some time to
understand why.  It would deserve a comment ;-)

Signed-off-by: Petr Mladek <pmladek@suse.cz>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Kay Sievers <kay@vrfy.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index db7a02e05241..012f3e40671d 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1561,6 +1561,11 @@ asmlinkage int vprintk_emit(int facility, int level,
                         case 'd':       /* KERN_DEFAULT */
                                 lflags |= LOG_PREFIX;
                         }
+                        /*
+                         * No need to check length here because vscnprintf
+                         * put '\0' at the end of the string. Only valid and
+                         * newly printed level is detected.
+                         */
                         text_len -= end_of_header - text;
                         text = (char *)end_of_header;
                 }