printk: Convert a use of sprintf to snprintf in console_unlock

When CONFIG_PRINTK is disabled (e.g. when building allnoconfig), clang
warns:

../kernel/printk/printk.c:2416:10: warning: 'sprintf' will always
overflow; destination buffer has size 0, but format string expands to at
least 33 [-Wfortify-source]
			len = sprintf(text,
			      ^
1 warning generated.

It is not wrong; text has a zero size when CONFIG_PRINTK is disabled
because LOG_LINE_MAX and PREFIX_MAX are both zero. Change to snprintf so
that this case is explicitly handled without any risk of overflow.

Link: https://github.com/ClangBuiltLinux/linux/issues/846
Link: https://github.com/llvm/llvm-project/commit/6d485ff455ea2b37fef9e06e426dae6c1241b231
Link: http://lkml.kernel.org/r/20200130221644.2273-1-natechancellor@gmail.com
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org
Cc: clang-built-linux@googlegroups.com
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index fada22d..a440947 100644 (file)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2413,9 +2413,9 @@ again:
                printk_safe_enter_irqsave(flags);
                raw_spin_lock(&logbuf_lock);
                if (console_seq < log_first_seq) {
-                       len = sprintf(text,
-                                     "** %llu printk messages dropped **\n",
-                                     log_first_seq - console_seq);
+                       len = snprintf(text, sizeof(text),
+                                      "** %llu printk messages dropped **\n",
+                                      log_first_seq - console_seq);
 
                        /* messages are gone, move to first one */
                        console_seq = log_first_seq;