linux-kernel - [PATCH] printk: Convert a use of sprintf to snprintf in console

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Date:   Thu, 30 Jan 2020 15:16:44 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>
Subject: [PATCH] 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: https://lore.kernel.org/lkml/20200130051711.GF115889@google.com/
Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
---
 kernel/printk/printk.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index fada22dc4ab6..a44094727a5c 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2413,9 +2413,9 @@ void console_unlock(void)
 		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;
-- 
2.25.0