linux-kernel - [PATCH] checkpatch: add check for snprintf to scnprintf

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Message-ID: <20240221-snprintf-checkpatch-v1-1-3ac5025b5961@google.com>
Date: Wed, 21 Feb 2024 22:05:16 +0000
From: Justin Stitt <justinstitt@...gle.com>
To: Andy Whitcroft <apw@...onical.com>, Joe Perches <joe@...ches.com>, 
	Dwaipayan Ray <dwaipayanray1@...il.com>, Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc: linux-kernel@...r.kernel.org, Lee Jones <lee@...nel.org>, 
	linux-hardening@...r.kernel.org, Kees Cook <keescook@...omium.org>, 
	Finn Thain <fthain@...ux-m68k.org>, Justin Stitt <justinstitt@...gle.com>
Subject: [PATCH] checkpatch: add check for snprintf to scnprintf

I am going to quote Lee Jones who has been doing some snprintf ->
scnprintf refactorings:

"There is a general misunderstanding amongst engineers that
{v}snprintf() returns the length of the data *actually* encoded into the
destination array.  However, as per the C99 standard {v}snprintf()
really returns the length of the data that *would have been* written if
there were enough space for it.  This misunderstanding has led to
buffer-overruns in the past.  It's generally considered safer to use the
{v}scnprintf() variants in their place (or even sprintf() in simple
cases).  So let's do that."

To help prevent new instances of snprintf() from popping up, let's add a
check to checkpatch.pl.

Suggested-by: Finn Thain <fthain@...ux-m68k.org>
Signed-off-by: Justin Stitt <justinstitt@...gle.com>
---
>From a discussion here [1].

[1]: https://lore.kernel.org/all/0f9c95f9-2c14-eee6-7faf-635880edcea4@linux-m68k.org/
---
 scripts/checkpatch.pl | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 9c4c4a61bc83..bb4e99c818a9 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -7012,6 +7012,12 @@ sub process {
 			     "Prefer strscpy, strscpy_pad, or __nonstring over strncpy - see: https://github.com/KSPP/linux/issues/90\n" . $herecurr);
 		}
 
+# snprintf uses that should likely be {v}scnprintf
+		if ($line =~ /\snprintf\s*\(\s*/) {
+				WARN("SNPRINTF",
+				     "Prefer scnprintf over snprintf\n" . $herecurr);
+		}
+
 # ethtool_sprintf uses that should likely be ethtool_puts
 		if ($line =~ /\bethtool_sprintf\s*\(\s*$FuncArg\s*,\s*$FuncArg\s*\)/) {
 			if (WARN("PREFER_ETHTOOL_PUTS",

---
base-commit: b401b621758e46812da61fa58a67c3fd8d91de0d
change-id: 20240221-snprintf-checkpatch-a864ed67ebd0

Best regards,
--
Justin Stitt <justinstitt@...gle.com>