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Date: Mon, 22 Apr 2024 21:33:31 +0000
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Message-ID: <20240422-snprintf-checkpatch-v5-1-f1e90bf7164e@google.com>
Subject: [PATCH v5] checkpatch: add check for snprintf to scnprintf
From: Justin Stitt <justinstitt@google.com>
To: Andy Whitcroft <apw@canonical.com>, Joe Perches <joe@perches.com>, 
	Dwaipayan Ray <dwaipayanray1@gmail.com>, Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: linux-kernel@vger.kernel.org, Lee Jones <lee@kernel.org>, 
	linux-hardening@vger.kernel.org, Kees Cook <keescook@chromium.org>, 
	Finn Thain <fthain@linux-m68k.org>, Justin Stitt <justinstitt@google.com>
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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@linux-m68k.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
---
Changes in v5:
- use capture groups to let the user know which variation they used
- Link to v4: https://lore.kernel.org/r/20240408-snprintf-checkpatch-v4-1-8697c96ac94b@google.com

Changes in v4:
- also check for vsnprintf variant (thanks Bill)
- Link to v3: https://lore.kernel.org/r/20240315-snprintf-checkpatch-v3-1-a451e7664306@google.com

Changes in v3:
- fix indentation
- add reference link (https://github.com/KSPP/linux/issues/105) (thanks Joe)
- Link to v2: https://lore.kernel.org/r/20240221-snprintf-checkpatch-v2-1-9baeb59dae30@google.com

Changes in v2:
- Had a vim moment and deleted a character before sending the patch.
- Replaced the character :)
- Link to v1: https://lore.kernel.org/r/20240221-snprintf-checkpatch-v1-1-3ac5025b5961@google.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..b7404e53c554 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);
 		}
 
+# {v}snprintf uses that should likely be {v}scnprintf
+		if ($line =~ /\b((v|)snprintf\s*\()/) {
+			WARN("SNPRINTF",
+			     "Prefer ${2}scnprintf over $1 - see: https://github.com/KSPP/linux/issues/105\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@google.com>