linux-hardening - [PATCH] checkpatch: Warn about 0-length and 1-element arrays

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Date:   Wed, 17 May 2023 13:45:37 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andy Whitcroft <apw@...onical.com>
Cc:     Kees Cook <keescook@...omium.org>, Joe Perches <joe@...ches.com>,
        Dwaipayan Ray <dwaipayanray1@...il.com>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: [PATCH] checkpatch: Warn about 0-length and 1-element arrays

Fake flexible arrays have been deprecated since last millennium. Proper
C99 flexible arrays must be used throughout the kernel so
CONFIG_FORTIFY_SOURCE and CONFIG_UBSAN_BOUNDS can provide proper array
bounds checking.

Cc: Andy Whitcroft <apw@...onical.com>
Cc: Joe Perches <joe@...ches.com>
Cc: Dwaipayan Ray <dwaipayanray1@...il.com>
Cc: Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc: Gustavo A. R. Silva <gustavoars@...nel.org>
Signed-off-by: Kees Cook <keescook@...omium.org>
---
I couldn't find a better way to figure out if a patch line is part of
a structure definition. I'm open to ideas! e.g. this would have
caught commit f5823fe6897c.
---
 scripts/checkpatch.pl | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index a90e0ede53ad..415b3c14589b 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -7430,6 +7430,20 @@ sub process {
 			}
 		}
 
+# check for fake flexible array declarations (i.e. not in function scope)
+		if (!defined($context_function)) {
+			if ($line =~ /^\+\s*((struct|union|enum)\s+$Ident|$Type|})\s+$Ident\s*\[0\][^;]*;/) {
+				if (WARN("ZERO_LENGTH_ARRAY",
+				     "Use C99 flexible arrays instead of zero-length arrays - see https://github.com/KSPP/linux/issues/78\n" . $herecurr) && $fix) {
+					$fixed[$fixlinenr] =~ s/\[0\]/[]/g;
+				}
+			}
+			if ($line =~ /^\+\s*((struct|union|enum)\s+$Ident|$Type|})\s+$Ident\s*\[1\][^;]*;/) {
+				WARN("ONE_ELEMENT_ARRAY",
+				     "Use C99 flexible arrays instead of one-element arrays - see https://github.com/KSPP/linux/issues/79\n" . $herecurr);
+			}
+		}
+
 # nested likely/unlikely calls
 		if ($line =~ /\b(?:(?:un)?likely)\s*\(\s*!?\s*(IS_ERR(?:_OR_NULL|_VALUE)?|WARN)/) {
 			WARN("LIKELY_MISUSE",
-- 
2.34.1