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

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

[Kees Cook]

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@canonical.com>
Cc: Joe Perches <joe@perches.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.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

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

[Joe Perches]

 * On Wed, 2023-05-17 at 13:45 -0700, Kees Cook wrote:
> 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.

I don't think this does what you want.

After adding this patch to checkpatch with this diff: No warning

-----------------------
diff --git a/drivers/net/netconsole.c b/drivers/net/netconsole.c
index 4f4f79532c6cf..513cb102b3b38 100644
--- a/drivers/net/netconsole.c
+++ b/drivers/net/netconsole.c
@@ -73,6 +73,11 @@ static DEFINE_SPINLOCK(target_list_lock);
 */
 static struct console netconsole_ext;
+struct foobar {
+ int a;
+ int b[1];
+};
+
 /**
 * struct netconsole_target - Represents a configured netconsole target.
 * @list: Links this target into the target_list.
-----------------------

because $context_function is DEFINE_SPINLOCK

I think you want something like:
---
 scripts/checkpatch.pl | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index b30114d637c40..778a0808c37cf 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -7418,6 +7418,18 @@ sub process {
 }
 }
+ # check for array definition/declarations that should use flexible arrays instead
+ if ($sline =~ /^[\+ ]\s*}\s*;\s*$/ &&
+ $prevline =~ /^\+\s*$Type\s*$Ident\s*\[\s*(0|1)\s*\]\s*;\s*$/) {
+ if ($1 =~ '0') {
+ WARN("ZERO_LENGTH_ARRAY",
+ "Prefer C99 flexible arrays instead of zero-length arrays - see https://github.com/KSPP/linux/issues/78\n" . $hereprev);
+ } else {
+ WARN("ONE_ELEMENT_ARRAY",
+ "Prefer C99 flexible arrays instead of one-element arrays - see https://github.com/KSPP/linux/issues/79\n" . $hereprev);
+ }
+ }
+
 # nested likely/unlikely calls
 if ($line =~ /\b(?:(?:un)?likely)\s*\(\s*!?\s*(IS_ERR(?:_OR_NULL|_VALUE)?|WARN)/) {
 WARN("LIKELY_MISUSE",