checkpatch: Check for 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@canonical.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Fixed-by: Joe Perches <joe@perches.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Joe Perches <joe@perches.com>
Link: https://lore.kernel.org/r/20230517204530.never.151-kees@kernel.org

scripts/checkpatch.pl

@@ -7430,6 +7430,16 @@ sub process {
 			}
 		}
 
+# check for array definition/declarations that should use flexible arrays instead
+		if ($sline =~ /^[\+ ]\s*\}(?:\s*__packed)?\s*;\s*$/ &&
+		    $prevline =~ /^\+\s*(?:\}(?:\s*__packed\s*)?|$Type)\s*$Ident\s*\[\s*(0|1)\s*\]\s*;\s*$/) {
+			if (ERROR("FLEXIBLE_ARRAY",
+				  "Use C99 flexible arrays - see https://docs.kernel.org/process/deprecated.html#zero-length-and-one-element-arrays\n" . $hereprev) &&
+			    $1 == '0' && $fix) {
+				$fixed[$fixlinenr - 1] =~ s/\[\s*0\s*\]/[]/;
+			}
+		}
+
 # nested likely/unlikely calls
 		if ($line =~ /\b(?:(?:un)?likely)\s*\(\s*!?\s*(IS_ERR(?:_OR_NULL|_VALUE)?|WARN)/) {
 			WARN("LIKELY_MISUSE",