coccinelle: misc: add flexible_array.cocci script

One-element and zero-length arrays are deprecated [1]. Kernel
code should always use "flexible array members" instead, except
for existing uapi definitions.

The script warns about one-element and zero-length arrays in structs.

[1] commit 68e4cd17 ("docs: deprecated.rst: Add zero-length and
    one-element arrays")

Cc: Kees Cook <keescook@chromium.org>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Denis Efremov <efremov@linux.com>
Signed-off-by: Julia Lawall <julia.lawall@inria.fr>

scripts/coccinelle/misc/flexible_array.cocci

+// SPDX-License-Identifier: GPL-2.0-only
+///
+/// Zero-length and one-element arrays are deprecated, see
+/// Documentation/process/deprecated.rst
+/// Flexible-array members should be used instead.
+///
+//
+// Confidence: High
+// Copyright: (C) 2020 Denis Efremov ISPRAS.
+// Comments:
+// Options: --no-includes --include-headers
+
+virtual context
+virtual report
+virtual org
+virtual patch
+
+@initialize:python@
+@@
+def relevant(positions):
+    for p in positions:
+        if "uapi" in p.file:
+             return False
+    return True
+
+@r depends on !patch@
+identifier name, array;
+type T;
+position p : script:python() { relevant(p) };
+@@
+
+(
+  struct name {
+    ...
+*   T array@p[\(0\|1\)];
+  };
+|
+  struct {
+    ...
+*   T array@p[\(0\|1\)];
+  };
+|
+  union name {
+    ...
+*   T array@p[\(0\|1\)];
+  };
+|
+  union {
+    ...
+*   T array@p[\(0\|1\)];
+  };
+)
+
+@depends on patch@
+identifier name, array;
+type T;
+position p : script:python() { relevant(p) };
+@@
+
+(
+  struct name {
+    ...
+    T array@p[
+-       0
+    ];
+  };
+|
+  struct {
+    ...
+    T array@p[
+-       0
+    ];
+  };
+)
+
+@script: python depends on report@
+p << r.p;
+@@
+
+msg = "WARNING use flexible-array member instead (https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays)"
+coccilib.report.print_report(p[0], msg)
+
+@script: python depends on org@
+p << r.p;
+@@
+
+msg = "WARNING use flexible-array member instead (https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays)"
+coccilib.org.print_todo(p[0], msg)