[Cocci] [PATCH v2] coccinelle: misc: add flexible_array.cocci script

[Denis Efremov <efremov@linux.com>]

Commit 68e4cd17e218 ("docs: deprecated.rst: Add zero-length and one-element
arrays") marks one-element and zero-length arrays as deprecated. Kernel
code should always use "flexible array members" instead.

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

Cc: Kees Cook <keescook@chromium.org>
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Denis Efremov <efremov@linux.com>
---
Changes in v2:
 - all uapi headers are now filtered-out. Unfortunately, coccinelle
   doesn't provide structure names in Location.current_element.
   For structures the field is always "something_else". Thus, there is
   no easy way to create a list of existing structures in uapi headers
   and suppress the warning only for them, but not for the newly added
   uapi structures.
 - The pattern doesn't require 2+ fields in a structure/union anymore.
   Now it also checks single field structures/unions.
 - The pattern simplified and now uses disjuction in array elements
   (Thanks, Markus)
 - Unions are removed from patch mode
 - one-element arrays are removed from patch mode. Correct patch may
   involve turning the array to a simple field instead of a flexible
   array.

On the current master branch, the rule generates:
 - context: https://gist.github.com/evdenis/e2b4323491f9eff35376372df07f723c
 - patch: https://gist.github.com/evdenis/46081da9d68ecefd07edc3769cebcf32

 scripts/coccinelle/misc/flexible_array.cocci | 88 ++++++++++++++++++++
 1 file changed, 88 insertions(+)
 create mode 100644 scripts/coccinelle/misc/flexible_array.cocci

diff --git a/scripts/coccinelle/misc/flexible_array.cocci b/scripts/coccinelle/misc/flexible_array.cocci
new file mode 100644
index 000000000000..bf6dcda1783e
--- /dev/null
+++ b/scripts/coccinelle/misc/flexible_array.cocci
@@ -0,0 +1,88 @@
+// 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 exists@
+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"
+coccilib.report.print_report(p[0], msg)
+
+@script: python depends on org@
+p << r.p;
+@@
+
+msg = "WARNING: use flexible-array member instead"
+coccilib.org.print_todo(p, msg)
-- 
2.26.2

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