[Cocci] [RFC PATCH] 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>
---

Currently, it's just a draft. I've placed a number of questions in the
script and marked them as TODO. Kees, Gustavo, if you could help me with
my questions I think that this rule will be enough to close:
https://github.com/KSPP/linux/issues/76

BTW, I it's possible to not warn about files in uapi folder if
this is relevant. Do I need to do it in the script?

 scripts/coccinelle/misc/flexible_array.cocci | 158 +++++++++++++++++++
 1 file changed, 158 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..1e7165c79e60
--- /dev/null
+++ b/scripts/coccinelle/misc/flexible_array.cocci
@@ -0,0 +1,158 @@
+// 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
+
+@r depends on !patch@
+identifier name, size, array;
+// TODO: We can additionally restrict size and array to:
+// identifier size =~ ".*(num|len|count|size|ncpus).*";
+// identifier array !~ ".*(pad|reserved).*";
+// Do we need it?
+type TS, TA;
+position p;
+@@
+
+(
+  // This will also match: typedef struct name { ...
+  // However nested structs are not matched, i.e.:
+  //   struct name1 { struct name2 { int s; int a[0]; } st; int i; }
+  // will not be matched. Do we need to handle it?
+  struct name {
+    ...      // TODO: Maybe simple ... is enough? It will match structs with a
+    TS size; // single field, e.g.
+    ...      // https://elixir.bootlin.com/linux/v5.8/source/arch/arm/include/uapi/asm/setup.h#L127
+(
+*    TA array@p[0];
+|
+     // TODO: It seems that there are exception cases for array[1], e.g.
+     //  https://elixir.bootlin.com/linux/v5.8/source/arch/powerpc/boot/rs6000.h#L152
+     //  https://elixir.bootlin.com/linux/v5.8/source/include/uapi/linux/cdrom.h#L292
+     //  https://elixir.bootlin.com/linux/v5.8/source/drivers/net/wireless/ath/ath6kl/usb.c#L108
+     // We could either drop array[1] checking from this rule or
+     // restrict array name with regexp and add, for example, an "allowlist"
+     // with struct names where we allow this code pattern.
+     // TODO: How to handle: u8 data[1][MAXLEN_PSTR6]; ?
+*    TA array@p[1];
+)
+  };
+|
+  struct {
+    ...
+    TS size;
+    ...
+(
+*    TA array@p[0];
+|
+*    TA array@p[1];
+)
+  };
+|
+  // TODO: do we need to handle unions?
+  union name {
+    ...
+    TS size;
+    ...
+(
+*    TA array@p[0];
+|
+*    TA array@p[1];
+)
+  };
+|
+  union {
+    ...
+    TS size;
+    ...
+(
+*    TA array@p[0];
+|
+*    TA array@p[1];
+)
+  };
+)
+
+// FIXME: Patch mode doesn't work as expected.
+// Coccinelle handles formatting incorrectly.
+// Patch mode in this rule should be disabled until
+// proper formatting will be supported.
+@depends on patch exists@
+identifier name, size, array;
+type TS, TA;
+@@
+
+(
+  struct name {
+    ...
+    TS size;
+    ...
+(
+-    TA array[0];
+|
+-    TA array[1];
+)
++    TA array[];
+  };
+|
+  struct {
+    ...
+    TS size;
+    ...
+(
+-    TA array[0];
+|
+-    TA array[1];
+)
++    TA array[];
+  };
+|
+  union name {
+    ...
+    TS size;
+    ...
+(
+-    TA array[0];
+|
+-    TA array[1];
+)
++    TA array[];
+  };
+|
+  union {
+    ...
+    TS size;
+    ...
+(
+-    TA array[0];
+|
+-    TA array[1];
+)
++    TA array[];
+  };
+)
+
+@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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