linux-kernel - [PATCH] tools/testing/nvdimm: Replace zero-length array with flexible-array

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Date:   Thu, 7 May 2020 13:57:44 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Ira Weiny <ira.weiny@...el.com>
Cc:     linux-nvdimm@...ts.01.org, linux-kernel@...r.kernel.org
Subject: [PATCH] tools/testing/nvdimm: Replace zero-length array with
 flexible-array

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

sizeof(flexible-array-member) triggers a warning because flexible array
members have incomplete type[1]. There are some instances of code in
which the sizeof operator is being incorrectly/erroneously applied to
zero-length arrays and the result is zero. Such instances may be hiding
some bugs. So, this work (flexible-array member conversions) will also
help to get completely rid of those sorts of issues.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
---
 tools/testing/nvdimm/test/nfit_test.h |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/tools/testing/nvdimm/test/nfit_test.h b/tools/testing/nvdimm/test/nfit_test.h
index db3c07beb9d1..b5f7a996c4d0 100644
--- a/tools/testing/nvdimm/test/nfit_test.h
+++ b/tools/testing/nvdimm/test/nfit_test.h
@@ -51,7 +51,7 @@ struct nd_cmd_translate_spa {
 		__u32 nfit_device_handle;
 		__u32 _reserved;
 		__u64 dpa;
-	} __packed devices[0];
+	} __packed devices[];

 } __packed;

@@ -74,7 +74,7 @@ struct nd_cmd_ars_err_inj_stat {
 	struct nd_error_stat_query_record {
 		__u64 err_inj_stat_spa_range_base;
 		__u64 err_inj_stat_spa_range_length;
-	} __packed record[0];
+	} __packed record[];
 } __packed;

 #define ND_INTEL_SMART			 1
@@ -180,7 +180,7 @@ struct nd_intel_fw_send_data {
 	__u32 context;
 	__u32 offset;
 	__u32 length;
-	__u8 data[0];
+	__u8 data[];
 /* this field is not declared due ot variable data from input */
 /*	__u32 status; */
 } __packed;