linux-kernel - [PATCH] gcov: gcc_3_4: Replace zero-length array with flexible-array member

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Message-ID: <20200302224501.GA14175@embeddedor>
Date:   Mon, 2 Mar 2020 16:45:01 -0600
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Peter Oberparleiter <oberpar@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: [PATCH] gcov: gcc_3_4: Replace zero-length array with flexible-array
 member

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]

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 <gustavo@...eddedor.com>
---
 kernel/gcov/gcc_3_4.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/gcov/gcc_3_4.c b/kernel/gcov/gcc_3_4.c
index 801ee4b0b969..acb83558e5df 100644
--- a/kernel/gcov/gcc_3_4.c
+++ b/kernel/gcov/gcc_3_4.c
@@ -38,7 +38,7 @@ static struct gcov_info *gcov_info_head;
 struct gcov_fn_info {
 	unsigned int ident;
 	unsigned int checksum;
-	unsigned int n_ctrs[0];
+	unsigned int n_ctrs[];
 };
 
 /**
@@ -78,7 +78,7 @@ struct gcov_info {
 	unsigned int			n_functions;
 	const struct gcov_fn_info	*functions;
 	unsigned int			ctr_mask;
-	struct gcov_ctr_info		counts[0];
+	struct gcov_ctr_info		counts[];
 };
 
 /**
@@ -352,7 +352,7 @@ struct gcov_iterator {
 	unsigned int count;
 
 	int num_types;
-	struct type_info type_info[0];
+	struct type_info type_info[];
 };
 
 static struct gcov_fn_info *get_func(struct gcov_iterator *iter)
-- 
2.25.0