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Date:   Thu, 13 Feb 2020 09:22:41 -0600
From:   "Gustavo A. R. Silva" <gustavo@embeddedor.com>
To:     Peter Oberparleiter <oberpar@linux.ibm.com>
Cc:     linux-kernel@vger.kernel.org,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Subject: [PATCH] gcov: gcc_4_7: Replace zero-length array with flexible-array
 member
Message-ID: <20200213152241.GA877@embeddedor>
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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@embeddedor.com>
---
 kernel/gcov/gcc_4_7.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/gcov/gcc_4_7.c b/kernel/gcov/gcc_4_7.c
index ec37563674d6..908fdf5098c3 100644
--- a/kernel/gcov/gcc_4_7.c
+++ b/kernel/gcov/gcc_4_7.c
@@ -68,7 +68,7 @@ struct gcov_fn_info {
 	unsigned int ident;
 	unsigned int lineno_checksum;
 	unsigned int cfg_checksum;
-	struct gcov_ctr_info ctrs[0];
+	struct gcov_ctr_info ctrs[];
 };
 
 /**
-- 
2.25.0