[patch 147/166] gcov: gcc_4_7: replace zero-length array with flexible-array member

Andrew Morton
2020-04-07T03:11:56Z

From: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Subject: gcov: gcc_4_7: 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")

Link: http://lkml.kernel.org/r/20200213152241.GA877@embeddedor
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

kernel/gcov/gcc_4_7.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/gcov/gcc_4_7.c~gcov-gcc_4_7-replace-zero-length-array-with-flexible-array-member
+++ a/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[];
};

/**
_