[patch 148/166] gcov: gcc_3_4: replace zero-length array with flexible-array member

Andrew Morton
2020-04-07T03:11:59Z

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

Link: http://lkml.kernel.org/r/20200302224501.GA14175@embeddedor
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

kernel/gcov/gcc_3_4.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

--- a/kernel/gcov/gcc_3_4.c~gcov-gcc_3_4-replace-zero-length-array-with-flexible-array-member
+++ a/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)
_