[PATCH] libceph: Replace zero-length array with flexible-array

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

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@kernel.org>
---
 include/linux/ceph/mon_client.h |    2 +-
 include/linux/crush/crush.h     |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/ceph/mon_client.h b/include/linux/ceph/mon_client.h
index dbb8a6959a73..ce4ffeb384d7 100644
--- a/include/linux/ceph/mon_client.h
+++ b/include/linux/ceph/mon_client.h
@@ -19,7 +19,7 @@ struct ceph_monmap {
 	struct ceph_fsid fsid;
 	u32 epoch;
 	u32 num_mon;
-	struct ceph_entity_inst mon_inst[0];
+	struct ceph_entity_inst mon_inst[];
 };
 
 struct ceph_mon_client;
diff --git a/include/linux/crush/crush.h b/include/linux/crush/crush.h
index 54741295c70b..38b0e4d50ed9 100644
--- a/include/linux/crush/crush.h
+++ b/include/linux/crush/crush.h
@@ -87,7 +87,7 @@ struct crush_rule_mask {
 struct crush_rule {
 	__u32 len;
 	struct crush_rule_mask mask;
-	struct crush_rule_step steps[0];
+	struct crush_rule_step steps[];
 };
 
 #define crush_rule_size(len) (sizeof(struct crush_rule) + \