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libceph, rbd: replace zero-length array with flexible-array

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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]

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>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
This commit is contained in:
Gustavo A. R. Silva 2020-05-07 13:51:38 -05:00 committed by Ilya Dryomov
parent dffdcd7145
commit 53ab8e7cd2
3 changed files with 3 additions and 3 deletions
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2
drivers/block/rbd_types.h
View File

@ -93,7 +93,7 @@ struct rbd_image_header_ondisk {
__le32 snap_count;
__le32 reserved;
__le64 snap_names_len;
struct rbd_image_snap_ondisk snaps[0];
struct rbd_image_snap_ondisk snaps[];
} __attribute__((packed));

2
include/linux/ceph/mon_client.h
View File

@ -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;

2
include/linux/crush/crush.h
View File

@ -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) + \