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 <[email protected]>
---
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) + \
On Thu, May 7, 2020 at 8:47 PM Gustavo A. R. Silva
<[email protected]> wrote:
>
> 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 <[email protected]>
> ---
> 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) + \
>
Applied.
Thanks,
Ilya