* [PATCH][next] bpf, sockmap: Replace zero-length array with flexible-array member
@ 2020-02-17 20:01 Gustavo A. R. Silva
2020-02-19 6:31 ` John Fastabend
0 siblings, 1 reply; 2+ messages in thread
From: Gustavo A. R. Silva @ 2020-02-17 20:01 UTC (permalink / raw)
To: John Fastabend, Daniel Borkmann, David S. Miller, Jakub Kicinski
Cc: netdev, bpf, linux-kernel, Gustavo A. R. Silva
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")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
net/core/sock_map.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index 085cef5857bb..3a7a96ab088a 100644
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c
@@ -518,7 +518,7 @@ struct bpf_htab_elem {
u32 hash;
struct sock *sk;
struct hlist_node node;
- u8 key[0];
+ u8 key[];
};
struct bpf_htab_bucket {
--
2.25.0
^ permalink raw reply related [flat|nested] 2+ messages in thread
* RE: [PATCH][next] bpf, sockmap: Replace zero-length array with flexible-array member
2020-02-17 20:01 [PATCH][next] bpf, sockmap: Replace zero-length array with flexible-array member Gustavo A. R. Silva
@ 2020-02-19 6:31 ` John Fastabend
0 siblings, 0 replies; 2+ messages in thread
From: John Fastabend @ 2020-02-19 6:31 UTC (permalink / raw)
To: Gustavo A. R. Silva, John Fastabend, Daniel Borkmann,
David S. Miller, Jakub Kicinski
Cc: netdev, bpf, linux-kernel, Gustavo A. R. Silva
Gustavo A. R. Silva 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]
>
> 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 <gustavo@embeddedor.com>
> ---
> net/core/sock_map.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> index 085cef5857bb..3a7a96ab088a 100644
> --- a/net/core/sock_map.c
> +++ b/net/core/sock_map.c
> @@ -518,7 +518,7 @@ struct bpf_htab_elem {
> u32 hash;
> struct sock *sk;
> struct hlist_node node;
> - u8 key[0];
> + u8 key[];
> };
>
> struct bpf_htab_bucket {
> --
> 2.25.0
>
The same pattern is in ./kernel/bpf/hashtab.c can you also change
it here then if this is the case so sockmap is aligned with bpf
coding style.
.John
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