* [PATCH][next] sunrpc: Replace zero-length array with flexible-array member
@ 2020-02-28 13:23 Gustavo A. R. Silva
2020-03-02 19:58 ` J. Bruce Fields
0 siblings, 1 reply; 3+ messages in thread
From: Gustavo A. R. Silva @ 2020-02-28 13:23 UTC (permalink / raw)
To: Trond Myklebust, Anna Schumaker, J. Bruce Fields, Chuck Lever
Cc: linux-nfs, 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>
---
include/linux/sunrpc/svc.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
index 1afe38eb33f7..7f0a83451bc0 100644
--- a/include/linux/sunrpc/svc.h
+++ b/include/linux/sunrpc/svc.h
@@ -380,7 +380,7 @@ struct svc_deferred_req {
struct cache_deferred_req handle;
size_t xprt_hlen;
int argslen;
- __be32 args[0];
+ __be32 args[];
};
struct svc_process_info {
--
2.25.0
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH][next] sunrpc: Replace zero-length array with flexible-array member
2020-02-28 13:23 [PATCH][next] sunrpc: Replace zero-length array with flexible-array member Gustavo A. R. Silva
@ 2020-03-02 19:58 ` J. Bruce Fields
2020-03-02 20:05 ` Gustavo A. R. Silva
0 siblings, 1 reply; 3+ messages in thread
From: J. Bruce Fields @ 2020-03-02 19:58 UTC (permalink / raw)
To: Gustavo A. R. Silva
Cc: Trond Myklebust, Anna Schumaker, Chuck Lever, linux-nfs, linux-kernel
On Fri, Feb 28, 2020 at 07:23:23AM -0600, 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]
I don't understand the quoted sentences at all. But I assume you're
telling me that sizeof(struct svc_deferred_req) won't be changed by this
patch, so, good, applied. Thanks!
--b.
>
> 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>
> ---
> include/linux/sunrpc/svc.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
> index 1afe38eb33f7..7f0a83451bc0 100644
> --- a/include/linux/sunrpc/svc.h
> +++ b/include/linux/sunrpc/svc.h
> @@ -380,7 +380,7 @@ struct svc_deferred_req {
> struct cache_deferred_req handle;
> size_t xprt_hlen;
> int argslen;
> - __be32 args[0];
> + __be32 args[];
> };
>
> struct svc_process_info {
> --
> 2.25.0
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH][next] sunrpc: Replace zero-length array with flexible-array member
2020-03-02 19:58 ` J. Bruce Fields
@ 2020-03-02 20:05 ` Gustavo A. R. Silva
0 siblings, 0 replies; 3+ messages in thread
From: Gustavo A. R. Silva @ 2020-03-02 20:05 UTC (permalink / raw)
To: J. Bruce Fields
Cc: Trond Myklebust, Anna Schumaker, Chuck Lever, linux-nfs, linux-kernel
On 3/2/20 13:58, J. Bruce Fields wrote:
> On Fri, Feb 28, 2020 at 07:23:23AM -0600, 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]
>
> I don't understand the quoted sentences at all. But I assume you're
> telling me that sizeof(struct svc_deferred_req) won't be changed by this
> patch, so, good, applied. Thanks!
>
Correct! :)
Thanks
--
Gustavo
> --b.
>
>>
>> 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>
>> ---
>> include/linux/sunrpc/svc.h | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
>> index 1afe38eb33f7..7f0a83451bc0 100644
>> --- a/include/linux/sunrpc/svc.h
>> +++ b/include/linux/sunrpc/svc.h
>> @@ -380,7 +380,7 @@ struct svc_deferred_req {
>> struct cache_deferred_req handle;
>> size_t xprt_hlen;
>> int argslen;
>> - __be32 args[0];
>> + __be32 args[];
>> };
>>
>> struct svc_process_info {
>> --
>> 2.25.0
^ permalink raw reply [flat|nested] 3+ messages in thread
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