[next] sunrpc: Replace zero-length array with flexible-array member
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Message ID 20200228132323.GA20181@embeddedor
State New
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  • [next] sunrpc: Replace zero-length array with flexible-array member
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Commit Message

Gustavo A. R. Silva Feb. 28, 2020, 1:23 p.m. UTC
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(-)

Comments

J. Bruce Fields March 2, 2020, 7:58 p.m. UTC | #1
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
Gustavo A. R. Silva March 2, 2020, 8:05 p.m. UTC | #2
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

Patch
diff mbox series

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 {