[PATCH] tools/testing: Replace zero-length array with flexible-array

[PATCH] tools/testing: Replace zero-length array with flexible-array

[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]

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>
---
 tools/testing/selftests/nsfs/pidns.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/nsfs/pidns.c b/tools/testing/selftests/nsfs/pidns.c
index e0d86e1668c0..e3c772c6a7c7 100644
--- a/tools/testing/selftests/nsfs/pidns.c
+++ b/tools/testing/selftests/nsfs/pidns.c
@@ -27,7 +27,7 @@
 #define __stack_aligned__	__attribute__((aligned(16)))
 struct cr_clone_arg {
 	char stack[128] __stack_aligned__;
-	char stack_ptr[0];
+	char stack_ptr[];
 };
 
 static int child(void *args)

Re: [PATCH] tools/testing: Replace zero-length array with flexible-array

[Shuah Khan]

On 5/7/20 12:56 PM, 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]
> 
> 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>
> ---
>   tools/testing/selftests/nsfs/pidns.c |    2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/nsfs/pidns.c b/tools/testing/selftests/nsfs/pidns.c
> index e0d86e1668c0..e3c772c6a7c7 100644
> --- a/tools/testing/selftests/nsfs/pidns.c
> +++ b/tools/testing/selftests/nsfs/pidns.c
> @@ -27,7 +27,7 @@
>   #define __stack_aligned__	__attribute__((aligned(16)))
>   struct cr_clone_arg {
>   	char stack[128] __stack_aligned__;
> -	char stack_ptr[0];
> +	char stack_ptr[];
>   };
>   
>   static int child(void *args)
> 

Thanks for the patch. I will pull this in for 5.7-rc6

thanks,
-- Shuah

Re: [PATCH] tools/testing: Replace zero-length array with flexible-array

[Gustavo A. R. Silva]

On Thu, May 07, 2020 at 01:04:14PM -0600, Shuah Khan wrote:
> > 
> > diff --git a/tools/testing/selftests/nsfs/pidns.c b/tools/testing/selftests/nsfs/pidns.c
> > index e0d86e1668c0..e3c772c6a7c7 100644
> > --- a/tools/testing/selftests/nsfs/pidns.c
> > +++ b/tools/testing/selftests/nsfs/pidns.c
> > @@ -27,7 +27,7 @@
> >   #define __stack_aligned__	__attribute__((aligned(16)))
> >   struct cr_clone_arg {
> >   	char stack[128] __stack_aligned__;
> > -	char stack_ptr[0];
> > +	char stack_ptr[];
> >   };
> >   static int child(void *args)
> > 
> 
> Thanks for the patch. I will pull this in for 5.7-rc6
> 

Thanks, Shuah.

--
Gustavo