[PATCH] IB/mlx4: Replace zero-length array with flexible-array

[PATCH] IB/mlx4: 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>
---
 include/linux/mlx4/qp.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/mlx4/qp.h b/include/linux/mlx4/qp.h
index 8e2828d48d7f..9db93e487496 100644
--- a/include/linux/mlx4/qp.h
+++ b/include/linux/mlx4/qp.h
@@ -362,7 +362,7 @@ struct mlx4_wqe_datagram_seg {
 
 struct mlx4_wqe_lso_seg {
 	__be32			mss_hdr_size;
-	__be32			header[0];
+	__be32			header[];
 };
 
 enum mlx4_wqe_bind_seg_flags2 {

Re: [PATCH] IB/mlx4: Replace zero-length array with flexible-array

[Jakub Kicinski]

On Thu, 7 May 2020 13:59:21 -0500 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[];
> };
>
> ...

Applied, thank you!

Re: [PATCH] IB/mlx4: Replace zero-length array with flexible-array

[Jason Gunthorpe]

On Sat, May 09, 2020 at 08:51:50PM -0700, Jakub Kicinski wrote:
> On Thu, 7 May 2020 13:59:21 -0500 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[];
> > };
> >
> > ...
> 
> Applied, thank you!

Jakub,

Please don't take RDMA patches in netdev unless it is a special
case. There is alot of cross posting and they often get into both
patchworks.

Thanks,
Jason

Re: [PATCH] IB/mlx4: Replace zero-length array with flexible-array

[Jakub Kicinski]

On Wed, 13 May 2020 15:43:16 -0300 Jason Gunthorpe wrote:
> On Sat, May 09, 2020 at 08:51:50PM -0700, Jakub Kicinski wrote:
> > On Thu, 7 May 2020 13:59:21 -0500 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[];
> > > };
> > >
> > > ...  
> > 
> > Applied, thank you!  
> 
> Jakub,
> 
> Please don't take RDMA patches in netdev unless it is a special
> case. There is alot of cross posting and they often get into both
> patchworks.

Sorry about that, I only looked at the subject after applying. 

Re: [PATCH] IB/mlx4: Replace zero-length array with flexible-array

[Jason Gunthorpe]

On Thu, May 07, 2020 at 01:59:21PM -0500, 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>
> ---
>  include/linux/mlx4/qp.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Applied to for-next, thanks

Jason

Re: [PATCH] IB/mlx4: Replace zero-length array with flexible-array

[Leon Romanovsky]

On Wed, May 13, 2020 at 03:33:35PM -0300, Jason Gunthorpe wrote:
> On Thu, May 07, 2020 at 01:59:21PM -0500, 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>
> > ---
> >  include/linux/mlx4/qp.h |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
>
> Applied to for-next, thanks

Jason,

Please be cautious here, Jakub already applied this patch.
https://lore.kernel.org/lkml/20200509205151.209bdc9d@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com/

>
> Jason