linux-kernel - Re: [PATCH] IB/core: Replace zero-length array with flexible-array member

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Message-ID: <20200213183850.GM31668@ziepe.ca>
Date:   Thu, 13 Feb 2020 14:38:50 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Doug Ledford <dledford@...hat.com>, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] IB/core: Replace zero-length array with flexible-array
 member

On Thu, Feb 13, 2020 at 12:37:15PM -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]
> 
> 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@...eddedor.com>
> ---
>  drivers/infiniband/core/cache.c     | 2 +-
>  drivers/infiniband/core/cm.c        | 4 ++--
>  drivers/infiniband/core/multicast.c | 2 +-
>  drivers/infiniband/core/sa_query.c  | 2 +-
>  4 files changed, 5 insertions(+), 5 deletions(-)

Any reason to skip these two?

drivers/infiniband/core/mad_priv.h:     u8 mad[0];
drivers/infiniband/core/mad_priv.h:     u8 data[0];

And may as well touch these in the subsystem headers too:

include/rdma/ib_fmr_pool.h:     u64                 page_list[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/ib_verbs.h:        u8      real_sz[0];
include/rdma/opa_vnic.h:        char *dev_priv[0];
include/rdma/rdmavt_mr.h:       struct rvt_segarray *map[0];    /* the segments */
include/rdma/rdmavt_qp.h:       struct rvt_sge sg_list[0];

?

Jason