linux-kernel - Re: [PATCH] RDMA/siw: Replace one-element array and use struct

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Message-ID: <20200519235231.GA31402@ziepe.ca>
Date:   Tue, 19 May 2020 20:52:31 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     "Gustavo A. R. Silva" <gustavoars@...nel.org>
Cc:     Bernard Metzler <bmt@...ich.ibm.com>,
        Doug Ledford <dledford@...hat.com>, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Subject: Re: [PATCH] RDMA/siw: Replace one-element array and use
 struct_size() helper

On Tue, May 19, 2020 at 06:30:18PM -0500, Gustavo A. R. Silva wrote:
> The current codebase makes use of one-element arrays in the following
> form:
> 
> struct something {
>     int length;
>     u8 data[1];
> };
> 
> struct something *instance;
> 
> instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
> instance->length = size;
> memcpy(instance->data, source, size);
> 
> 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. So, replace
> the one-element array with a flexible-array member.
> 
> Also, make use of the new struct_size() helper to properly calculate the
> size of struct siw_pbl.
> 
> This issue was found with the help of Coccinelle and, audited and fixed
> _manually_.
> 
> [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@...nel.org>
> ---
>  drivers/infiniband/sw/siw/siw.h     | 2 +-
>  drivers/infiniband/sw/siw/siw_mem.c | 5 +----
>  2 files changed, 2 insertions(+), 5 deletions(-)

Applied to for-next, thanks

Jason