linux-kernel - Re: [PATCH v2] tty: n_hdlc: Use flexible-array member and struct

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Date:   Thu, 23 Jan 2020 07:16:29 +0100
From:   Jiri Slaby <jslaby@...e.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mikael Magnusson <mikachu@...il.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] tty: n_hdlc: Use flexible-array member and
 struct_size() helper

On 21. 01. 20, 18:21, Gustavo A. R. Silva wrote:
> Old code in the kernel uses 1-byte and 0-byte arrays to indicate the
> presence of a "variable length array":
> 
> 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);
> 
> There is also 0-byte arrays. Both cases pose confusion for things like
> sizeof(), CONFIG_FORTIFY_SOURCE, etc.[1] Instead, the preferred mechanism
> to declare variable-length types such as the one above is a flexible array
> member[2] which need to be the last member of a structure and empty-sized:
> 
> struct something {
>         int stuff;
>         u8 data[];
> };
> 
> Also, 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
> inadvertenly introduced[3] to the codebase from now on.
> 
> Lastly, make use of the struct_size() helper to safely calculate the
> allocation size for instances of struct n_hdlc_buf and avoid any potential
> type mistakes[4][5].
> 
> [1] https://github.com/KSPP/linux/issues/21
> [2] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
> [3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")
> [4] https://lore.kernel.org/lkml/60e14fb7-8596-e21c-f4be-546ce39e7bdb@embeddedor.com/
> [5] commit 553d66cb1e86 ("iommu/vt-d: Use struct_size() helper")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>

Now it looks good to me:
Reviewed-by: Jiri Slaby <jslaby@...e.cz>

thanks,
-- 
js
suse labs