Re: [PATCH] char: hpet: Use flexible-array member - Greg Kroah-Hartman

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Cc: Clemens Ladisch <clemens@ladisch.de>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] char: hpet: Use flexible-array member
Date: Thu, 23 Jan 2020 19:25:45 +0100	[thread overview]
Message-ID: <20200123182545.GA1954152@kroah.com> (raw)
In-Reply-To: <20200120235326.GA29231@embeddedor.com>

On Mon, Jan 20, 2020 at 05:53:26PM -0600, 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
> unadvertenly introduced[3] to the codebase from now on.
> 
> [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")
> 
> Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
> ---
>  drivers/char/hpet.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/char/hpet.c b/drivers/char/hpet.c
> index 9ac6671bb514..aed2c45f7968 100644
> --- a/drivers/char/hpet.c
> +++ b/drivers/char/hpet.c
> @@ -110,7 +110,7 @@ struct hpets {
>  	unsigned long hp_delta;
>  	unsigned int hp_ntimer;
>  	unsigned int hp_which;
> -	struct hpet_dev hp_dev[1];
> +	struct hpet_dev hp_dev[];

Are you sure the allocation size is the same again?  Much like the
n_hdlc patch was, I think you need to adjust the variable size here.
Maybe, it's a bit of a pain to figure out at a quick glance, I just want
to make sure you at least do look at that :)

thanks,

greg k-h