[PATCH master] fs: /dev/mem: handle copy at offset 0 correctly

[Sascha Hauer]

On Mon, Nov 29, 2021 at 08:14:54AM +0100, Ahmad Fatoum wrote:
> Despite that /dev/mem has a size of 0, the check preventing
> out-of-bounds access works for /dev/mem as well, because of unsigned
> wrap around. Corner case is when offset is zero. There will be no wrap
> around and
> 
>   count = min(count, 0 - 0) = 0
> 
> Leading to unexpected behavior with e.g.
> 
>   memcmp -s /dev/mem 0
> 
> on systems without MMU. Fix this.
> 
> Signed-off-by: Ahmad Fatoum <ahmad at a3f.at>
> ---
>  fs/devfs-core.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)

Applied, thanks

Sascha

> 
> diff --git a/fs/devfs-core.c b/fs/devfs-core.c
> index 3715e543e632..2d016e0e4861 100644
> --- a/fs/devfs-core.c
> +++ b/fs/devfs-core.c
> @@ -573,7 +573,10 @@ ssize_t mem_copy(struct device_d *dev, void *dst, const void *src,
>  	if (!dev || dev->num_resources < 1)
>  		return -1;
>  
> -	count = size = min(count, resource_size(&dev->resource[0]) - offset);
> +	if (resource_size(&dev->resource[0]) > 0 || offset != 0)
> +		count = min(count, resource_size(&dev->resource[0]) - offset);
> +
> +	size = count;
>  
>  	/* no rwsize specification given. Do whatever memcpy likes best */
>  	if (!rwsize) {
> -- 
> 2.33.0
> 
> 
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