Re: [PATCH v2] vt: fix memory overlapping when deleting chars in the buffer

[Jiri Slaby]

On 27. 06. 22, 14:54, Yangxi Xiang wrote:
> A memory overlapping copy occurs when deleting a long line. This memory
> overlapping copy can cause data corruption when scr_memcpyw is optimized
> to memcpy because memcpy does not ensure its behavior if the destination
> buffer overlaps with the source buffer. The line buffer is not always
> broken, because the memcpy utilizes the hardware acceleration, whose
> result is not deterministic.
>
> Fix this problem by using replacing the scr_memcpyw with scr_memmovew, and
> preserving the memcpy optimization when the buffers are not overlapping.
>
> Fixes: 81732c3b2fed ("Fix line garbage in virtual console").
> Signed-off-by: Yangxi Xiang <xyangxi5@xxxxxxxxx>
> ---
>   drivers/tty/vt/vt.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index f8c87c4d7399..d87bff9d8ed5 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -853,9 +853,13 @@ static void insert_char(struct vc_data *vc, unsigned int nr)
>   static void delete_char(struct vc_data *vc, unsigned int nr)
>   {
>   	unsigned short *p = (unsigned short *) vc->vc_pos;
> +	unsigned short cp = (vc->vc_cols - vc->state.x - nr) * 2;
>   
>   	vc_uniscr_delete(vc, nr);
> -	scr_memcpyw(p, p + nr, (vc->vc_cols - vc->state.x - nr) * 2);
> +	if (cp > nr)
> +		scr_memmovew(p, p + nr, cp);
> +	else
> +		scr_memcpyw(p, p + nr, cp);

Why not to use memmove in both cases? I.e. simply switch scr_memcpyw to 
scr_memmovew?

thanks,
--
js
suse labs