Re: [PATCH] mm: mremap - Fix extent calculation

[Guenter Roeck <linux@roeck-us.net>]

On Sat, Dec 19, 2020 at 05:04:33PM +0000, Kalesh Singh wrote:
> When `next < old_addr`, `next - old_addr` arithmetic underflows
> causing `extent` to be incorrect.
> 
> Make `extent` the smaller of `next - old_addr` or `old_end - old_addr`.
> 
> Reported-by: Guenter Roeck <linux@roeck-us.net>
> Signed-off-by: Kalesh Singh <kaleshsingh@google.com>

This patch fixes the problem I had observed when booting 'parisc'
images.

Tested-by: Guenter Roeck <linux@roeck-us.net>

Guenter

> ---
>  mm/mremap.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/mremap.c b/mm/mremap.c
> index c5590afe7165..f554320281cc 100644
> --- a/mm/mremap.c
> +++ b/mm/mremap.c
> @@ -358,7 +358,9 @@ static unsigned long get_extent(enum pgt_entry entry, unsigned long old_addr,
>  
>  	next = (old_addr + size) & mask;
>  	/* even if next overflowed, extent below will be ok */
> -	extent = (next > old_end) ? old_end - old_addr : next - old_addr;
> +	extent = next - old_addr;
> +	if (extent > old_end - old_addr)
> +		extent = old_end - old_addr;
>  	next = (new_addr + size) & mask;
>  	if (extent > next - new_addr)
>  		extent = next - new_addr;
> -- 
> 2.29.2.729.g45daf8777d-goog
>