Re: [PATCH v3] xen/privcmd: fix error exit of privcmd_ioctl_dm_op()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.08.2022 13:40, Juergen Gross wrote:
> --- a/drivers/xen/privcmd.c
> +++ b/drivers/xen/privcmd.c
> @@ -581,7 +581,7 @@ static int lock_pages(
>       struct privcmd_dm_op_buf kbufs[], unsigned int num,
>       struct page *pages[], unsigned int nr_pages, unsigned int *pinned)
>  {
> -     unsigned int i;
> +     unsigned int i, off = 0;
>  
>       for (i = 0; i < num; i++) {
>               unsigned int requested;
> @@ -589,19 +589,23 @@ static int lock_pages(
>  
>               requested = DIV_ROUND_UP(
>                       offset_in_page(kbufs[i].uptr) + kbufs[i].size,
> -                     PAGE_SIZE);
> +                     PAGE_SIZE) - off;
>               if (requested > nr_pages)
>                       return -ENOSPC;
>  
>               page_count = pin_user_pages_fast(
> -                     (unsigned long) kbufs[i].uptr,
> +                     (unsigned long)kbufs[i].uptr + off * PAGE_SIZE,
>                       requested, FOLL_WRITE, pages);
> -             if (page_count < 0)
> -                     return page_count;
> +             if (page_count <= 0)
> +                     return page_count ? : -EFAULT;
>  
>               *pinned += page_count;
>               nr_pages -= page_count;
>               pages += page_count;
> +
> +             off = requested - page_count;
> +             if (off)
> +                     i--;
>       }

Initially I thought this would go wrong only on the 3rd iteration, but
meanwhile I think it's wrong already on the 2nd. What I think you need
is

                if (page_count < requested)
                        i--;
                off += page_count;

or with the i++ from the loop header absorbed here

                if (page_count == requested)
                        i++;
                off += page_count;

Plus of course off needs resetting to zero whenever i advances. I.e.

                if (page_count == requested) {
                        i++;
                        off = 0;
                } else {
                        off += page_count;
                }

Jan