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

[Juergen Gross <jgross@xxxxxxxx>]

On 25.08.22 13:58, Jan Beulich wrote:
> 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;
> 		}

Yeah, or:

		off = (page_count == requested) ? 0 : off + page_count;
		i += !off;


Juergen

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