Re: rnfs: rq_respages pointer is bad

[Tom Tucker <tom-7bPotxP6k4+P2YhJcF5u+vpXobYPEAuW@public.gmane.org>]

David J. Wilder wrote:
> Tom
>
> I have been chasing an rnfs related Oops in svc_process().  I have found
> the source of the Oops but I am not sure of my fix.  I am seeing the
> problem on ppc64, kernel 2.6.32, I have not tried other arch yet.
>
> The source of the problem is in rdma_read_complete(), I am finding that
> rqstp->rq_respages is set to point past the end of the rqstp->rq_pages
> page list.  This results in a NULL reference in svc_process() when
> passing rq_respages[0] to page_address().
>
> In rdma_read_complete() we are using rqstp->rq_arg.pages as the base of
> the page list then indexing by page_no, however rq_arg.pages is not
> pointing to the start of the list so rq_respages ends up pointing to:
>
> rqstp->rq_pages[(head->count+1) + head->hdr_count]
>
> In my case, it ends up pointing one past the end of the list by one.
>
> Here is the change I made.
>
> static int rdma_read_complete(struct svc_rqst *rqstp,
>                               struct svc_rdma_op_ctxt *head)
> {
>         int page_no;
>         int ret;
>
>         BUG_ON(!head);
>
>         /* Copy RPC pages */
>         for (page_no = 0; page_no < head->count; page_no++) {
>                 put_page(rqstp->rq_pages[page_no]);
>                 rqstp->rq_pages[page_no] = head->pages[page_no];
>         }
>         /* Point rq_arg.pages past header */
>         rqstp->rq_arg.pages = &rqstp->rq_pages[head->hdr_count];
>         rqstp->rq_arg.page_len = head->arg.page_len;
>         rqstp->rq_arg.page_base = head->arg.page_base;
>
>         /* rq_respages starts after the last arg page */
> -       rqstp->rq_respages = &rqstp->rq_arg.pages[page_no];
> +	rqstp->rq_respages = &rqstp->rq_pages[page_no];
>   

This might be clearer as:

        rqstp->rq_respages = &rqstp->rq_pages[head->count];

> .
> .
> .
>
> The change works for me, but I am not sure it is safe to assume the
> rqstp->rq_pages[head->count] will always point to the last arg page.
>
> Dave.
>
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