nfsd/nfsd3_proc_readdir: fix buffer count and page pointers

After this commit
f875a79 nfsd: allow nfsv3 readdir request to be larger.
nfsv3 readdir request size can be larger than PAGE_SIZE. So if the
directory been read is large enough, we can use multiple pages
in rq_respages. Update buffer count and page pointers like we do
in readdirplus to make this happen.

Now listing a directory within 3000 files will panic because we
are counting in a wrong way and would write on random page.

Fixes: f875a79 "nfsd: allow nfsv3 readdir request to be larger"
Signed-off-by: Murphy Zhou <[e-mail]>
Signed-off-by: J. Bruce Fields <[e-mail]>

fs/nfsd/nfs3proc.c

···
 	struct nfsd3_readdirargs *argp = rqstp->rq_argp;
 	struct nfsd3_readdirres  *resp = rqstp->rq_resp;
 	__be32		nfserr;
-	int		count;
+	int		count = 0;
+	struct page	**p;
+	caddr_t		page_addr = NULL;
 
 	dprintk("nfsd: READDIR(3)  %s %d bytes at %d\n",
 				SVCFH_fmt(&argp->fh),
···
 	nfserr = nfsd_readdir(rqstp, &resp->fh, (loff_t*) &argp->cookie, 
 					&resp->common, nfs3svc_encode_entry);
 	memcpy(resp->verf, argp->verf, 8);
-	resp->count = resp->buffer - argp->buffer;
+	count = 0;
+	for (p = rqstp->rq_respages + 1; p < rqstp->rq_next_page; p++) {
+		page_addr = page_address(*p);
+
+		if (((caddr_t)resp->buffer >= page_addr) &&
+		    ((caddr_t)resp->buffer < page_addr + PAGE_SIZE)) {
+			count += (caddr_t)resp->buffer - page_addr;
+			break;
+		}
+		count += PAGE_SIZE;
+	}
+	resp->count = count >> 2;
 	if (resp->offset) {
 		loff_t offset = argp->cookie;
 

fs/nfsd/nfs3xdr.c

···
 nfs3svc_decode_readdirargs(struct svc_rqst *rqstp, __be32 *p)
 {
 	struct nfsd3_readdirargs *args = rqstp->rq_argp;
+	int len;
 	u32 max_blocksize = svc_max_payload(rqstp);
 
 	p = decode_fh(p, &args->fh);
···
 	args->verf   = p; p += 2;
 	args->dircount = ~0;
 	args->count  = ntohl(*p++);
-	args->count  = min_t(u32, args->count, max_blocksize);
-	args->buffer = page_address(*(rqstp->rq_next_page++));
+	len = args->count  = min_t(u32, args->count, max_blocksize);
+
+	while (len > 0) {
+		struct page *p = *(rqstp->rq_next_page++);
+		if (!args->buffer)
+			args->buffer = page_address(p);
+		len -= PAGE_SIZE;
+	}
 
 	return xdr_argsize_check(rqstp, p);
 }