nfsd: eliminate an unnecessary acl size limit

We're unnecessarily limiting the size of an ACL to less than what most
filesystems will support.  Some users do hit the limit and it's
confusing and unnecessary.

It still seems prudent to impose some limit on the number of ACEs the
client gives us before passing it straight to kmalloc().  So, let's just
limit it to the maximum number that would be possible given the amount
of data left in the argument buffer.

That will still leave one limit beyond whatever the filesystem imposes:
the client and server negotiate a limit on the size of a request, which
we have to respect.

But we're no longer imposing any additional arbitrary limit.

struct nfs4_ace is 20 bytes on my system and the maximum call size we'll
negotiate is about a megabyte, so in practice this is limiting the
allocation here to about a megabyte.
Reported-by: "de Vandiere, Louis" <louis.devandiere@atos.net>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/acl.h

@@ -39,14 +39,6 @@ struct nfs4_acl;
 struct svc_fh;
 struct svc_rqst;
 
-/*
- * Maximum ACL we'll accept from a client; chosen (somewhat
- * arbitrarily) so that kmalloc'ing the ACL shouldn't require a
- * high-order allocation.  This allows 204 ACEs on x86_64:
- */
-#define NFS4_ACL_MAX ((PAGE_SIZE - sizeof(struct nfs4_acl)) \
-			/ sizeof(struct nfs4_ace))
-
 int nfs4_acl_bytes(int entries);
 int nfs4_acl_get_whotype(char *, u32);
 __be32 nfs4_acl_write_who(struct xdr_stream *xdr, int who);

fs/nfsd/nfs4xdr.c

@@ -204,6 +204,13 @@ static __be32 *read_buf(struct nfsd4_compoundargs *argp, u32 nbytes)
 	return p;
 }
 
+static unsigned int compoundargs_bytes_left(struct nfsd4_compoundargs *argp)
+{
+	unsigned int this = (char *)argp->end - (char *)argp->p;
+
+	return this + argp->pagelen;
+}
+
 static int zero_clientid(clientid_t *clid)
 {
 	return (clid->cl_boot == 0) && (clid->cl_id == 0);
@@ -348,7 +355,12 @@ nfsd4_decode_fattr(struct nfsd4_compoundargs *argp, u32 *bmval,
 		READ_BUF(4); len += 4;
 		nace = be32_to_cpup(p++);
 
-		if (nace > NFS4_ACL_MAX)
+		if (nace > compoundargs_bytes_left(argp)/20)
+			/*
+			 * Even with 4-byte names there wouldn't be
+			 * space for that many aces; something fishy is
+			 * going on:
+			 */
 			return nfserr_fbig;
 
 		*acl = svcxdr_tmpalloc(argp, nfs4_acl_bytes(nace));