io_uring: allow sparse fixed file sets

This is in preparation for allowing updates to fixed file sets without
requiring a full unregister+register.
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2379,6 +2379,8 @@ static int io_req_set_file(struct io_ring_ctx *ctx, const struct sqe_submit *s,
 		if (unlikely(!ctx->user_files ||
 		    (unsigned) fd >= ctx->nr_user_files))
 			return -EBADF;
+		if (!ctx->user_files[fd])
+			return -EBADF;
 		req->file = ctx->user_files[fd];
 		req->flags |= REQ_F_FIXED_FILE;
 	} else {
@@ -2999,6 +3001,7 @@ static void __io_sqe_files_unregister(struct io_ring_ctx *ctx)
 	int i;
 
 	for (i = 0; i < ctx->nr_user_files; i++)
+		if (ctx->user_files[i])
 			fput(ctx->user_files[i]);
 #endif
 }
@@ -3067,7 +3070,7 @@ static int __io_sqe_files_scm(struct io_ring_ctx *ctx, int nr, int offset)
 	struct sock *sk = ctx->ring_sock->sk;
 	struct scm_fp_list *fpl;
 	struct sk_buff *skb;
-	int i;
+	int i, nr_files;
 
 	if (!capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) {
 		unsigned long inflight = ctx->user->unix_inflight + nr;
@@ -3087,21 +3090,31 @@ static int __io_sqe_files_scm(struct io_ring_ctx *ctx, int nr, int offset)
 	}
 
 	skb->sk = sk;
-	skb->destructor = io_destruct_skb;
 
+	nr_files = 0;
 	fpl->user = get_uid(ctx->user);
 	for (i = 0; i < nr; i++) {
-		fpl->fp[i] = get_file(ctx->user_files[i + offset]);
-		unix_inflight(fpl->user, fpl->fp[i]);
+		if (!ctx->user_files[i + offset])
+			continue;
+		fpl->fp[nr_files] = get_file(ctx->user_files[i + offset]);
+		unix_inflight(fpl->user, fpl->fp[nr_files]);
+		nr_files++;
 	}
 
-	fpl->max = fpl->count = nr;
+	if (nr_files) {
+		fpl->max = SCM_MAX_FD;
+		fpl->count = nr_files;
 		UNIXCB(skb).fp = fpl;
+		skb->destructor = io_destruct_skb;
 		refcount_add(skb->truesize, &sk->sk_wmem_alloc);
 		skb_queue_head(&sk->sk_receive_queue, skb);
 
-	for (i = 0; i < nr; i++)
+		for (i = 0; i < nr_files; i++)
 			fput(fpl->fp[i]);
+	} else {
+		kfree_skb(skb);
+		kfree(fpl);
+	}
 
 	return 0;
 }
@@ -3132,6 +3145,7 @@ static int io_sqe_files_scm(struct io_ring_ctx *ctx)
 		return 0;
 
 	while (total < ctx->nr_user_files) {
+		if (ctx->user_files[total])
 			fput(ctx->user_files[total]);
 		total++;
 	}
@@ -3163,10 +3177,15 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
 	if (!ctx->user_files)
 		return -ENOMEM;
 
-	for (i = 0; i < nr_args; i++) {
+	for (i = 0; i < nr_args; i++, ctx->nr_user_files++) {
 		ret = -EFAULT;
 		if (copy_from_user(&fd, &fds[i], sizeof(fd)))
 			break;
+		/* allow sparse sets */
+		if (fd == -1) {
+			ret = 0;
+			continue;
+		}
 
 		ctx->user_files[i] = fget(fd);
 
@@ -3184,12 +3203,12 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
 			fput(ctx->user_files[i]);
 			break;
 		}
-		ctx->nr_user_files++;
 		ret = 0;
 	}
 
 	if (ret) {
 		for (i = 0; i < ctx->nr_user_files; i++)
+			if (ctx->user_files[i])
 				fput(ctx->user_files[i]);
 
 		kfree(ctx->user_files);