anonfd: Allow making anon files read-only

It seems a couple places such as arch/ia64/kernel/perfmon.c and
drivers/infiniband/core/uverbs_main.c could use anon_inode_getfile()
instead of a private pseudo-fs + alloc_file(), if only there were a way
to get a read-only file.  So provide this by having anon_inode_getfile()
create a read-only file if we pass O_RDONLY in flags.
Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/anon_inodes.c

@@ -89,11 +89,19 @@ struct file *anon_inode_getfile(const char *name,
 	struct qstr this;
 	struct path path;
 	struct file *file;
+	fmode_t mode;
 	int error;
 
 	if (IS_ERR(anon_inode_inode))
 		return ERR_PTR(-ENODEV);
 
+	switch (flags & O_ACCMODE) {
+	case O_RDONLY: mode = FMODE_READ;		break;
+	case O_WRONLY: mode = FMODE_WRITE;		break;
+	case O_RDWR:   mode = FMODE_READ | FMODE_WRITE;	break;
+	default:       return ERR_PTR(-EINVAL);
+	}
+
 	if (fops->owner && !try_module_get(fops->owner))
 		return ERR_PTR(-ENOENT);
 
@@ -121,13 +129,13 @@ struct file *anon_inode_getfile(const char *name,
 	d_instantiate(path.dentry, anon_inode_inode);
 
 	error = -ENFILE;
-	file = alloc_file(&path, FMODE_READ | FMODE_WRITE, fops);
+	file = alloc_file(&path, mode, fops);
 	if (!file)
 		goto err_dput;
 	file->f_mapping = anon_inode_inode->i_mapping;
 
 	file->f_pos = 0;
-	file->f_flags = O_RDWR | (flags & O_NONBLOCK);
+	file->f_flags = flags & (O_ACCMODE | O_NONBLOCK);
 	file->f_version = 0;
 	file->private_data = priv;
 

fs/eventfd.c

@@ -339,7 +339,7 @@ struct file *eventfd_file_create(unsigned int count, int flags)
 	ctx->flags = flags;
 
 	file = anon_inode_getfile("[eventfd]", &eventfd_fops, ctx,
-				  flags & EFD_SHARED_FCNTL_FLAGS);
+				  O_RDWR | (flags & EFD_SHARED_FCNTL_FLAGS));
 	if (IS_ERR(file))
 		eventfd_free_ctx(ctx);
 

fs/eventpoll.c

@@ -1206,7 +1206,7 @@ SYSCALL_DEFINE1(epoll_create1, int, flags)
 	 * a file structure and a free file descriptor.
 	 */
 	error = anon_inode_getfd("[eventpoll]", &eventpoll_fops, ep,
-				 flags & O_CLOEXEC);
+				 O_RDWR | (flags & O_CLOEXEC));
 	if (error < 0)
 		ep_free(ep);
 

fs/signalfd.c

@@ -236,7 +236,7 @@ SYSCALL_DEFINE4(signalfd4, int, ufd, sigset_t __user *, user_mask,
 		 * anon_inode_getfd() will install the fd.
 		 */
 		ufd = anon_inode_getfd("[signalfd]", &signalfd_fops, ctx,
-				       flags & (O_CLOEXEC | O_NONBLOCK));
+				       O_RDWR | (flags & (O_CLOEXEC | O_NONBLOCK)));
 		if (ufd < 0)
 			kfree(ctx);
 	} else {

fs/timerfd.c

@@ -200,7 +200,7 @@ SYSCALL_DEFINE2(timerfd_create, int, clockid, int, flags)
 	hrtimer_init(&ctx->tmr, clockid, HRTIMER_MODE_ABS);
 
 	ufd = anon_inode_getfd("[timerfd]", &timerfd_fops, ctx,
-			       flags & TFD_SHARED_FCNTL_FLAGS);
+			       O_RDWR | (flags & TFD_SHARED_FCNTL_FLAGS));
 	if (ufd < 0)
 		kfree(ctx);
 

kernel/perf_event.c

@@ -4724,7 +4724,7 @@ SYSCALL_DEFINE5(perf_event_open,
 	if (IS_ERR(event))
 		goto err_put_context;
 
-	err = anon_inode_getfd("[perf_event]", &perf_fops, event, 0);
+	err = anon_inode_getfd("[perf_event]", &perf_fops, event, O_RDWR);
 	if (err < 0)
 		goto err_free_put_context;
 

virt/kvm/kvm_main.c

@@ -1177,7 +1177,7 @@ static struct file_operations kvm_vcpu_fops = {
  */
 static int create_vcpu_fd(struct kvm_vcpu *vcpu)
 {
-	return anon_inode_getfd("kvm-vcpu", &kvm_vcpu_fops, vcpu, 0);
+	return anon_inode_getfd("kvm-vcpu", &kvm_vcpu_fops, vcpu, O_RDWR);
 }
 
 /*
@@ -1638,7 +1638,7 @@ static int kvm_dev_ioctl_create_vm(void)
 	kvm = kvm_create_vm();
 	if (IS_ERR(kvm))
 		return PTR_ERR(kvm);
-	fd = anon_inode_getfd("kvm-vm", &kvm_vm_fops, kvm, 0);
+	fd = anon_inode_getfd("kvm-vm", &kvm_vm_fops, kvm, O_RDWR);
 	if (fd < 0)
 		kvm_put_kvm(kvm);