[PATCH] btrfs: 'usertrans' mount option to allow unprivileged userspace transactions

[PATCH] btrfs: 'usertrans' mount option to allow unprivileged userspace transactions

[Sage Weil]

This lets an administrator give non-root users access to the btrfs 
transaction start/end ioctls via a mount option.

Currently any process using the ioctls must run as root.  That's 
appropriate in general, since the ioctls allow let any process to hang fs 
commits by holding an open transaction indefinitely.  But on locked down 
hosts (i.e. no users), that can be inconvenient because daemons using 
transactions have to run as root.

Signed-off-by: Sage Weil <sage@newdream.net>
---
 fs/btrfs/ctree.h |    1 +
 fs/btrfs/ioctl.c |    3 ++-
 fs/btrfs/super.c |    9 ++++++++-
 3 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 03441a9..eb1f3b7 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1102,6 +1102,7 @@ struct btrfs_root {
 #define BTRFS_MOUNT_FLUSHONCOMMIT       (1 << 7)
 #define BTRFS_MOUNT_SSD_SPREAD		(1 << 8)
 #define BTRFS_MOUNT_NOSSD		(1 << 9)
+#define BTRFS_MOUNT_USERTRANS           (1 << 10)
 
 #define btrfs_clear_opt(o, opt)		((o) &= ~BTRFS_MOUNT_##opt)
 #define btrfs_set_opt(o, opt)		((o) |= BTRFS_MOUNT_##opt)
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index eff18f5..2aadc14 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1183,7 +1183,8 @@ static long btrfs_ioctl_trans_start(struct file *file)
 	struct btrfs_trans_handle *trans;
 	int ret = 0;
 
-	if (!capable(CAP_SYS_ADMIN))
+	if (!capable(CAP_SYS_ADMIN) &&
+	    !btrfs_test_opt(root, USERTRANS))
 		return -EPERM;
 
 	if (file->private_data) {
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 708ac06..1be8567 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -67,7 +67,8 @@ enum {
 	Opt_degraded, Opt_subvol, Opt_device, Opt_nodatasum, Opt_nodatacow,
 	Opt_max_extent, Opt_max_inline, Opt_alloc_start, Opt_nobarrier,
 	Opt_ssd, Opt_nossd, Opt_ssd_spread, Opt_thread_pool, Opt_noacl,
-	Opt_compress, Opt_notreelog, Opt_ratio, Opt_flushoncommit, Opt_err,
+	Opt_compress, Opt_notreelog, Opt_ratio, Opt_flushoncommit,
+	Opt_usertrans, Opt_err,
 };
 
 static match_table_t tokens = {
@@ -89,6 +90,7 @@ static match_table_t tokens = {
 	{Opt_notreelog, "notreelog"},
 	{Opt_flushoncommit, "flushoncommit"},
 	{Opt_ratio, "metadata_ratio=%d"},
+	{Opt_usertrans, "usertrans"},
 	{Opt_err, NULL},
 };
 
@@ -257,6 +259,9 @@ int btrfs_parse_options(struct btrfs_root *root, char *options)
 				printk(KERN_INFO "btrfs: metadata ratio %d\n",
 				       info->metadata_ratio);
 			}
+		case Opt_usertrans:
+			printk(KERN_INFO "btrfs: enabling unprivileged user transactions\n");
+			btrfs_set_opt(info->mount_opt, USERTRANS);
 			break;
 		default:
 			break;
@@ -449,6 +454,8 @@ static int btrfs_show_options(struct seq_file *seq, struct vfsmount *vfs)
 		seq_puts(seq, ",notreelog");
 	if (btrfs_test_opt(root, FLUSHONCOMMIT))
 		seq_puts(seq, ",flushoncommit");
+	if (btrfs_test_opt(root, USERTRANS))
+		seq_puts(seq, ",usertrans");
 	if (!(root->fs_info->sb->s_flags & MS_POSIXACL))
 		seq_puts(seq, ",noacl");
 	return 0;
-- 
1.5.6.5

Re: [PATCH] btrfs: 'usertrans' mount option to allow unprivileged userspace transactions

[Christoph Hellwig]

On Thu, Jun 25, 2009 at 02:45:39PM -0700, Sage Weil wrote:
> This lets an administrator give non-root users access to the btrfs 
> transaction start/end ioctls via a mount option.
> 
> Currently any process using the ioctls must run as root.  That's 
> appropriate in general, since the ioctls allow let any process to hang fs 
> commits by holding an open transaction indefinitely.  But on locked down 
> hosts (i.e. no users), that can be inconvenient because daemons using 
> transactions have to run as root.

If the system is locked down there is no problem giving the capability
to the daemon user.  I'm strong against adding any kind of
privilegue-evading options to filesystems.

Re: [PATCH] btrfs: 'usertrans' mount option to allow unprivileged userspace transactions

[Sage Weil]

On Fri, 26 Jun 2009, Christoph Hellwig wrote:
> On Thu, Jun 25, 2009 at 02:45:39PM -0700, Sage Weil wrote:
> > This lets an administrator give non-root users access to the btrfs 
> > transaction start/end ioctls via a mount option.
> > 
> > Currently any process using the ioctls must run as root.  That's 
> > appropriate in general, since the ioctls allow let any process to hang fs 
> > commits by holding an open transaction indefinitely.  But on locked down 
> > hosts (i.e. no users), that can be inconvenient because daemons using 
> > transactions have to run as root.
> 
> If the system is locked down there is no problem giving the capability
> to the daemon user.  I'm strong against adding any kind of
> privilegue-evading options to filesystems.

Well, I was hoping for finer granularity that CAP_SYS_ADMIN, but i can 
live with it.  Thanks.

sage