Re: [RFC v3 2/2] fuse: Add posix acl support

[ebiederm@xmission.com (Eric W. Biederman)]

Seth Forshee <seth.forshee@canonical.com> writes:

> On Sat, Aug 06, 2016 at 11:09:54PM +0200, Miklos Szeredi wrote:
>> On Sat, Aug 6, 2016 at 3:52 AM, Seth Forshee <seth.forshee@canonical.com> wrote:
>> > On Fri, Aug 05, 2016 at 06:07:44PM -0500, Eric W. Biederman wrote:
>> > What I'm not convinced of is that the userspace visible changes in
>> > behavior won't break someone's software, even if they aren't really
>> > getting acl enforcement.
>> 
>> That's a key point.  Backward compatibility is important, and not even
>> hard to do because fuse can negotiate supported features with the
>> userspace filesystem.
>> 
>> So we can have a new FUSE_POSIX_ACL feature flag in INIT, sent if
>> "default_permissions" is on.
>> 
>> If not set in INIT reply just pass all xattrs through to the
>> filesystem.  Caching should not be done. Don't think about whether
>> it's logical or not, or if anyone could use it for anything sane.
>> Just do what we are doing currently.  Translating uids still makes
>> sense, but that's another story.
>
> Translating uids is actually central to the differing positions that you
> and Eric have. What Eric wants is for the only path for supporting posix
> acls to be via the helpers, that way all concerns about translating uids
> can be addressed there. If fuse is to allow the xattrs to be passed
> directly through to the filesystem then there has to be a second
> mechanism which translates the uids for that case.

I think I will agree with Miklos.  Translating uids is another story and
worst case we can work on that in September when I get back from
vacation.  Nothing of what you are talking about will make things worse
for translating uids, so it is perfectly fine to merge posix acl support
into fuse and then handle uid translation.

Until we set FS_USER_NS fuse it is fine to not get the uid translation.

Which yields a very simple implementation:

In fuse_fill_super:
	if (!fc->posix_acl) {
        	sb->s_xattr = fuse_xattr_handlers;
        } else {
        	sb->s_xattr = fuse_acl_xattr_handlers;
        }

Where fuse_acl_xattr_handlers are a different array that
includes the posix acl interrcept (aka the array in patch 1 or the array
in patch 2).

Then fuse_get_acl and fuse_set_acl can just test fc->posix_acl
and fail if that is not set.

I think that is all you need to do, and we can worry about the other
details after posix acl support has landed in fuse.

>> If the flag is set in INIT reply, then that means userspace filesystem
>> wants handling of posix acl permission checking in kernel.  It would
>> also mean that caching of posix acl are allowed (lifetime linked to
>> attribute lifetime).
>> 
>> If filesystem wants to explicitly disable posix acl support, then it
>> can reply EOPNOTSUPP to getxattr and setxattr on "system.posix_acl_*".
>>   Alternatively we can add a FUSE_NO_POSIX_ACL feature flag, that
>> filesystem can return in reply to FUSE_POSIX_ACL.
>> 
>> I agree that adding CONFIG_FUSE_FS_POSIX_ACL is probably not worth it,
>> just make any such code dependent on CONFIG_FS_POSIX_ACL.
>
> But CONFIG_FS_POSIX_ACL doesn't have an input prompt and thus isn't
> displayed in menuconfig, etc. If that's what you want, fine, but it
> seems like an unusual situation.

Then in Kconfig I would have FUSE_FS "select FS_POSIX_ACL".  That
simplifies the problem.  Unless Miklos wants something more granular.

Eric