[Ocfs2-devel] [RFC] The reflink(2) system call v4.

[Stephen Smalley <sds@tycho.nsa.gov>]

On Fri, 2009-05-15 at 13:53 -0700, Joel Becker wrote:
> On Fri, May 15, 2009 at 09:42:09AM -0700, Joel Becker wrote:
> > On Fri, May 15, 2009 at 11:55:25AM -0400, Stephen Smalley wrote:
> > > Consider a program that wants to always preserve attributes on the
> > > reflinks it creates.  If the interface allows the program to explicitly
> > > request that behavior and returns an error when the request cannot be
> > > honored, then the program knows that upon a successful return, the
> > > attributes were in fact preserved.  If the interface instead silently
> > > selects a behavior based on the current privileges of the process and
> > > gives no indication to the caller as to what behavior was selected, then
> > > the opportunity for error is great.
> > 
> > 	I get that.  I'm looking at what the programming interface is.
> > What's the standard function for "I want the fallback behavior" called?
> > What's the standard function for "I want preserve security" called?
> > "int reflink(oldpath, newpath)" has to pick one of the behaviors.  Which
> > is it?
> 
> 	Ok, I've been casting about how to solve the concern and provide
> a decent interface.  I'm not about to give up on either.  I think,
> though, that we do have to let the application signal its intent to the
> system.  And if we're doing that, let's add a little flexibility.
> 	I think the interface will be this (ignoring the reflinkat(2)
> bit for now):
> 
> int reflink(const char *oldpath, const char *newpath, int preserve);
> 
> - Data and xattrs are reflinked always.
> - 'preserve is a bitfield describing which attributes to keep across the
>   reflink:
>  * REFLINK_ATTR_OWNER - Keeps uid/gid the same.  Requires ownership or
>    CAP_CHOWN.
>  * REFLINK_ATTR_SECURITY - Keeps the security state (SELinux/SMACK/etc)
>    the same.  This requires REFLINK_ATTR_OWNER (the security state makes
>    no sense if the ownership changes).  If not set, the filesystem wipes
>    all security.* xattrs and reinitializes with
>    security_inode_init_security() just like a new file.
>  * REFLINK_ATTR_MODE - Keeps the mode bits the same.  Requires ownership
>    or CAP_FOWNER.
>  * REFLINK_ATTR_ACL - Keeps the ACLs the same.  Requires
>    REFLINK_ATTR_MODE, as ACLs have to get adjusted when the mode
>    changes, and so you can't keep them the same if the mode wasn't
>    preserved.  If not set, the filesystem reinits the ACLs as for a new
>    file.
> - REFLINK_ATTR_NONE is 0 and REFLINK_ATTR_ALL is ~0.
> 
> 	That's all the relevant attributes.  The timestamps behave as
> already described (ctime is now, mtime matches the source), which is the
> only sane behavior for this sort of thing.
> 	So, a copy program would reflink(source, target,
> REFLINK_ATTR_NONE), a snapshot program would reflink(source, target,
> REFLINK_ATTR_ALL), and someone wanting the fallback behavior can do it
> easily.
> 	In the kernel, security_inode_reflink() gets passed the preserve
> bits.  It's responsible for determining whether REFLINK_ATTR_SECURITY is
> allowed (vfs_reflink() will already have asserted REFLINK_ATTR_OWNER).
> It may do other checks on the reflink and the preserve bits, that's up
> to the LSM.
>         For scripting, we add the we add the '-p' and '-P' to "ln -r":
> 
> - ln -r == reflink(source, target, REFLINK_ATTR_NONE);
> - ln -r -P == reflink(source, target, REFLINK_ATTR_ALL);
> - ln -r -p == the fallback behavior.  This is like cp(1), where "cp -p"
>   is best-effort.
> 
> 	Does this make everyone happy?

For simplicity and robustness, I would only support the none or all
flags, i.e. preserve can be a simple bool.  I don't think you really
want to deal with the individual flags, and I don't see a use case for
them.

-- 
Stephen Smalley
National Security Agency