Re: [PATCH v8 RESEND 3/8] vfs: plumb i_version handling into struct kstat

[Christian Brauner <brauner@kernel.org>]

On Wed, Jan 25, 2023 at 01:30:07PM -0500, Jeff Layton wrote:
> On Wed, 2023-01-25 at 16:50 +0100, Christian Brauner wrote:
> > On Tue, Jan 24, 2023 at 02:30:20PM -0500, Jeff Layton wrote:
> > > The NFS server has a lot of special handling for different types of
> > > change attribute access, depending on the underlying filesystem. In
> > > most cases, it's doing a getattr anyway and then fetching that value
> > > after the fact.
> > > 
> > > Rather that do that, add a new STATX_CHANGE_COOKIE flag that is a
> > > kernel-only symbol (for now). If requested and getattr can implement it,
> > > it can fill out this field. For IS_I_VERSION inodes, add a generic
> > > implementation in vfs_getattr_nosec. Take care to mask
> > > STATX_CHANGE_COOKIE off in requests from userland and in the result
> > > mask.
> > > 
> > > Since not all filesystems can give the same guarantees of monotonicity,
> > > claim a STATX_ATTR_CHANGE_MONOTONIC flag that filesystems can set to
> > > indicate that they offer an i_version value that can never go backward.
> > > 
> > > Eventually if we decide to make the i_version available to userland, we
> > > can just designate a field for it in struct statx, and move the
> > > STATX_CHANGE_COOKIE definition to the uapi header.
> > > 
> > > Reviewed-by: NeilBrown <neilb@suse.de>
> > > Signed-off-by: Jeff Layton <jlayton@kernel.org>
> > > ---
> > >  fs/stat.c            | 17 +++++++++++++++--
> > >  include/linux/stat.h |  9 +++++++++
> > >  2 files changed, 24 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/fs/stat.c b/fs/stat.c
> > > index d6cc74ca8486..f43afe0081fe 100644
> > > --- a/fs/stat.c
> > > +++ b/fs/stat.c
> > > @@ -18,6 +18,7 @@
> > >  #include <linux/syscalls.h>
> > >  #include <linux/pagemap.h>
> > >  #include <linux/compat.h>
> > > +#include <linux/iversion.h>
> > >  
> > >  #include <linux/uaccess.h>
> > >  #include <asm/unistd.h>
> > > @@ -122,6 +123,11 @@ int vfs_getattr_nosec(const struct path *path, struct kstat *stat,
> > >  	stat->attributes_mask |= (STATX_ATTR_AUTOMOUNT |
> > >  				  STATX_ATTR_DAX);
> > >  
> > > +	if ((request_mask & STATX_CHANGE_COOKIE) && IS_I_VERSION(inode)) {
> > > +		stat->result_mask |= STATX_CHANGE_COOKIE;
> > > +		stat->change_cookie = inode_query_iversion(inode);
> > > +	}
> > > +
> > >  	mnt_userns = mnt_user_ns(path->mnt);
> > >  	if (inode->i_op->getattr)
> > >  		return inode->i_op->getattr(mnt_userns, path, stat,
> > > @@ -602,9 +608,11 @@ cp_statx(const struct kstat *stat, struct statx __user *buffer)
> > >  
> > >  	memset(&tmp, 0, sizeof(tmp));
> > >  
> > > -	tmp.stx_mask = stat->result_mask;
> > > +	/* STATX_CHANGE_COOKIE is kernel-only for now */
> > > +	tmp.stx_mask = stat->result_mask & ~STATX_CHANGE_COOKIE;
> > >  	tmp.stx_blksize = stat->blksize;
> > > -	tmp.stx_attributes = stat->attributes;
> > > +	/* STATX_ATTR_CHANGE_MONOTONIC is kernel-only for now */
> > > +	tmp.stx_attributes = stat->attributes & ~STATX_ATTR_CHANGE_MONOTONIC;
> > >  	tmp.stx_nlink = stat->nlink;
> > >  	tmp.stx_uid = from_kuid_munged(current_user_ns(), stat->uid);
> > >  	tmp.stx_gid = from_kgid_munged(current_user_ns(), stat->gid);
> > > @@ -643,6 +651,11 @@ int do_statx(int dfd, struct filename *filename, unsigned int flags,
> > >  	if ((flags & AT_STATX_SYNC_TYPE) == AT_STATX_SYNC_TYPE)
> > >  		return -EINVAL;
> > >  
> > > +	/* STATX_CHANGE_COOKIE is kernel-only for now. Ignore requests
> > > +	 * from userland.
> > > +	 */
> > > +	mask &= ~STATX_CHANGE_COOKIE;
> > > +
> > >  	error = vfs_statx(dfd, filename, flags, &stat, mask);
> > >  	if (error)
> > >  		return error;
> > > diff --git a/include/linux/stat.h b/include/linux/stat.h
> > > index ff277ced50e9..52150570d37a 100644
> > > --- a/include/linux/stat.h
> > > +++ b/include/linux/stat.h
> > 
> > Sorry being late to the party once again...
> > 
> > > @@ -52,6 +52,15 @@ struct kstat {
> > >  	u64		mnt_id;
> > >  	u32		dio_mem_align;
> > >  	u32		dio_offset_align;
> > > +	u64		change_cookie;
> > >  };
> > >  
> > > +/* These definitions are internal to the kernel for now. Mainly used by nfsd. */
> > > +
> > > +/* mask values */
> > > +#define STATX_CHANGE_COOKIE		0x40000000U	/* Want/got stx_change_attr */
> > > +
> > > +/* file attribute values */
> > > +#define STATX_ATTR_CHANGE_MONOTONIC	0x8000000000000000ULL /* version monotonically increases */
> > 
> > maybe it would be better to copy what we do for SB_* vs SB_I_* flags and
> > at least rename them to:
> > 
> > STATX_I_CHANGE_COOKIE
> > STATX_I_ATTR_CHANGE_MONOTONIC
> > i_change_cookie
> 
> An "i_"/"I_" prefix says "inode" to me. Maybe I've been at this too
> long. ;)
> 
> > 
> > to visually distinguish internal and external flags.
> > 
> > And also if possible it might be useful to move STATX_I_* flags to the
> > higher 32 bits and then one can use upper_32_bits to retrieve kernel
> > internal flags and lower_32_bits for userspace flags in tiny wrappers.
> > 
> > (I did something similar for clone3() a few years ago but there to
> > distinguish between flags available both in clone() and clone3() and
> > such that are only available in clone3().)
> > 
> > But just a thought. I mostly worry about accidently leaking this to
> > userspace so ideally we'd even have separate fields in struct kstat for
> > internal and external attributes but that might bump kstat size, though
> > I don't think struct kstat is actually ever really allocated all that
> > much.
> 
> I'm not sure that the internal/external distinction matters much for
> filesystem providers or consumers of it. The place that it matters is at
> the userland interface level, where statx or something similar is
> called.
> 
> At some point we may want to make STATX_CHANGE_COOKIE queryable via
> statx, at which point we'll have to have a big flag day where we do
> s/STATX_I_CHANGE_COOKIE/STATX_CHANGE_COOKIE/.
> 
> I don't think it's worth it here.

I'm not fond of internal and external STATX_* flags having the exact
same name but fine. :)