Re: [RFC PATCH 06/11] xfs: initialize fs-verity on file open and cleanup on inode destruction

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Dec 14, 2022 at 12:35:24PM +1100, Dave Chinner wrote:
> On Tue, Dec 13, 2022 at 06:29:30PM +0100, Andrey Albershteyn wrote:
> > fs-verity will read and attach metadata (not the tree itself) from
> > a disk for those inodes which already have fs-verity enabled.
> > 
> > Signed-off-by: Andrey Albershteyn <aalbersh@redhat.com>
> > ---
> >  fs/xfs/xfs_file.c  | 8 ++++++++
> >  fs/xfs/xfs_super.c | 2 ++
> >  2 files changed, 10 insertions(+)
> > 
> > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > index 242165580e682..5eadd9a37c50e 100644
> > --- a/fs/xfs/xfs_file.c
> > +++ b/fs/xfs/xfs_file.c
> > @@ -32,6 +32,7 @@
> >  #include <linux/mman.h>
> >  #include <linux/fadvise.h>
> >  #include <linux/mount.h>
> > +#include <linux/fsverity.h>
> >  
> >  static const struct vm_operations_struct xfs_file_vm_ops;
> >  
> > @@ -1170,9 +1171,16 @@ xfs_file_open(
> >  	struct inode	*inode,
> >  	struct file	*file)
> >  {
> > +	int		error = 0;
> > +
> >  	if (xfs_is_shutdown(XFS_M(inode->i_sb)))
> >  		return -EIO;
> >  	file->f_mode |= FMODE_NOWAIT | FMODE_BUF_RASYNC | FMODE_BUF_WASYNC;
> > +
> > +	error = fsverity_file_open(inode, file);
> > +	if (error)
> > +		return error;
> 
> This is a hot path, so shouldn't we elide the function call
> altogether if verity is not enabled on the inode? i.e:
> 
> 	if (IS_VERITY(inode)) {
> 		error = fsverity_file_open(inode, file);
> 		if (error)
> 			return error;
> 	}
> 
> It doesn't really matter for a single file open, but when you're
> opening a few million inodes every second the function call overhead
> only to immediately return because IS_VERITY() is false adds up...
> 
> >  	return generic_file_open(inode, file);
> >  }
> >  
> > diff --git a/fs/xfs/xfs_super.c b/fs/xfs/xfs_super.c
> > index 8f1e9b9ed35d9..50c2c819ba940 100644
> > --- a/fs/xfs/xfs_super.c
> > +++ b/fs/xfs/xfs_super.c
> > @@ -45,6 +45,7 @@
> >  #include <linux/magic.h>
> >  #include <linux/fs_context.h>
> >  #include <linux/fs_parser.h>
> > +#include <linux/fsverity.h>
> >  
> >  static const struct super_operations xfs_super_operations;
> >  
> > @@ -647,6 +648,7 @@ xfs_fs_destroy_inode(
> >  	ASSERT(!rwsem_is_locked(&inode->i_rwsem));
> >  	XFS_STATS_INC(ip->i_mount, vn_rele);
> >  	XFS_STATS_INC(ip->i_mount, vn_remove);
> > +	fsverity_cleanup_inode(inode);
> 
> Similarly, shouldn't this be:
> 
> 	if (fsverity_active(inode))
> 		fsverity_cleanup_inode(inode);
> 

If you actually want to do that, then we should instead make these functions
inline functions that do the "is anything needed?" check, then call a
double-underscored version that does the actual work.  Some of the fscrypt
functions are like that.  Then all filesystems would get the benefit.

Funnily enough, I had actually wanted to do that for fsverity_file_open()
originally, but Ted had preferred the simpler version.

Anyway, if this is something you want, I can change it to be that way.

- Eric