Re: [PATCH V8 2/3] fuse: Introduce synchronous read and write for passthrough

[Alessio Balsini]

Hi Amir,

On Sat, Sep 12, 2020 at 12:55:35PM +0300, Amir Goldstein wrote:
> On Fri, Sep 11, 2020 at 7:34 PM Alessio Balsini <balsini@xxxxxxxxxxx> wrote:
> > +ssize_t fuse_passthrough_read_iter(struct kiocb *iocb_fuse,
> > +                                  struct iov_iter *iter)
> > +{
> > +       ssize_t ret;
> > +       struct file *fuse_filp = iocb_fuse->ki_filp;
> > +       struct fuse_file *ff = fuse_filp->private_data;
> > +       struct file *passthrough_filp = ff->passthrough_filp;
> > +
> > +       if (!iov_iter_count(iter))
> > +               return 0;
> > +
> > +       if (is_sync_kiocb(iocb_fuse)) {
> > +               struct kiocb iocb;
> > +
> > +               kiocb_clone(&iocb, iocb_fuse, passthrough_filp);
> > +               ret = call_read_iter(passthrough_filp, &iocb, iter);
> > +               iocb_fuse->ki_pos = iocb.ki_pos;
> > +               if (ret >= 0)
> > +                       fuse_copyattr(fuse_filp, passthrough_filp, false);
> > +
> > +       } else {
> > +               ret = -EIO;
> > +       }
> > +
> > +       return ret;
> > +}
> > +
> > +ssize_t fuse_passthrough_write_iter(struct kiocb *iocb_fuse,
> > +                                   struct iov_iter *iter)
> > +{
> > +       ssize_t ret;
> > +       struct file *fuse_filp = iocb_fuse->ki_filp;
> > +       struct fuse_file *ff = fuse_filp->private_data;
> > +       struct inode *fuse_inode = file_inode(fuse_filp);
> > +       struct file *passthrough_filp = ff->passthrough_filp;
> > +
> > +       if (!iov_iter_count(iter))
> > +               return 0;
> > +
> > +       inode_lock(fuse_inode);
> > +
> > +       if (is_sync_kiocb(iocb_fuse)) {
> > +               struct kiocb iocb;
> > +
> > +               kiocb_clone(&iocb, iocb_fuse, passthrough_filp);
> > +
> > +               file_start_write(passthrough_filp);
> > +               ret = call_write_iter(passthrough_filp, &iocb, iter);
> 
> Why not vfs_iter_write()/vfs_iter_read()?
> 
> You are bypassing many internal VFS checks that seem pretty important.
> 

I've been thinking a lot about this and decided to go for the VFS bypassing
solution because:
1. it looked odd to me to perform VFS checks twice, both for FUSE and lower
   FS and it seemed to me that we found a tradeoff with Jann about doing
   this lower FS call, and
2. in our Android use case (I just saw you asking for more details about
   this in, I'll reply on the other thread), the user might have the right
   credentials to access the FUSE file system, but not to access the lower
   file system, so the VFS checkings would fail. So that would have created
   the need for a credential bypassing that looked hacky.

But I agree and I would probably sleep better knowing that VFS checks are
not skipped :) So I decided to implemented the vfs_iter_{read,write}()
variant.

I again picked a lot from the overlayfs solution. In a few words, I get the
FUSE daemon credential reference at FUSE FS creation time and, when
passthrough read/write operations are triggered, the kernel temporarily
overrides the requesting process' credentials with those of the FUSE
daemon. Credentials are reverted as soon as the operation completes.

I have a temporary development branch where I'm developing the V9 of this
patch, plus the VFS variant (git history may change):

https://github.com/balsini/linux/tree/fuse-passthrough-stable-v5.8-v9-vfs

For now, I'm happy to say that I like this VFS solution, it also simplified
the lower file system notifications, so I'll probably go with this in the
V9.

Thanks again Amir, much appreciated.
Alessio