Re: [PATCH v3] ovl: fix null pointer when filesystem doesn't support direct IO

[Huang Jianan]

在 2021/9/22 16:06, Chengguang Xu 写道:
> 在 2021/9/22 15:23, Huang Jianan 写道:
> > From: Huang Jianan <huangjianan@xxxxxxxx>
> >
> > At present, overlayfs provides overlayfs inode to users. Overlayfs
> > inode provides ovl_aops with noop_direct_IO to avoid open failure
> > with O_DIRECT. But some compressed filesystems, such as erofs and
> > squashfs, don't support direct_IO.
> >
> > Users who use f_mapping->a_ops->direct_IO to check O_DIRECT support,
> > will read file through this way. This will cause overlayfs to access
> > a non-existent direct_IO function and cause panic due to null pointer:
>
> I just looked around the code more closely, in open_with_fake_path(),
>
> do_dentry_open() has already checked O_DIRECT open flag and 
> a_ops->direct_IO of underlying real address_space.
>
> Am I missing something?

It seems that loop_update_dio will set lo->use_dio after open file 
without set O_DIRECT.
loop_update_dio will check f_mapping->a_ops->direct_IO but it deal with 
ovl_aops with
noop _direct_IO.

So I think we still need a new aops?

Thanks,
Jianan

> Thanks,
>
> Chengguang
>
>
> > Kernel panic - not syncing: CFI failure (target: 0x0)
> > CPU: 6 PID: 247 Comm: loop0
> > Call Trace:
> >   panic+0x188/0x45c
> >   __cfi_slowpath+0x0/0x254
> >   __cfi_slowpath+0x200/0x254
> >   generic_file_read_iter+0x14c/0x150
> >   vfs_iocb_iter_read+0xac/0x164
> >   ovl_read_iter+0x13c/0x2fc
> >   lo_rw_aio+0x2bc/0x458
> >   loop_queue_work+0x4a4/0xbc0
> >   kthread_worker_fn+0xf8/0x1d0
> >   loop_kthread_worker_fn+0x24/0x38
> >   kthread+0x29c/0x310
> >   ret_from_fork+0x10/0x30
> >
> > The filesystem may only support direct_IO for some file types. For
> > example, erofs supports direct_IO for uncompressed files. So return
> > -EINVAL when the file doesn't support direct_IO to fix this problem.
> >
> > Fixes: 5b910bd615ba ("ovl: fix GPF in swapfile_activate of file from 
> > overlayfs over xfs")
> > Signed-off-by: Huang Jianan <huangjianan@xxxxxxxx>
> > ---
> > change since v2:
> >   - Return error in ovl_open directly. (Chengguang Xu)
> >
> > Change since v1:
> >   - Return error to user rather than fall back to buffered io. 
> > (Chengguang Xu)
> >
> >   fs/overlayfs/file.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> > index d081faa55e83..a0c99ea35daf 100644
> > --- a/fs/overlayfs/file.c
> > +++ b/fs/overlayfs/file.c
> > @@ -157,6 +157,10 @@ static int ovl_open(struct inode *inode, struct 
> > file *file)
> >       if (IS_ERR(realfile))
> >           return PTR_ERR(realfile);
> >   +    if ((f->f_flags & O_DIRECT) && (!realfile->f_mapping->a_ops ||
> > +        !realfile->f_mapping->a_ops->direct_IO))
> > +        return -EINVAL;
> > +
> >       file->private_data = realfile;
> >         return 0;