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

[Huang Jianan]

在 2021/9/22 9:56, Chengguang Xu 写道:
> 在 2021/9/18 20:13, 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:
> >
> > 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 fall
> > back to buffered io only when the file doesn't support direct_IO to
> > fix this problem.
>
>
> IMO, return error to user seems better option than fall back to
>
> buffered io directly.
Agreed, I will send v2 to fix it.

Thanks,
Jianan
> Thanks,
>
> Chengguang
>
>
> > Fixes: 5b910bd615ba ("ovl: fix GPF in swapfile_activate of file from 
> > overlayfs over xfs")
> > Signed-off-by: Huang Jianan <huangjianan@xxxxxxxx>
> > ---
> >   fs/overlayfs/file.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> > index d081faa55e83..998c60770b81 100644
> > --- a/fs/overlayfs/file.c
> > +++ b/fs/overlayfs/file.c
> > @@ -296,6 +296,10 @@ static ssize_t ovl_read_iter(struct kiocb *iocb, 
> > struct iov_iter *iter)
> >       if (ret)
> >           return ret;
> >   +    if ((iocb->ki_flags & IOCB_DIRECT) && 
> > (!real.file->f_mapping->a_ops ||
> > +        !real.file->f_mapping->a_ops->direct_IO))
> > +        iocb->ki_flags &= ~IOCB_DIRECT;
> > +
> >       old_cred = ovl_override_creds(file_inode(file)->i_sb);
> >       if (is_sync_kiocb(iocb)) {
> >           ret = vfs_iter_read(real.file, iter, &iocb->ki_pos,