Re: [PATCH] fs: fuse: dax: set fc->dax to NULL in fuse_dax_conn_free()

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Dec 01, 2023 at 10:42:53AM +0800, Hangyu Hua wrote:
> On 30/11/2023 18:54, Miklos Szeredi wrote:
> > On Thu, 16 Nov 2023 at 08:57, Hangyu Hua <hbh25y@gmail.com> wrote:
> > > 
> > > fuse_dax_conn_free() will be called when fuse_fill_super_common() fails
> > > after fuse_dax_conn_alloc(). Then deactivate_locked_super() in
> > > virtio_fs_get_tree() will call virtio_kill_sb() to release the discarded
> > > superblock. This will call fuse_dax_conn_free() again in fuse_conn_put(),
> > > resulting in a possible double free.
> > > 
> > > Fixes: 1dd539577c42 ("virtiofs: add a mount option to enable dax")
> > > Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
> > > ---
> > >   fs/fuse/dax.c | 1 +
> > >   1 file changed, 1 insertion(+)
> > > 
> > > diff --git a/fs/fuse/dax.c b/fs/fuse/dax.c
> > > index 23904a6a9a96..12ef91d170bb 100644
> > > --- a/fs/fuse/dax.c
> > > +++ b/fs/fuse/dax.c
> > > @@ -1222,6 +1222,7 @@ void fuse_dax_conn_free(struct fuse_conn *fc)
> > >          if (fc->dax) {
> > >                  fuse_free_dax_mem_ranges(&fc->dax->free_ranges);
> > >                  kfree(fc->dax);
> > > +               fc->dax = NULL;
> > 
> > Is there a reason not to simply remove the fuse_dax_conn_free() call
> > from the cleanup path in fuse_fill_super_common()?
> 
> I think setting fc->dax to NULL keeps the memory allocation and release
> functions together in fuse_fill_super_common more readable. What do you
> think?

I agree with this. fuse_fill_super_common() calls fuse_dax_conn_alloc()
which in-turn initializes fc->dax. If fuse_fill_super_common() fails
later after calling fuse_dax_conn_alloc(), then cleanup of fc->dax
and other associated stuff in same function makes sense.

As a code reader I would like to know how fc->dax is being freed in
case of error and its right there in the error path (err_free_dax:).

I think I set the fc->dax upon initialization. Upon failure I freed
the data structures but did not set fc->dax back to NULL.

To me, this patch looks reasonable.

Acked-by: Vivek Goyal <vgoyal@redhat.com>

Thanks
Vivek

> 
> Thanks,
> Hangyu
> 
> > 
> > Thanks,
> > Miklos
> > 
> > 
> > >          }
> > >   }
> > > 
> > > --
> > > 2.34.1
> > > 
>