Re: [PATCH v3] ovl: use a dedicated semaphore for dir upperfile caching

[Amir Goldstein <amir73il@gmail.com>]

On Wed, Jan 20, 2021 at 12:20 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Tue, Jan 05, 2021 at 08:47:41AM +0200, Amir Goldstein wrote:
> > On Tue, Jan 5, 2021 at 2:36 AM Icenowy Zheng <icenowy@aosc.io> wrote:
> > >
> > > The function ovl_dir_real_file() currently uses the semaphore of the
> > > inode to synchronize write to the upperfile cache field.
> >
> > Although the inode lock is a rw_sem it is referred to as the "inode lock"
> > and you also left semaphore in the commit subject.
> > No need to re-post. This can be fixed on commit.
> >
> > >
> > > However, this function will get called by ovl_ioctl_set_flags(), which
> > > utilizes the inode semaphore too. In this case ovl_dir_real_file() will
> > > try to claim a lock that is owned by a function in its call stack, which
> > > won't get released before ovl_dir_real_file() returns.
> > >
> > > Define a dedicated semaphore for the upperfile cache, so that the
> > > deadlock won't happen.
> > >
> > > Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR ioctls for directories")
> > > Cc: stable@vger.kernel.org # v5.10
> > > Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
> > > ---
> > > Changes in v2:
> > > - Fixed missing replacement in error handling path.
> > > Changes in v3:
> > > - Use mutex instead of semaphore.
> > >
> > >  fs/overlayfs/readdir.c | 10 +++++-----
> > >  1 file changed, 5 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> > > index 01620ebae1bd..3980f9982f34 100644
> > > --- a/fs/overlayfs/readdir.c
> > > +++ b/fs/overlayfs/readdir.c
> > > @@ -56,6 +56,7 @@ struct ovl_dir_file {
> > >         struct list_head *cursor;
> > >         struct file *realfile;
> > >         struct file *upperfile;
> > > +       struct mutex upperfile_mutex;
> >
> > That's a very specific name.
> > This mutex protects members of struct ovl_dir_file, which could evolve
> > into struct ovl_file one day (because no reason to cache only dir upper file),
> > so I would go with a more generic name, but let's leave it to Miklos to decide.
> >
> > He could have a different idea altogether for fixing this bug.
>
> How about this (untested) patch?
>

Much better :)

> It's a cleanup as well as a fix, but maybe we should separate the cleanup from
> the fix...
>
> Thanks,
> Miklos
> ---
>
>  fs/overlayfs/readdir.c |   23 +++++++----------------
>  1 file changed, 7 insertions(+), 16 deletions(-)
>
> --- a/fs/overlayfs/readdir.c
> +++ b/fs/overlayfs/readdir.c
> @@ -865,7 +865,7 @@ struct file *ovl_dir_real_file(const str
>
>         struct ovl_dir_file *od = file->private_data;
>         struct dentry *dentry = file->f_path.dentry;
> -       struct file *realfile = od->realfile;
> +       struct file *old, *realfile = od->realfile;
>
>         if (!OVL_TYPE_UPPER(ovl_path_type(dentry)))
>                 return want_upper ? NULL : realfile;
> @@ -874,29 +874,20 @@ struct file *ovl_dir_real_file(const str
>          * Need to check if we started out being a lower dir, but got copied up
>          */
>         if (!od->is_upper) {
> -               struct inode *inode = file_inode(file);
> -
>                 realfile = READ_ONCE(od->upperfile);
>                 if (!realfile) {
>                         struct path upperpath;
>
>                         ovl_path_upper(dentry, &upperpath);
>                         realfile = ovl_dir_open_realfile(file, &upperpath);
> +                       if (IS_ERR(realfile))
> +                               return realfile;
>
> -                       inode_lock(inode);
> -                       if (!od->upperfile) {
> -                               if (IS_ERR(realfile)) {
> -                                       inode_unlock(inode);
> -                                       return realfile;
> -                               }
> -                               smp_store_release(&od->upperfile, realfile);
> -                       } else {
> -                               /* somebody has beaten us to it */
> -                               if (!IS_ERR(realfile))
> -                                       fput(realfile);
> -                               realfile = od->upperfile;
> +                       old = cmpxchg_release(&od->upperfile, NULL, realfile);
> +                       if (old) {
> +                               fput(realfile);
> +                               realfile = old;
>                         }
> -                       inode_unlock(inode);
>                 }
>         }
>