linux-kernel - Re: [PATCH] ovl: use a dedicated semaphore for dir upperfile caching

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Message-ID: <CAOQ4uxgNWkzVphdB7cAkwdUXagM_NsCUYDRT1f-=X1rn1-KpUQ@mail.gmail.com>
Date:   Sun, 3 Jan 2021 16:10:55 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Icenowy Zheng <icenowy@...c.io>
Cc:     Miklos Szeredi <miklos@...redi.hu>,
        Xiao Yang <yangx.jy@...fujitsu.com>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] ovl: use a dedicated semaphore for dir upperfile caching

On Fri, Jan 1, 2021 at 10:12 PM Icenowy Zheng <icenowy@...c.io> wrote:
>
> The function ovl_dir_real_file() currently uses the semaphore of the
> inode to synchronize write to the upperfile cache field.
>
> 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.

oops. I wondered why I didn't see any warnings on this from lockdep.
Ah! because the xfstest that exercises ovl_ioctl_set_flags() on directory,
generic/079, starts with an already upper dir.

And the xfstest that checks chattr+i on lower/upper files, overlay/040,
does not check chattr on dirs (ioctl on overlay dirs wasn't supported at
the time the test was written).

Would you be able to create a variant of test overlay/040 that also tests
chattr +i on lower/upper dirs to test your patch and confirm that the test
fails on master with the appropriate Kconfig debug options.

>
> 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@...r.kernel.org # v5.10
> Signed-off-by: Icenowy Zheng <icenowy@...c.io>
> ---
>  fs/overlayfs/readdir.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
> index 01620ebae1bd..f10701aabb71 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 semaphore upperfile_sem;

mutex please

>  };
>
>  static struct ovl_cache_entry *ovl_cache_entry_from_node(struct rb_node *n)
> @@ -883,7 +884,7 @@ struct file *ovl_dir_real_file(const struct file *file, bool want_upper)
>                         ovl_path_upper(dentry, &upperpath);
>                         realfile = ovl_dir_open_realfile(file, &upperpath);
>
> -                       inode_lock(inode);
> +                       down(&od->upperfile_sem);
>                         if (!od->upperfile) {
>                                 if (IS_ERR(realfile)) {
>                                         inode_unlock(inode);

You missed this unlock

Thanks,
Amir.