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

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Message-ID: <CAOQ4uxiFoQhrMbs91ZUNXqbJUXb5XRBgRrcq1rmChLKQGKg5xg@mail.gmail.com>
Date:   Tue, 5 Jan 2021 08:47:41 +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 v3] ovl: use a dedicated semaphore for dir upperfile caching

On Tue, Jan 5, 2021 at 2:36 AM 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.

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@...r.kernel.org # v5.10
> Signed-off-by: Icenowy Zheng <icenowy@...c.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.

Thanks,
Amir.