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

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Date:   Thu, 21 Jan 2021 09:07:26 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Icenowy Zheng <icenowy@...c.io>
Cc:     Amir Goldstein <amir73il@...il.com>,
        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 Thu, Jan 21, 2021 at 4:43 AM Icenowy Zheng <icenowy@...c.io> wrote:
>
> 在 2021-01-20星期三的 11:20 +0100，Miklos Szeredi写道：
> > 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@...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.
> >
> > How about this (untested) patch?
> >
> > It's a cleanup as well as a fix, but maybe we should separate the
> > cleanup from
> > the fix...
>
> If you are going to post this, feel free to add
>
> Tested-by: Icenowy Zheng <icenowy@...c.io>

Okay, thanks.

> (And if you remove the IS_ERR(realfile) part, the tested-by tag still
> applies.)

Dropping the IS_ERR(realfile) here would mean having to add the same
check before relevant fput() calls, which would make it more complex
not less.

Or did you mean something else?

Thanks,
Miklos