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From: Amir Goldstein <amir73il@gmail.com>
Date: Mon, 27 Aug 2018 22:05:00 +0300
Message-ID: <CAOQ4uxjE-XtGC=D-C+329snTs7u7bGgcrns2N-rR34ZXAdm=0g@mail.gmail.com>
Subject: Re: [PATCH v3 6/6] ovl: add ovl_fadvise()
To: Vivek Goyal <vgoyal@redhat.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Dave Chinner <david@fromorbit.com>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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On Mon, Aug 27, 2018 at 9:52 PM Vivek Goyal <vgoyal@redhat.com> wrote:
>
> On Mon, Aug 27, 2018 at 03:56:04PM +0300, Amir Goldstein wrote:
> > Implement stacked fadvise to fix syscalls readahead(2) and fadvise64(2)
> > on an overlayfs file.
> >
> > Suggested-by: Miklos Szeredi <mszeredi@redhat.com>
> > Fixes: d1d04ef8572b ("ovl: stack file ops")
> > Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> > ---
> >  fs/overlayfs/file.c | 18 ++++++++++++++++++
> >  1 file changed, 18 insertions(+)
> >
> > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
> > index a4acd84591d4..42d2d034d85c 100644
> > --- a/fs/overlayfs/file.c
> > +++ b/fs/overlayfs/file.c
> > @@ -331,6 +331,23 @@ static long ovl_fallocate(struct file *file, int mode, loff_t offset, loff_t len
> >       return ret;
> >  }
> >
> > +int ovl_fadvise(struct file *file, loff_t offset, loff_t len, int advice)
> > +{
> > +     struct fd real;
> > +     int ret;
> > +
> > +     ret = ovl_real_fdget(file, &real);
> > +     if (ret)
> > +             return ret;
> > +
> > +     /* XXX: do we need mounter credentials? */
>
> Given we are switching creds to mounter for rest of the file operations,
> so I would think we need to do it here as well to be consistent with
> this security model.
>

Yeh, I guess so, although I did not see any security checks in
fadvise64(2) syscall. readahead(2) at least checks for FMODE_READ
on the open file fadvise doesn't even bother with that...

Miklos, let me know if you want me to add override_creds or will
you add it yourself.

Thanks,
Amir.