linux-kernel - Re: [PATCH v2] ovl: provide real_file() and overlayfs get_unmapped

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Date:   Thu, 4 Jun 2020 11:16:49 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     kbuild test robot <lkp@...el.com>, kbuild-all@...ts.01.org,
        Colin Walters <walters@...bum.org>,
        syzbot <syzbot+d6ec23007e951dadf3de@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        overlayfs <linux-unionfs@...r.kernel.org>
Subject: Re: [PATCH v2] ovl: provide real_file() and overlayfs get_unmapped_area()

On Thu, May 28, 2020 at 11:01 PM Mike Kravetz <mike.kravetz@...cle.com> wrote:
>
> On 5/28/20 1:37 AM, kbuild test robot wrote:
> > Hi Mike,
> >
> > I love your patch! Yet something to improve:
> >
> > [auto build test ERROR on miklos-vfs/overlayfs-next]
> > [also build test ERROR on linus/master v5.7-rc7]
> > [cannot apply to linux/master next-20200526]
> > [if your patch is applied to the wrong git tree, please drop us a note to help
> > improve the system. BTW, we also suggest to use '--base' option to specify the
> > base tree in git format-patch, please see https://stackoverflow.com/a/37406982]
> >
> > url:    https://github.com/0day-ci/linux/commits/Mike-Kravetz/ovl-provide-real_file-and-overlayfs-get_unmapped_area/20200528-080533
> > base:   https://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs.git overlayfs-next
> > config: h8300-randconfig-r036-20200528 (attached as .config)
> > compiler: h8300-linux-gcc (GCC) 9.3.0
> > reproduce (this is a W=1 build):
> >         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> >         chmod +x ~/bin/make.cross
> >         # save the attached .config to linux build tree
> >         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross ARCH=h8300
> >
> > If you fix the issue, kindly add following tag as appropriate
> > Reported-by: kbuild test robot <lkp@...el.com>
> >
> > All error/warnings (new ones prefixed by >>, old ones prefixed by <<):
> >
> > fs/overlayfs/file.c: In function 'ovl_get_unmapped_area':
> >>> fs/overlayfs/file.c:768:14: error: 'struct mm_struct' has no member named 'get_unmapped_area'
> > 768 |   current->mm->get_unmapped_area)(realfile,
> > |              ^~
> >>> fs/overlayfs/file.c:770:1: warning: control reaches end of non-void function [-Wreturn-type]
> > 770 | }
> > | ^
> >
> > vim +768 fs/overlayfs/file.c
> >
> >    760
> >    761        static unsigned long ovl_get_unmapped_area(struct file *file,
> >    762                                        unsigned long uaddr, unsigned long len,
> >    763                                        unsigned long pgoff, unsigned long flags)
> >    764        {
> >    765                struct file *realfile = real_file(file);
> >    766
> >    767                return (realfile->f_op->get_unmapped_area ?:
> >  > 768                        current->mm->get_unmapped_area)(realfile,
> >    769                                                        uaddr, len, pgoff, flags);
> >  > 770        }
> >    771
> >
> > ---
> > 0-DAY CI Kernel Test Service, Intel Corporation
> > https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org
> >
>
> Well yuck!  get_unmapped_area is not part of mm_struct if !CONFIG_MMU.
>
> Miklos, would adding '#ifdef CONFIG_MMU' around the overlayfs code be too
> ugly for you?  Another option is to use real_file() in the mmap code as
> done in [1].

I think the proper fix is to add an inline helper
(call_get_unmapped_area()?) in linux/mm.h, and make that work properly
for the NOMMU case as well.

Thanks,
Miklos