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From:   Souptick Joarder <jrdr.linux@gmail.com>
Date:   Thu, 3 Sep 2020 12:42:44 +0530
Message-ID: <CAFqt6zb2GjO9KOVdKT1P0P-Wn+isnArous5gpw-AJ3va+fi9rw@mail.gmail.com>
Subject: Re: [PATCH] mm/gup: don't permit users to call get_user_pages with FOLL_LONGTERM
To:     John Hubbard <jhubbard@nvidia.com>
Cc:     Barry Song <song.bao.hua@hisilicon.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux-MM <linux-mm@kvack.org>, linux-kernel@vger.kernel.org,
        linuxarm@huawei.com, Jan Kara <jack@suse.cz>,
        =?UTF-8?B?SsOpcsO0bWUgR2xpc3Nl?= <jglisse@redhat.com>,
        "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Chinner <david@fromorbit.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Jonathan Corbet <corbet@lwn.net>,
        Michal Hocko <mhocko@suse.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Shuah Khan <shuah@kernel.org>, Vlastimil Babka <vbabka@suse.cz>
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On Wed, Aug 19, 2020 at 11:45 PM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 8/19/20 4:01 AM, Barry Song wrote:
> > gug prohibits users from calling get_user_pages() with FOLL_PIN. But it
>
> Maybe Andrew can fix the typo above: gug --> gup.
>
>
> > allows users to call get_user_pages() with FOLL_LONGTERM only. It seems
> > insensible.
> >
> > since FOLL_LONGTERM is a stricter case of FOLL_PIN, we should prohibit
> > users from calling get_user_pages() with FOLL_LONGTERM while not with
> > FOLL_PIN.
> >
> > mm/gup_benchmark.c used to be the only user who did this improperly.
> > But it has been fixed by moving to use pin_user_pages().
>
> For future patches, you don't have to write everything in the
> commit log. Some things are better placed in a cover letter or after
> the "---" line, because they don't need to be recorded forever.
>
> Anyway, the diffs seem fine, assuming that you've audited the call sites.

We can use is_valid_gup_flags() inside ->
get_user_pages_locked(),
get_user_pages_unlocked(),
pin_user_pages_locked() as well.

Are you planning to add it in future patches ?

>
> thanks,
> --
> John Hubbard
> NVIDIA
>
> >
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > Cc: Jan Kara <jack@suse.cz>
> > Cc: J=C3=A9r=C3=B4me Glisse <jglisse@redhat.com>
> > Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> > Cc: Al Viro <viro@zeniv.linux.org.uk>
> > Cc: Christoph Hellwig <hch@infradead.org>
> > Cc: Dan Williams <dan.j.williams@intel.com>
> > Cc: Dave Chinner <david@fromorbit.com>
> > Cc: Jason Gunthorpe <jgg@ziepe.ca>
> > Cc: Jonathan Corbet <corbet@lwn.net>
> > Cc: Michal Hocko <mhocko@suse.com>
> > Cc: Mike Kravetz <mike.kravetz@oracle.com>
> > Cc: Shuah Khan <shuah@kernel.org>
> > Cc: Vlastimil Babka <vbabka@suse.cz>
> > Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
> > ---
> >   mm/gup.c | 37 ++++++++++++++++++++++---------------
> >   1 file changed, 22 insertions(+), 15 deletions(-)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index ae096ea7583f..4da669f79566 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1789,6 +1789,25 @@ static long __get_user_pages_remote(struct mm_st=
ruct *mm,
> >                                      gup_flags | FOLL_TOUCH | FOLL_REMO=
TE);
> >   }
> >
> > +static bool is_valid_gup_flags(unsigned int gup_flags)
> > +{
> > +     /*
> > +      * FOLL_PIN must only be set internally by the pin_user_pages*() =
APIs,
> > +      * never directly by the caller, so enforce that with an assertio=
n:
> > +      */
> > +     if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> > +             return false;
> > +     /*
> > +      * FOLL_PIN is a prerequisite to FOLL_LONGTERM. Another way of sa=
ying
> > +      * that is, FOLL_LONGTERM is a specific case, more restrictive ca=
se of
> > +      * FOLL_PIN.
> > +      */
> > +     if (WARN_ON_ONCE(gup_flags & FOLL_LONGTERM))
> > +             return false;
> > +
> > +     return true;
> > +}
> > +
> >   /**
> >    * get_user_pages_remote() - pin user pages in memory
> >    * @mm:             mm_struct of target mm
> > @@ -1854,11 +1873,7 @@ long get_user_pages_remote(struct mm_struct *mm,
> >               unsigned int gup_flags, struct page **pages,
> >               struct vm_area_struct **vmas, int *locked)
> >   {
> > -     /*
> > -      * FOLL_PIN must only be set internally by the pin_user_pages*() =
APIs,
> > -      * never directly by the caller, so enforce that with an assertio=
n:
> > -      */
> > -     if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> > +     if (!is_valid_gup_flags(gup_flags))
> >               return -EINVAL;
> >
> >       return __get_user_pages_remote(mm, start, nr_pages, gup_flags,
> > @@ -1904,11 +1919,7 @@ long get_user_pages(unsigned long start, unsigne=
d long nr_pages,
> >               unsigned int gup_flags, struct page **pages,
> >               struct vm_area_struct **vmas)
> >   {
> > -     /*
> > -      * FOLL_PIN must only be set internally by the pin_user_pages*() =
APIs,
> > -      * never directly by the caller, so enforce that with an assertio=
n:
> > -      */
> > -     if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> > +     if (!is_valid_gup_flags(gup_flags))
> >               return -EINVAL;
> >
> >       return __gup_longterm_locked(current->mm, start, nr_pages,
> > @@ -2810,11 +2821,7 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast_only);
> >   int get_user_pages_fast(unsigned long start, int nr_pages,
> >                       unsigned int gup_flags, struct page **pages)
> >   {
> > -     /*
> > -      * FOLL_PIN must only be set internally by the pin_user_pages*() =
APIs,
> > -      * never directly by the caller, so enforce that:
> > -      */
> > -     if (WARN_ON_ONCE(gup_flags & FOLL_PIN))
> > +     if (!is_valid_gup_flags(gup_flags))
> >               return -EINVAL;
> >
> >       /*
> >
>
>