linux-kernel - Re: [PATCH v16 12/16] IB, arm64: untag user pointers in ib_uverbs_(re)reg

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Message-ID: <CAAeHK+xyqwuJyviGhvU7L1wPZQF7Mf9g2vgKSsYmML3fV6NrXg@mail.gmail.com>
Date:   Tue, 4 Jun 2019 14:45:32 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
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        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
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        Yishai Hadas <yishaih@...lanox.com>,
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        Szabolcs Nagy <Szabolcs.Nagy@....com>
Subject: Re: [PATCH v16 12/16] IB, arm64: untag user pointers in ib_uverbs_(re)reg_mr()

On Tue, Jun 4, 2019 at 2:27 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> On Tue, Jun 04, 2019 at 02:18:19PM +0200, Andrey Konovalov wrote:
> > On Mon, Jun 3, 2019 at 7:46 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
> > >
> > > On Mon, Jun 03, 2019 at 06:55:14PM +0200, Andrey Konovalov wrote:
> > > > This patch is a part of a series that extends arm64 kernel ABI to allow to
> > > > pass tagged user pointers (with the top byte set to something else other
> > > > than 0x00) as syscall arguments.
> > > >
> > > > ib_uverbs_(re)reg_mr() use provided user pointers for vma lookups (through
> > > > e.g. mlx4_get_umem_mr()), which can only by done with untagged pointers.
> > > >
> > > > Untag user pointers in these functions.
> > > >
> > > > Signed-off-by: Andrey Konovalov <andreyknvl@...gle.com>
> > > >  drivers/infiniband/core/uverbs_cmd.c | 4 ++++
> > > >  1 file changed, 4 insertions(+)
> > > >
> > > > diff --git a/drivers/infiniband/core/uverbs_cmd.c b/drivers/infiniband/core/uverbs_cmd.c
> > > > index 5a3a1780ceea..f88ee733e617 100644
> > > > +++ b/drivers/infiniband/core/uverbs_cmd.c
> > > > @@ -709,6 +709,8 @@ static int ib_uverbs_reg_mr(struct uverbs_attr_bundle *attrs)
> > > >       if (ret)
> > > >               return ret;
> > > >
> > > > +     cmd.start = untagged_addr(cmd.start);
> > > > +
> > > >       if ((cmd.start & ~PAGE_MASK) != (cmd.hca_va & ~PAGE_MASK))
> > > >               return -EINVAL;
> > >
> > > I feel like we shouldn't thave to do this here, surely the cmd.start
> > > should flow unmodified to get_user_pages, and gup should untag it?
> > >
> > > ie, this sort of direction for the IB code (this would be a giant
> > > patch, so I didn't have time to write it all, but I think it is much
> > > saner):
> >
> > Hi Jason,
> >
> > ib_uverbs_reg_mr() passes cmd.start to mlx4_get_umem_mr(), which calls
> > find_vma(), which only accepts untagged addresses. Could you explain
> > how your patch helps?
>
> That mlx4 is just a 'weird duck', it is not the normal flow, and I
> don't think the core code should be making special consideration for
> it.

How do you think we should do untagging (or something else) to deal
with this 'weird duck' case?

>
> Jason