linux-kernel - Re: [PATCH v3] drivers/vfio: Fix a redundant copy bug

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Message-ID: <CAAa=b7eGPmfaVV+8UCVfQmELE6ZnkW+Yggfk=Xx1e_qS=gDc5A@mail.gmail.com>
Date:   Wed, 17 Oct 2018 14:10:37 -0500
From:   Wenwen Wang <wang6495@....edu>
To:     alex.williamson@...hat.com
Cc:     Kangjie Lu <kjlu@....edu>, kvm@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>, aik@...abs.ru,
        david@...son.dropbear.id.au, Wenwen Wang <wang6495@....edu>
Subject: Re: [PATCH v3] drivers/vfio: Fix a redundant copy bug

On Wed, Oct 17, 2018 at 2:05 PM Alex Williamson
<alex.williamson@...hat.com> wrote:
>
> On Wed, 17 Oct 2018 12:58:26 -0500
> Wenwen Wang <wang6495@....edu> wrote:
>
> > On Wed, Oct 17, 2018 at 10:45 AM Alex Williamson
> > <alex.williamson@...hat.com> wrote:
> > >
> > > On Wed, 17 Oct 2018 09:32:04 -0500
> > > Wenwen Wang <wang6495@....edu> wrote:
> > >
> > > > In vfio_spapr_iommu_eeh_ioctl(), if the ioctl command is VFIO_EEH_PE_OP,
> > > > the user-space buffer 'arg' is copied to the kernel object 'op' and the
> > > > 'argsz' and 'flags' fields of 'op' are checked. If the check fails, an
> > > > error code EINVAL is returned. Otherwise, 'op.op' is further checked
> > > > through a switch statement to invoke related handlers. If 'op.op' is
> > > > VFIO_EEH_PE_INJECT_ERR, the whole user-space buffer 'arg' is copied again
> > > > to 'op' to obtain the err information. However, in the following execution
> > > > of this case, the fields of 'op', except the field 'err', are actually not
> > > > used. That is, the second copy has a redundant part. Therefore, for both
> > > > performance consideration, the redundant part of the second copy should be
> > > > removed.
> > > >
> > > > This patch removes such a part in the second copy. It only copies from
> > > > 'err.type' to 'err.mask', which is exactly required by the
> > > > VFIO_EEH_PE_INJECT_ERR op.
> > > >
> > > > This patch also adds a 4-byte reserved field in the structure
> > > > vfio_eeh_pe_op to make sure that the u64 fields in the structure
> > > > vfio_eeh_pe_err are 8-byte aligned.
> > > >
> > > > Signed-off-by: Wenwen Wang <wang6495@....edu>
> > > > ---
> > > >  drivers/vfio/vfio_spapr_eeh.c | 9 ++++++---
> > > >  include/uapi/linux/vfio.h     | 1 +
> > > >  2 files changed, 7 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/drivers/vfio/vfio_spapr_eeh.c b/drivers/vfio/vfio_spapr_eeh.c
> > > > index 38edeb4..66634c6 100644
> > > > --- a/drivers/vfio/vfio_spapr_eeh.c
> > > > +++ b/drivers/vfio/vfio_spapr_eeh.c
> > > > @@ -37,6 +37,7 @@ long vfio_spapr_iommu_eeh_ioctl(struct iommu_group *group,
> > > >       struct eeh_pe *pe;
> > > >       struct vfio_eeh_pe_op op;
> > > >       unsigned long minsz;
> > > > +     unsigned long start, end;
> > > >       long ret = -EINVAL;
> > > >
> > > >       switch (cmd) {
> > > > @@ -86,10 +87,12 @@ long vfio_spapr_iommu_eeh_ioctl(struct iommu_group *group,
> > > >                       ret = eeh_pe_configure(pe);
> > > >                       break;
> > > >               case VFIO_EEH_PE_INJECT_ERR:
> > > > -                     minsz = offsetofend(struct vfio_eeh_pe_op, err.mask);
> > > > -                     if (op.argsz < minsz)
> > > > +                     start = offsetof(struct vfio_eeh_pe_op, err.type);
> > >
> > > I noted in the previous version that we already have this in minsz, so
> > > you're fixing a redundant copy with a redundant operation.
> >
> > The value in start is different from the value in minsz. So why is
> > this a redundant operation?
>
> I suppose that's true given the alignment issue below, so we're
> actually avoiding 16 bytes rather than 12.  The benefit of this change
> still seems pretty thin to me, but it is more correct, so I guess it's
> ok.  Do you want to send a new version or shall I just drop the vfio.h
> changes and the last paragraph of the commit log in favor of the
> separate patch?  Alexey or David, do you want to provide an Ack for
> these?  Thanks,

I can send a new version of the patch. Thanks!

Wenwen