linux-kernel - Re: [PATCH 08/10] ioctl_userfaultfd.2: clarify the state of the uffdio

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Message-ID: <CAJHvVcjTBmQewEUKPCa-ZFEEv_Lczx5Pp9=vgnnTPSrqc4RhVA@mail.gmail.com>
Date:   Tue, 26 Sep 2023 10:58:18 -0700
From:   Axel Rasmussen <axelrasmussen@...gle.com>
To:     Alejandro Colomar <alx@...nel.org>
Cc:     Peter Xu <peterx@...hat.com>, linux-man@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/10] ioctl_userfaultfd.2: clarify the state of the
 uffdio_api structure on error

On Mon, Sep 25, 2023 at 4:56 PM Alejandro Colomar <alx@...nel.org> wrote:
>
> Hi Axel,
>
> On Tue, Sep 19, 2023 at 12:02:04PM -0700, Axel Rasmussen wrote:
> > The old FIXME noted that the zeroing was done to differentiate the two
> > EINVAL cases. It's possible something like this was true historically,
> > but in current Linux we zero it in *both* EINVAL cases, so this is at
> > least no longer true.
> >
> > After reading the code, I can't determine any clear reason why we zero
> > it in some cases but not in others. So, some simple advice we can give
> > userspace is: if an error occurs, treat the contents of the structure as
> > unspecified. Just re-initialize it before retrying UFFDIO_API again.
> >
> > Signed-off-by: Axel Rasmussen <axelrasmussen@...gle.com>
>
> I can't apply this patch due to conflicts (due to not having applied two
> of the previous ones).  Please resend all remaining patches in following
> revisions of the patch set.
>
> The applied ones are here:
>
> <https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/log/?h=contrib>
>
> It's kind of like Linux's 'next' branch.

Thanks for the review Alex! I'll fix up the issues noted and send the
remaining few patches this week. :)

>
> Cheers,
> Alex
>
> > ---
> >  man2/ioctl_userfaultfd.2 | 16 ++++++++--------
> >  1 file changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/man2/ioctl_userfaultfd.2 b/man2/ioctl_userfaultfd.2
> > index 1aa9654be..29dca1f6b 100644
> > --- a/man2/ioctl_userfaultfd.2
> > +++ b/man2/ioctl_userfaultfd.2
> > @@ -272,6 +272,14 @@ operation returns 0 on success.
> >  On error, \-1 is returned and
> >  .I errno
> >  is set to indicate the error.
> > +If an error occurs,
> > +the kernel may zero the provided
> > +.I uffdio_api
> > +structure.
> > +The caller should treat its contents as unspecified,
> > +and reinitialize it before re-attempting another
> > +.B UFFDIO_API
> > +call.
> >  Possible errors include:
> >  .TP
> >  .B EFAULT
> > @@ -305,14 +313,6 @@ twice,
> >  the first time with no features set,
> >  is explicitly allowed
> >  as per the two-step feature detection handshake.
> > -.\" FIXME In the above error case, the returned 'uffdio_api' structure is
> > -.\" zeroed out. Why is this done? This should be explained in the manual page.
> > -.\"
> > -.\" Mike Rapoport:
> > -.\"     In my understanding the uffdio_api
> > -.\"     structure is zeroed to allow the caller
> > -.\"     to distinguish the reasons for -EINVAL.
> > -.\"
> >  .SS UFFDIO_REGISTER
> >  (Since Linux 4.3.)
> >  Register a memory address range with the userfaultfd object.
> > --
> > 2.42.0.459.ge4e396fd5e-goog
> >