linux-kernel - Re: [PATCH v2 3/5] ioctl_userfaultfd.2: clarify the state of the uffdio

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Message-ID: <20231009091256.GN3303@kernel.org>
Date:   Mon, 9 Oct 2023 12:12:56 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Alejandro Colomar <alx@...nel.org>
Cc:     Axel Rasmussen <axelrasmussen@...gle.com>,
        Peter Xu <peterx@...hat.com>, linux-man@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/5] ioctl_userfaultfd.2: clarify the state of the
 uffdio_api structure on error

On Sun, Oct 08, 2023 at 11:52:56PM +0200, Alejandro Colomar wrote:
> Hi Axel,
> 
> On Tue, Oct 03, 2023 at 12:45:45PM -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>
> > ---
> >  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 28dd2fcdd..2ee6a0532 100644
> > --- a/man2/ioctl_userfaultfd.2
> > +++ b/man2/ioctl_userfaultfd.2
> > @@ -248,6 +248,14 @@ 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
> > @@ -281,14 +289,6 @@ feature was enabled,
> >  but the calling process doesn't have the
> >  .B CAP_SYS_PTRACE
> >  capability.
> > -.\" 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.
> > -.\"
> 
> I've added Mike to the thread in case he wants to comment.

Thanks, Alex!

It looks like I reviewed v1 of the patchset though :)
 
> Thanks,
> Alex
> 
> >  .SS UFFDIO_REGISTER
> >  (Since Linux 4.3.)
> >  Register a memory address range with the userfaultfd object.
> > -- 
> > 2.42.0.609.gbb76f46606-goog
> > 
> 
> -- 
> <https://www.alejandro-colomar.es/>



-- 
Sincerely yours,
Mike.