Re: [PATCH 08/10] ioctl_userfaultfd.2: clarify the state of the uffdio_api structure on error

Axel Rasmussen
2023-09-26T17:59:00Z

On Mon, Sep 25, 2023 at 4:56 PM Alejandro Colomar <alx@kernel.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@google.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
> >