Re: [Qemu-devel] [PATCH 1/4] hostmem-memfd: enable seals only if supported

[Marc-André Lureau]

Hi

On Tue, Nov 27, 2018 at 4:02 PM Ilya Maximets <address@hidden> wrote:
>
> On 27.11.2018 15:00, Marc-André Lureau wrote:
> > Hi
> > On Tue, Nov 27, 2018 at 3:56 PM Ilya Maximets <address@hidden> wrote:
> >>
> >> On 27.11.2018 14:49, Marc-André Lureau wrote:
> >>> Hi
> >>> On Tue, Nov 27, 2018 at 3:11 PM Ilya Maximets <address@hidden> wrote:
> >>>>
> >>>> If seals are not supported, memfd_create() will fail.
> >>>> Furthermore, there is no way to disable it in this case because
> >>>> '.seal' property is not registered.
> >>>>
> >>>> This issue leads to vhost-user-test failures on RHEL 7.2:
> >>>>
> >>>>   qemu-system-x86_64: -object memory-backend-memfd,id=mem,size=2M,: \
> >>>>                       failed to create memfd: Invalid argument
> >>>>
> >>>> Signed-off-by: Ilya Maximets <address@hidden>
> >>>
> >>>
> >>> This will change the default behaviour of memfd backend, and may thus
> >>> me considered a break.
> >>
> >> This will change the default behaviour only on systems without sealing
> >> support. But current implementation is broken there anyway and does not
> >> work.
> >>
> >>>
> >>> Instead, memfd vhost-user-test should skipped (or tuned with
> >>> sealed=false if unsupported)
> >>
> >> vhost-user-test is just an example here. In fact memfd could not be
> >> used at all on system without sealing support. And there is no way
> >> to disable seals.
> >
> > which system supports memfd without sealing?
>
> RHEL 7.2. kernel version 3.10.0-327.el7.x86_64

Correct, it was backported without sealing for some reason.

I would rather have an explicit seal=off argument on such system
(because sealing is expected to be available with memfd in general)

>
> >
> >>
> >>>
> >>>> ---
> >>>>  backends/hostmem-memfd.c | 4 ++--
> >>>>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>>>
> >>>> diff --git a/backends/hostmem-memfd.c b/backends/hostmem-memfd.c
> >>>> index b6836b28e5..ee39bdbde6 100644
> >>>> --- a/backends/hostmem-memfd.c
> >>>> +++ b/backends/hostmem-memfd.c
> >>>> @@ -129,8 +129,8 @@ memfd_backend_instance_init(Object *obj)
> >>>>  {
> >>>>      HostMemoryBackendMemfd *m = MEMORY_BACKEND_MEMFD(obj);
> >>>>
> >>>> -    /* default to sealed file */
> >>>> -    m->seal = true;
> >>>> +    /* default to sealed file if supported */
> >>>> +    m->seal = qemu_memfd_check(MFD_ALLOW_SEALING);
> >>>>  }
> >>>>
> >>>>  static void
> >>>> --
> >>>> 2.17.1
> >>>>
> >>>
> >>>
> >>
> >
> >



-- 
Marc-André Lureau