Re: [LTP] [PATCH v1] security/dirtyc0w_shmem: Fix remaining cases where UFFD_FEATURE_MINOR_SHMEM is absent

[Petr Vorel <pvorel@suse.cz>]

> On 25.11.22 12:12, Petr Vorel wrote:
> > Hi David,

> > > When UFFD_FEATURE_MINOR_SHMEM is not defined, we still have to make the
> > > checkpoint happy, otherwise our parent process will run into a timeout.
> > > Further, we have to test if UFFD_FEATURE_MINOR_SHMEM is really returned by
> > > the UFFD_API ioctl: if the kernel knows about the feature but doesn't
> > > support it, it will be masked off.

> > > Reported-by: Martin Doucha <mdoucha@suse.cz>
> > > Cc: Petr Vorel <pvorel@suse.cz>
> > > Cc: Cyril Hrubis <chrubis@suse.cz>
> > > Signed-off-by: David Hildenbrand <david@redhat.com>
> > > ---
> > >   .../dirtyc0w_shmem/dirtyc0w_shmem_child.c        | 16 ++++++++++++----
> > >   1 file changed, 12 insertions(+), 4 deletions(-)

> > > diff --git a/testcases/kernel/security/dirtyc0w_shmem/dirtyc0w_shmem_child.c b/testcases/kernel/security/dirtyc0w_shmem/dirtyc0w_shmem_child.c
> > > index cb2e9df0c..c117c6f39 100644
> > > --- a/testcases/kernel/security/dirtyc0w_shmem/dirtyc0w_shmem_child.c
> > > +++ b/testcases/kernel/security/dirtyc0w_shmem/dirtyc0w_shmem_child.c
> > > @@ -24,12 +24,12 @@
> > >   #include <linux/userfaultfd.h>
> > >   #endif

> > > -#ifdef UFFD_FEATURE_MINOR_SHMEM
> > Shouldn't be the check and TST_TEST_TCONF() actually be in dirtyc0w_shmem.c?
> > I overlooked that, but IMHO test does not make sense at all if
> > UFFD_FEATURE_MINOR_SHMEM not defined, right?

> > Also Martin noted that ("The parent process should not even fork() when
> > UFFD_FEATURE_MINOR_SHMEM is not defined in config.h.").


> I tried that first, but then we can still run into the runtime absence of
> UFFD_FEATURE_MINOR_SHMEM. Checking that also in the parent resulted in some
> IMHO unpleasant code while I worked on that.

> This is certainly the easiest approach, because we still have to make the
> child program compile either way.

Right, it needs to be in child. Using TST_TEST_TCONF() also in master does not
look to me as too unpleasant code, but take it just a suggestion. Obviously the
only requirement is code compiles and works on both defined and undefined
UFFD_FEATURE_MINOR_SHMEM.

> Anyhow, I'll do whatever you decide, because I want to cross this off my
> list. So any guidance on how to complete this would be appreciated.

Understand.

Kind regards,
Petr

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