Re: [PATCH 1/3] bpf: verifier: Accept dynptr mem as mem in helpers

[Alexei Starovoitov]

On Thu, Apr 6, 2023 at 1:55 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Wed, Apr 5, 2023 at 5:40 PM Daniel Rosenberg <drosen@xxxxxxxxxx> wrote:
> >
> > This allows using memory retrieved from dynptrs with helper functions
> > that accept ARG_PTR_TO_MEM. For instance, results from bpf_dynptr_data
> > can be passed along to bpf_strncmp.
> >
> > Signed-off-by: Daniel Rosenberg <drosen@xxxxxxxxxx>
> > ---
>
> I think patches like this should be targeted against bpf-next, so for
> next revision please send them against bpf-next tree.
>
> >  kernel/bpf/verifier.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 56f569811f70..20beab52812a 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -7164,12 +7164,16 @@ static int check_reg_type(struct bpf_verifier_env *env, u32 regno,
> >          * ARG_PTR_TO_MEM + MAYBE_NULL is compatible with PTR_TO_MEM and PTR_TO_MEM + MAYBE_NULL,
> >          * but ARG_PTR_TO_MEM is compatible only with PTR_TO_MEM but NOT with PTR_TO_MEM + MAYBE_NULL
> >          *
> > +        * ARG_PTR_TO_MEM is compatible with PTR_TO_MEM that is tagged with a dynptr type.
> > +        *
> >          * Therefore we fold these flags depending on the arg_type before comparison.
> >          */
> >         if (arg_type & MEM_RDONLY)
> >                 type &= ~MEM_RDONLY;
> >         if (arg_type & PTR_MAYBE_NULL)
> >                 type &= ~PTR_MAYBE_NULL;
> > +       if (base_type(arg_type) == ARG_PTR_TO_MEM)
> > +               type &= ~DYNPTR_TYPE_FLAG_MASK;
>
> Something feels off here. Can you paste a bit of verifier log for the
> failure you were getting. And let's have a selftest for this situation
> as well.
>
> ARG_PTR_TO_MEM shouldn't be qualified with the DYNPTR_TYPE flag, it's
> just memory, there is no need to know what type of dynptr it was
> derived from. So if that happens, the problem is somewhere else. Let's
> root cause and fix that. Having a selftest that demonstrates the
> problem will help with that.

+1
All of the DYNPTR_TYPE_FLAG_MASK flags cannot appear in type == reg->type
here.
They are either dynamic flags inside bpf_dynptr_kern->size
or in arg_type.
Like in bpf_dynptr_from_mem_proto.