netdev - Re: [PATCHv17 bpf-next 5/6] selftests/bpf: Add verifier tests for bpf arg ARG_CONST_MAP_PTR_OR

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Date:   Thu, 28 Jan 2021 15:14:36 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Hangbin Liu <liuhangbin@...il.com>,
        John Fastabend <john.fastabend@...il.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Jiri Benc <jbenc@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>, ast@...nel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        David Ahern <dsahern@...il.com>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Subject: Re: [PATCHv17 bpf-next 5/6] selftests/bpf: Add verifier tests for bpf
 arg ARG_CONST_MAP_PTR_OR_NULL

Hangbin Liu wrote:
> On Wed, Jan 27, 2021 at 02:24:47PM -0800, John Fastabend wrote:
> > [...]
> > 
> > > +{
> > > +	"ARG_CONST_MAP_PTR_OR_NULL: null pointer for ex_map",
> > > +	.insns = {
> > > +		BPF_MOV64_IMM(BPF_REG_1, 0),
> > > +		/* bpf_redirect_map_multi arg1 (in_map) */
> > > +		BPF_LD_MAP_FD(BPF_REG_1, 0),
> > > +		/* bpf_redirect_map_multi arg2 (ex_map) */
> > > +		BPF_MOV64_IMM(BPF_REG_2, 0),
> > > +		/* bpf_redirect_map_multi arg3 (flags) */
> > > +		BPF_MOV64_IMM(BPF_REG_3, 0),
> > > +		BPF_EMIT_CALL(BPF_FUNC_redirect_map_multi),
> > > +		BPF_EXIT_INSN(),
> > > +	},
> > > +	.fixup_map_devmap = { 1 },
> > > +	.result = ACCEPT,
> > > +	.prog_type = BPF_PROG_TYPE_XDP,
> > > +	.retval = 4,
> > 
> > Do we need one more case where this is map_or_null? In above
> > ex_map will be scalar tnum_const=0 and be exactly a null. This
> > will push verifier here,
> > 
> >   meta->map_ptr = register_is_null(reg) ? NULL : reg->map_ptr;
> > 
> > In the below case it is known to be not null.
> > 
> > Is it also interesting to have a case where register_is_null(reg)
> > check fails and reg->map_ptr is set, but may be null.
> 
> Hi John,
> 
> I'm not familiar with the test_verifier syntax. Doesn't
> BPF_LD_MAP_FD(BPF_REG_1, 0) just assign the register with map NULL?

On second thought because we are only running the verifier here and
not actually calling the helper I guess both paths are in fact
covered here.

Acked-by: John Fastabend <john.fastabend@...il.com>