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 11:53:00 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     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

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?

Thanks
hangbin