netdev - Re: [PATCH v2 net-next] bpf: enable verifier to add 0 to packet ptr

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Message-ID: <20170203205513.GA24959@ast-mbp.thefacebook.com>
Date:   Fri, 3 Feb 2017 12:55:15 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     William Tu <u9012063@...il.com>
Cc:     netdev@...r.kernel.org, Mihai Budiu <mbudiu@...are.com>
Subject: Re: [PATCH v2 net-next] bpf: enable verifier to add 0 to packet ptr

On Fri, Feb 03, 2017 at 09:22:45AM -0800, William Tu wrote:
> The patch fixes the case when adding a zero value to the packet
> pointer.  The verifer reports the following error:
>   [...]
>     R0=imm0,min_value=0,max_value=0
>     R1=pkt(id=0,off=0,r=4)
>     R2=pkt_end R3=fp-12
>     R4=imm4,min_value=4,max_value=4
>     R5=pkt(id=0,off=4,r=4)
>   269: (bf) r2 = r0	// r2 becomes imm0
>   270: (77) r2 >>= 3
>   271: (bf) r4 = r1	// r4 becomes pkt ptr
>   272: (0f) r4 += r2	// r4 += 0
>   addition of negative constant to packet pointer is not allowed
> 
> Signed-off-by: William Tu <u9012063@...il.com>
> Signed-off-by: Mihai Budiu <mbudiu@...are.com>
> ---
>  kernel/bpf/verifier.c                       |  2 +-
>  tools/testing/selftests/bpf/test_verifier.c | 15 +++++++++++++++
>  2 files changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index fb3513b..1a754e5 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -1397,7 +1397,7 @@ static int check_packet_ptr_add(struct bpf_verifier_env *env,
>  		imm = insn->imm;
>  
>  add_imm:
> -		if (imm <= 0) {
> +		if (imm < 0) {
>  			verbose("addition of negative constant to packet pointer is not allowed\n");
>  			return -EACCES;
>  		}
> diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
> index 0d0912c..a2b5c7e 100644
> --- a/tools/testing/selftests/bpf/test_verifier.c
> +++ b/tools/testing/selftests/bpf/test_verifier.c
> @@ -2404,6 +2404,21 @@ static struct bpf_test tests[] = {
>  		.prog_type = BPF_PROG_TYPE_SCHED_CLS,
>  	},
>  	{
> +		"direct packet access: test14 (pkt_ptr += 0, good access)",
> +		.insns = {
> +			BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,
> +				    offsetof(struct __sk_buff, data)),
> +			BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1,
> +				    offsetof(struct __sk_buff, data_end)),
> +			BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
> +			BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 0),

wait. the test is bogus.
please write the proper test for the feature
and check that it fails before the patch and passes afterwards.

> +			BPF_MOV64_IMM(BPF_REG_0, 1),
> +			BPF_EXIT_INSN(),
> +		},
> +		.result = ACCEPT,
> +		.prog_type = BPF_PROG_TYPE_SCHED_CLS,
> +	},
> +	{
>  		"helper access to packet: test1, valid packet_ptr range",
>  		.insns = {
>  			BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,
> -- 
> 2.7.4
>