linux-kernel - Re: [PATCH bpf-next v2 2/2] selftests/bpf: check null propagation only neither reg is PTR_TO_BTF

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Message-ID: <18e1219a-d2b2-0373-1f30-fcf83acd328f@linux.dev>
Date:   Wed, 21 Dec 2022 13:21:25 -0800
From:   Martin KaFai Lau <martin.lau@...ux.dev>
To:     Hao Sun <sunhao.th@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        David Miller <davem@...emloft.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next v2 2/2] selftests/bpf: check null propagation
 only neither reg is PTR_TO_BTF_ID

On 12/21/22 5:46 AM, Hao Sun wrote:
> Hi,
> 
> I’ve tried something like the bellow, but soon realized that this
> won’t work because once compiler figures out `inner_map` equals
> to `val`, it can choose either reg to write into in the following
> path, meaning that this program can be rejected due to writing
> into read-only PTR_TO_BTF_ID reg, and this makes the test useless.

hmm... I read the above a few times but I still don't quite get it.  In 
particular, '...can be rejected due to writing into read-only PTR_TO_BTF_ID 
reg...'.  Where is it writing into a read-only PTR_TO_BTF_ID reg in the 
following bpf prog?  Did I overlook something?

> 
> Essentially, we want two regs, one points to PTR_TO_BTD_ID, one
> points to MAP_VALUR_OR_NULL, then compare them and deref map val.

If I read this request correctly, I guess the compiler has changed 'ret = *val' 
to 'ret = *inner_map'?  Thus, the verifier did not reject because it deref a 
PTR_TO_BTF_ID?

> It’s hard to implement this in C level because compilers decide
> which reg to use but not us, maybe we can just drop this test.

Have you tried inline assembly.  Something like this (untested):

         asm volatile (
                 "r8 = %[val];\n"
                 "r9 = %[inner_map];\n"
		"if r8 != r9 goto +1;\n"
                 "%[ret] = *(u64 *)(r8 +0);\n"
                 :[ret] "+r"(ret)
                 : [inner_map] "r"(inner_map), [val] "r"(val)
                 :"r8", "r9");

Please attach the verifier output in the future.  It will be easier to understand.

> 
> thoughts?
>    
> +struct {
> +	__uint(type, BPF_MAP_TYPE_HASH);
> +	__uint(max_entries, 1);
> +	__type(key, u64);
> +	__type(value, u64);
> +} m_hash SEC(".maps");
> +
> +SEC("?raw_tp")
> +__failure __msg("invalid mem access 'map_value_or_null")
> +int jeq_infer_not_null_ptr_to_btfid(void *ctx)
> +{
> +	struct bpf_map *map = (struct bpf_map *)&m_hash;
> +	struct bpf_map *inner_map = map->inner_map_meta;
> +	u64 key = 0, ret = 0, *val;
> +
> +	val = bpf_map_lookup_elem(map, &key);
> +	/* Do not mark ptr as non-null if one of them is
> +	 * PTR_TO_BTF_ID, reject because of invalid access
> +	 * to map value.
> +	 */
> +	if (val == inner_map)
> +		ret = *val;
> +
> +	return ret;
> +}