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SubjectRe: [PATCH bpf-next v2 2/2] selftests/bpf: check null propagation only neither reg is PTR_TO_BTF_ID
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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;
> +}

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