Messages in this thread | | | Date | Wed, 21 Dec 2022 13:21:25 -0800 | Subject | Re: [PATCH bpf-next v2 2/2] selftests/bpf: check null propagation only neither reg is PTR_TO_BTF_ID | From | Martin KaFai Lau <> |
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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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