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    SubjectRe: [PATCH bpf-next v2 2/2] selftests/bpf: check null propagation only neither reg is PTR_TO_BTF_ID
    Martin KaFai Lau <martin.lau@linux.dev> 于2022年12月22日周四 05:21写道:
    >
    > 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?
    >

    Yes, and if we do "*val = 0", it's rejected due to writing to read-only
    PTR_TO_BTF_ID reg.

    > > 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");
    >

    This would work, didn't realize that I can inline BPF insns this way.
    Thanks!

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

    Will do the next time.

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