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

[Hao Sun]

Martin KaFai Lau <martin.lau@xxxxxxxxx> 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.