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From:   Hao Sun <sunhao.th@gmail.com>
Date:   Thu, 22 Dec 2022 10:30:19 +0800
Message-ID: <CACkBjsa+gOh0pFPctnigXs9ugW06efCqZ+qn+dTcyRJbf6z-MQ@mail.gmail.com>
Subject: Re: [PATCH bpf-next v2 2/2] selftests/bpf: check null propagation
 only neither reg is PTR_TO_BTF_ID
To:     Martin KaFai Lau <martin.lau@linux.dev>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        John Fastabend <john.fastabend@gmail.com>,
        Andrii Nakryiko <andrii@kernel.org>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        David Miller <davem@davemloft.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>
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Martin KaFai Lau <martin.lau@linux.dev> =E4=BA=8E2022=E5=B9=B412=E6=9C=8822=
=E6=97=A5=E5=91=A8=E5=9B=9B 05:21=E5=86=99=E9=81=93=EF=BC=9A
>
> On 12/21/22 5:46 AM, Hao Sun wrote:
> > Hi,
> >
> > I=E2=80=99ve tried something like the bellow, but soon realized that th=
is
> > won=E2=80=99t 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 =
=3D *val'
> to 'ret =3D *inner_map'?  Thus, the verifier did not reject because it de=
ref a
> PTR_TO_BTF_ID?
>

Yes, and if we do "*val =3D 0", it's rejected due to writing to read-only
PTR_TO_BTF_ID reg.

> > It=E2=80=99s 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 =3D %[val];\n"
>                  "r9 =3D %[inner_map];\n"
>                 "if r8 !=3D r9 goto +1;\n"
>                  "%[ret] =3D *(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 un=
derstand.
>

Will do the next time.