Re: [PATCH bpf-next v2 09/15] selftests/bpf: Test assigning ID to scalars on spill

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Maxim Mikityanskiy <maxtram95@gmail.com>
Cc: Eduard Zingerman <eddyz87@gmail.com>,
	Alexei Starovoitov <ast@kernel.org>,
	 Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	 Shung-Hsi Yu <shung-hsi.yu@suse.com>,
	John Fastabend <john.fastabend@gmail.com>,
	 Martin KaFai Lau <martin.lau@linux.dev>,
	Song Liu <song@kernel.org>,
	 Yonghong Song <yonghong.song@linux.dev>,
	KP Singh <kpsingh@kernel.org>,
	 Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
	Jiri Olsa <jolsa@kernel.org>,  Mykola Lysenko <mykolal@fb.com>,
	Shuah Khan <shuah@kernel.org>,
	 "David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	 Jesper Dangaard Brouer <hawk@kernel.org>,
	bpf@vger.kernel.org, linux-kselftest@vger.kernel.org,
	 netdev@vger.kernel.org, Maxim Mikityanskiy <maxim@isovalent.com>
Subject: Re: [PATCH bpf-next v2 09/15] selftests/bpf: Test assigning ID to scalars on spill
Date: Tue, 9 Jan 2024 15:34:07 -0800	[thread overview]
Message-ID: <CAEf4Bzb4Gy1CTE7ebjA6b2vtZ7dhZnpsw25YnGTx6y2rYYFmcw@mail.gmail.com> (raw)
In-Reply-To: <20240108205209.838365-10-maxtram95@gmail.com>

On Mon, Jan 8, 2024 at 12:53 PM Maxim Mikityanskiy <maxtram95@gmail.com> wrote:
>
> From: Maxim Mikityanskiy <maxim@isovalent.com>
>
> The previous commit implemented assigning IDs to registers holding
> scalars before spill. Add the test cases to check the new functionality.
>
> Signed-off-by: Maxim Mikityanskiy <maxim@isovalent.com>
> Acked-by: Eduard Zingerman <eddyz87@gmail.com>
> ---
>  .../selftests/bpf/progs/verifier_spill_fill.c | 133 ++++++++++++++++++
>  1 file changed, 133 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> index f303ac19cf41..b05aab925ee5 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c
> @@ -766,4 +766,137 @@ l0_%=:    r0 = 0;                                         \
>         : __clobber_all);
>  }
>

[...]

> +
> +SEC("xdp")
> +__description("8-bit spill of 8-bit reg should assign ID")
> +__success __retval(0)
> +__naked void spill_8bit_of_8bit_ok(void)
> +{
> +       asm volatile ("                                 \
> +       /* Roll one bit to make the register inexact. */\
> +       call %[bpf_get_prandom_u32];                    \
> +       r0 &= 0x80;                                     \
> +       /* 8-bit spill r0 to stack - should assign an ID. */\
> +       *(u8*)(r10 - 8) = r0;                           \
> +       /* 8-bit fill r1 from stack - should preserve the ID. */\
> +       r1 = *(u8*)(r10 - 8);                           \
> +       /* Compare r1 with another register to trigger find_equal_scalars.\
> +        * Having one random bit is important here, otherwise the verifier cuts\
> +        * the corners.                                 \
> +        */                                             \
> +       r2 = 0;                                         \
> +       if r1 != r2 goto l0_%=;                         \
> +       /* The result of this comparison is predefined. */\
> +       if r0 == r2 goto l0_%=;                         \
> +       /* Dead branch: the verifier should prune it. Do an invalid memory\
> +        * access if the verifier follows it.           \
> +        */                                             \
> +       r0 = *(u64*)(r9 + 0);                           \
> +       exit;                                           \
> +l0_%=: r0 = 0;                                         \
> +       exit;                                           \
> +"      :
> +       : __imm(bpf_get_prandom_u32)
> +       : __clobber_all);
> +}
> +

Can you add a test where we spill register of one size and fill a
different size? And what should the behavior be? Should we or should
we not preserve linked IDs in such situation?

>  char _license[] SEC("license") = "GPL";
> --
> 2.43.0
>