linux-kernel - Re: [PATCH bpf-next 2/3] bpf: selftests: Add bpf_assert_if() and bpf_assert_with

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Date: Sat, 16 Dec 2023 14:47:33 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Daniel Xu <dxu@...uu.xyz>
Cc: Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Andrii Nakryiko <andrii@...nel.org>, Shuah Khan <shuah@...nel.org>, 
	Kumar Kartikeya Dwivedi <memxor@...il.com>, Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>, 
	Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko <mykolal@...com>, bpf <bpf@...r.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next 2/3] bpf: selftests: Add bpf_assert_if() and
 bpf_assert_with_if() macros

On Thu, Dec 14, 2023 at 7:10 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> Just realized that we can go the other way instead.
>
> We can get rid of bpf_assert_eq/ne/... and replace with:
>
> diff --git a/tools/testing/selftests/bpf/bpf_experimental.h
> b/tools/testing/selftests/bpf/bpf_experimental.h
> index 1386baf9ae4a..1c500287766d 100644
> --- a/tools/testing/selftests/bpf/bpf_experimental.h
> +++ b/tools/testing/selftests/bpf/bpf_experimental.h
> @@ -254,6 +254,15 @@ extern void bpf_throw(u64 cookie) __ksym;
>                 }
>                  \
>          })
>
> +#define _EQ(LHS, RHS) \
> +       ({ int var = 1;\
> +               asm volatile goto("if %[lhs] == %[rhs] goto %l[l_yes]" \
> +               :: [lhs] "r"(LHS), [rhs] "i"(RHS) :: l_yes);\
> +       var = 0;\
> +l_yes:\
> +       var;\
> +       })

Realized we can do much better.
We can take advantage that bpf assembly syntax resembles C and do:

bpf_assert(CMP(cookie, "!=", 0);

and use it as generic "volatile compare" that compiler cannot optimize out:

Replacing:
if (foo < bar) ...
with
if (CMP(foo, "<", bar)) ...
when the compare operator should be preserved.
I'll try to prototype it soon.

Might go further and use C++ for bpf programs :)
Override operator<, opreator==, ...
then if (foo < bar) will be in asm code as written in C++ bpf prog.