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

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Date: Thu, 14 Dec 2023 18:46:19 -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 2:56 PM Daniel Xu <dxu@...uu.xyz> wrote:
>
> These macros are a temporary stop-gap until bpf exceptions support
> unwinding acquired entities. Basically these macros act as if they take
> a callback which only get executed if the assertion fails.
>
> Signed-off-by: Daniel Xu <dxu@...uu.xyz>
> ---
>  .../testing/selftests/bpf/bpf_experimental.h  | 22 +++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
> index 1386baf9ae4a..d63f415bef26 100644
> --- a/tools/testing/selftests/bpf/bpf_experimental.h
> +++ b/tools/testing/selftests/bpf/bpf_experimental.h
> @@ -263,6 +263,17 @@ extern void bpf_throw(u64 cookie) __ksym;
>   */
>  #define bpf_assert(cond) if (!(cond)) bpf_throw(0);
>
> +/* Description
> + *     Assert that a conditional expression is true. If false, runs code in the
> + *     body before throwing.
> + * Returns
> + *     Void.
> + * Throws
> + *     An exception with the value zero when the assertion fails.
> + */
> +#define bpf_assert_if(cond) \
> +       for (int ___i = 0, ___j = !!(cond); !(___j) && !___i; bpf_throw(0), ___i++)

Kumar,

Is this approach reliable?
I suspect the compiler can still optimize it.
I feel it will be annoying to clean up if folks start using it now,
since there won't be a drop in replacement.
Every such bpf_assert_if() would need to be manually patched.

If 2nd part of exception is far, how about we add an equivalent
of __bpf_assert() macroses with conditional ops in asm,
but with extra 'asm volatile goto' that can be used to construct
release of resources.

bpf_do_assert_eq(var1, 0) { bpf_spin_unlock(...); }
bpf_do_assert_lt(var2, 0) { bpf_spin_unlock(...); }