linux-kernel - Re: [PATCH bpf-next v2 8/9] selftests/bpf: Add selftests for load-acquire and store-release instructions

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Message-ID: <Z6ajasn2k559SGNN@google.com>
Date: Sat, 8 Feb 2025 00:20:58 +0000
From: Peilin Ye <yepeilin@...gle.com>
To: bpf@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Cc: bpf@...f.org, Xu Kuohai <xukuohai@...weicloud.com>,
	Eduard Zingerman <eddyz87@...il.com>,
	David Vernet <void@...ifault.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	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@...ichev.me>,
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	Jonathan Corbet <corbet@....net>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Puranjay Mohan <puranjay@...nel.org>,
	Ilya Leoshkevich <iii@...ux.ibm.com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Quentin Monnet <qmo@...nel.org>,
	Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
	Ihor Solodrai <ihor.solodrai@...ux.dev>,
	Yingchi Long <longyingchi24s@....ac.cn>,
	Josh Don <joshdon@...gle.com>, Barret Rhoden <brho@...gle.com>,
	Neel Natu <neelnatu@...gle.com>,
	Benjamin Segall <bsegall@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 8/9] selftests/bpf: Add selftests for
 load-acquire and store-release instructions

On Fri, Feb 07, 2025 at 02:06:29AM +0000, Peilin Ye wrote:
> --- a/tools/testing/selftests/bpf/progs/arena_atomics.c
> +++ b/tools/testing/selftests/bpf/progs/arena_atomics.c
> @@ -6,6 +6,8 @@
>  #include <stdbool.h>
>  #include <stdatomic.h>
>  #include "bpf_arena_common.h"
> +#include "../../../include/linux/filter.h"
> +#include "bpf_misc.h"
>  
>  struct {
>  	__uint(type, BPF_MAP_TYPE_ARENA);
> @@ -274,4 +276,90 @@ int uaf(const void *ctx)
>  	return 0;
>  }
>  
> +__u8 __arena_global load_acquire8_value = 0x12;
   ~~~~

CI job x86_64-llvm-17 [1] failed because clang-17 crashed when compiling
this file (arena_atomics.c):

  fatal error: error in backend: unable to write nop sequence of 1 bytes

After some digging, I believe I am hitting a known issue that Yonghong
described in [2].  Changing __u8 and __u16 variables to __u32 seems to
resolve/work around the issue:

  +__u32 __arena_global load_acquire8_value = 0x12;

Will look into it more.

[1] https://github.com/kernel-patches/bpf/actions/runs/13191887466/job/36826207612
[2] https://lore.kernel.org/bpf/d56223f9-483e-fbc1-4564-44c0858a1e3e@meta.com/

> +__u16 __arena_global load_acquire16_value = 0x1234;
> +__u32 __arena_global load_acquire32_value = 0x12345678;
> +__u64 __arena_global load_acquire64_value = 0x1234567890abcdef;
> +
> +__u8 __arena_global load_acquire8_result = 0x12;
> +__u16 __arena_global load_acquire16_result = 0x1234;
> +__u32 __arena_global load_acquire32_result = 0x12345678;
> +__u64 __arena_global load_acquire64_result = 0x1234567890abcdef;

Thanks,
Peilin Ye