linux-kernel - Re: [PATCH bpf v2 2/2] selftests/bpf: Add socket filter attach test

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Message-ID: <mb61pv7m92evu.fsf@gmail.com>
Date: Wed, 27 Aug 2025 11:16:53 +0000
From: Puranjay Mohan <puranjay12@...il.com>
To: Eduard Zingerman <eddyz87@...il.com>
Cc: KaFai Wan <kafai.wan@...ux.dev>, xukuohai@...weicloud.com,
 ast@...nel.org, daniel@...earbox.net, john.fastabend@...il.com,
 andrii@...nel.org, martin.lau@...ux.dev, song@...nel.org,
 yonghong.song@...ux.dev, kpsingh@...nel.org, sdf@...ichev.me,
 haoluo@...gle.com, jolsa@...nel.org, mykolal@...com, shuah@...nel.org,
 mrpre@....com, linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH bpf v2 2/2] selftests/bpf: Add socket filter attach test

Eduard Zingerman <eddyz87@...il.com> writes:

> On Mon, 2025-08-25 at 21:27 +0200, Puranjay Mohan wrote:
>
> [...]
>
>> Hi Eduard,
>> 
>> You were right, I have verified that the program is hitting the 0xfff
>> boundary while doing the call to bpf_skb_load_helper_32
>> While jiting this call, emit_a32_mov_i(tmp[1], func, ctx); is called,
>> where this issue it triggered.
>> 
>> The offset in imm_offset() is calculated as:
>> ctx->offsets[ctx->prog->len - 1] * 4 + ctx->prologue_bytes +
>> ctx->epilogue_bytes + imm_i * 4
>> 
>> For this program, ctx->offsets[ctx->prog->len - 1] * 4 itself is
>> 0x1400 which is above 0xfff boundary.
>> So, this is not a bug and expected behaviour with the current
>> implementation of the JIT.
>> 
>> For now, we can merge this and later I will try to improve the JIT so
>> it works for bigger programs.
>
> Hi Puranjay,
>
> Thank you for checking this!
> What do you think about this test case, do we need it in the suite?

I don't think that we need this test as it is based on a missing feature
in the JIT. Once the arm JIT is improved, this test will silently stop
testing what it is supposed to test (fallback to interpreter).

Thanks,
Puranjay