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

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Message-ID: <35c18502a4870d8a833c1c9af20b85ca3f8a0ff6.camel@gmail.com>
Date: Thu, 14 Aug 2025 09:06:35 -0700
From: Eduard Zingerman <eddyz87@...il.com>
To: Puranjay Mohan <puranjay12@...il.com>
Cc: KaFai Wan <kafai.wan@...ux.dev>, puranjay@...nel.org, 
	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

On Thu, 2025-08-14 at 13:23 +0200, Puranjay Mohan wrote:
> On Thu, Aug 14, 2025 at 2:35 AM Eduard Zingerman <eddyz87@...il.com> wrote:
> > 
> > On Wed, 2025-08-13 at 23:29 +0800, KaFai Wan wrote:
> > > This test verifies socket filter attachment functionality on architectures
> > > supporting either BPF JIT compilation or the interpreter.
> > > 
> > > It specifically validates the fallback to interpreter behavior when JIT fails,
> > > particularly targeting ARMv6 devices with the following configuration:
> > >   # CONFIG_BPF_JIT_ALWAYS_ON is not set
> > >   CONFIG_BPF_JIT_DEFAULT_ON=y
> > > 
> > > Signed-off-by: KaFai Wan <kafai.wan@...ux.dev>
> > > ---
> > 
> > This test should not be landed as-is, first let's do an analysis for
> > why the program fails to jit compile on arm.
> > 
> > I modified kernel to dump BPF program before jit attempt, but don't
> > see anything obviously wrong with it.  The patch to get disassembly
> > and disassembly itself with resolved kallsyms are attached.
> > 
> > Can someone with access to ARM vm/machine take a looks at this?
> > Puranjay, Xu, would you have some time?
> 
> Hi Eduard,
> Thanks for the email, I will look into it.
> 
> Let me try to boot a kernel on ARMv6 qemu and reproduce this.

Thank you, Puranjay,

While looking at the code yesterday I found a legit case for failing
to jit on armv6:

https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/arch/arm/net/bpf_jit_32.c#n445
https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git/tree/arch/arm/net/bpf_jit_32.c#n2089

But attached program does not seem to be that big to hit 0xfff boundary.