linux-kernel - Re: [PATCH RFC bpf-next 1/4] bpf: add struct largest member size in func model

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Message-Id: <D9EWSDXHDGFJ.FIDSHIR1OP80@bootlin.com>
Date: Thu, 24 Apr 2025 15:38:35 +0200
From: Alexis Lothoré <alexis.lothore@...tlin.com>
To: "Xu Kuohai" <xukuohai@...weicloud.com>, "Andrii Nakryiko"
 <andrii.nakryiko@...il.com>
Cc: "Alexei Starovoitov" <ast@...nel.org>, "Daniel Borkmann"
 <daniel@...earbox.net>, "John Fastabend" <john.fastabend@...il.com>,
 "Andrii Nakryiko" <andrii@...nel.org>, "Martin KaFai Lau"
 <martin.lau@...ux.dev>, "Eduard Zingerman" <eddyz87@...il.com>, "Song Liu"
 <song@...nel.org>, "Yonghong Song" <yonghong.song@...ux.dev>, "KP Singh"
 <kpsingh@...nel.org>, "Stanislav Fomichev" <sdf@...ichev.me>, "Hao Luo"
 <haoluo@...gle.com>, "Jiri Olsa" <jolsa@...nel.org>, "Puranjay Mohan"
 <puranjay@...nel.org>, "Catalin Marinas" <catalin.marinas@....com>, "Will
 Deacon" <will@...nel.org>, "Mykola Lysenko" <mykolal@...com>, "Shuah Khan"
 <shuah@...nel.org>, "Maxime Coquelin" <mcoquelin.stm32@...il.com>,
 "Alexandre Torgue" <alexandre.torgue@...s.st.com>, "Florent Revest"
 <revest@...omium.org>, "Bastien Curutchet" <bastien.curutchet@...tlin.com>,
 <ebpf@...uxfoundation.org>, "Thomas Petazzoni"
 <thomas.petazzoni@...tlin.com>, <bpf@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, <linux-arm-kernel@...ts.infradead.org>,
 <linux-kselftest@...r.kernel.org>,
 <linux-stm32@...md-mailman.stormreply.com>
Subject: Re: [PATCH RFC bpf-next 1/4] bpf: add struct largest member size in
 func model

Hi Xu,

On Thu Apr 24, 2025 at 2:00 PM CEST, Xu Kuohai wrote:
> On 4/24/2025 3:24 AM, Alexis Lothoré wrote:
>> Hi Andrii,
>> 
>> On Wed Apr 23, 2025 at 7:15 PM CEST, Andrii Nakryiko wrote:
>>> On Thu, Apr 17, 2025 at 12:14 AM Alexis Lothoré
>>> <alexis.lothore@...tlin.com> wrote:
>>>>
>>>> Hi Andrii,
>>>>
>>>> On Wed Apr 16, 2025 at 11:24 PM CEST, Andrii Nakryiko wrote:
>>>>> On Fri, Apr 11, 2025 at 1:32 PM Alexis Lothoré (eBPF Foundation)
>>>>> <alexis.lothore@...tlin.com> wrote:

[...]

>> Thanks for the pointer, I'll take a look at it. The more we discuss this
>> series, the less member size sounds relevant for what I'm trying to achieve
>> here.
>> 
>> Following Xu's comments, I have been thinking about how I could detect the
>> custom alignments and packing on structures, and I was wondering if I could
>> somehow benefit from __attribute__ encoding in BTF info ([1]). But
>> following your hint, I also see some btf_is_struct_packed() in
>> tools/lib/bpf/btf_dump.c that could help. I'll dig this further and see if
>> I can manage to make something work with all of this.
>>
>
> With DWARF info, we might not need to detect the structure alignment anymore,
> since the DW_AT_location attribute tells us where the structure parameter is
> located on the stack, and DW_AT_byte_size gives us the size of the structure.

I am not sure to follow you here, because DWARF info is not accessible
from kernel at runtime, right ? Or are you meaning that we could, at build
time, enrich the BTF info embedded in the kernel thanks to DWARF info ?

Thanks,

Alexis

-- 
Alexis Lothoré, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com