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

["Alexis Lothoré" <alexis.lothore@bootlin.com>]

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@bootlin.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@bootlin.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