Re: [PATCH RFC] btf_encoder: skip functions consuming structs passed by value on stack

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

Hi Alan,

On Thu Jun 19, 2025 at 5:41 PM CEST, Alan Maguire wrote:
> On 19/06/2025 14:12, Alexis Lothoré wrote:
>> On Wed Jun 18, 2025 at 6:28 PM CEST, Alexei Starovoitov wrote:
>>> On Wed, Jun 18, 2025 at 8:02 AM Alexis Lothoré
>>> <alexis.lothore@bootlin.com> wrote:

[...]

>>> though it's passed on the stack it fits into normal calling convention.
>>> It doesn't have align or packed attributes, so no need to exclude it ?
>> 
>> I went for the simplest solution, assuming that there were cases involving
>> packing/alignent customization that we would not be able to detect (eg: the
>> packed attr that does not change size but reduce alignment). But thinking
>> more about it, those cases need really specific conditions thay may not
>> exist currently in the kernel (eg: having some __int128 embedded in a
>> struct).
>> 
>> I see that pahole already has some logic to check if a struct is
>> altered (eg class__infer_packed_attributes), I'll check if I can come with
>> something more selective.
>>
>
> sounds good; one additional suggestion is given that these sorts of
> functions are rare to nonexistent in vmlinux, perhaps we could add some
> tests to the tests/ directory that compile C code and generate BTF from
> the associated DWARF, verifying that functions are (or are not) encoded
> as expected?
>
> I'm working on adding automatic comparison of vmlinux BTF function
> encoding for candidate patch series to pahole's CI (so we can see if
> functions appear/disappear), but in a case like this a few explicit
> tests would be great to have. Thanks!

Yes, sure. I did not consider at all tests while the series is in RFC, but
I had it in mind for the next steps. I'll take into account the need to
add and build automatically some custom C code exposing those "exotic"
functions and structs.

Alexis.

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