On Thu, Jun 5, 2025 at 12:35 AM Alexis Lothoré alexis.lothore@bootlin.com wrote:
Hi Ihor,
On Wed Jun 4, 2025 at 7:31 PM CEST, Ihor Solodrai wrote:
On 6/4/25 2:02 AM, Alexis Lothoré wrote:
[...]
Could I be missing something obvious ? Or did I misunderstand the actual attribute encoding feature ?
Hi Alexis.
The changes recently landed in pahole and libbpf re attributes had a very narrow goal: passing through particular attributes for some BPF kfuncs from the kernel source to vmlinux.h
BTF now has a way of encoding any attribute (as opposed to only bpf type/decl tags) by setting type/decl tag kind flag [1]. So it is possible to represent attributes like packed and aligned in BTF.
However, the BTF tags need to be generated by something, in case of vmlinux by pahole. Pahole generates BTF by parsing DWARF. And, as far as I understand, attributes are not (can not be?) represented in DWARF in a generic way, it really depends on specifics of the attribute.
In order to support packed/aligned, pahole needs to know how to figure them out from DWARF input and add the tags to BTF. And this does not happen right now, which is why you don't see anything in bpftool output.
[1] https://lore.kernel.org/bpf/20250130201239.1429648-1-ihor.solodrai@linux.dev...
Thanks for the details ! I have missed this possibility, as I have been assuming that DWARF info was exposing the needed info. I'll take a look at it, but if those attributes can not be represented by DWARF, I'll have to find another way of getting those packing/alignment modifications on data type (eg: re-use/share btf__align_of from libbpf, as suggested by Andrii, but it may not able to cover all cases).
Not sure all the trouble is worth it. I feel it's a corner case. Something we don't need to fix.