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