On Thu, Jun 5, 2025 at 12:35 AM Alexis Lothoré <alexis.loth...@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.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).
Not sure all the trouble is worth it. I feel it's a corner case. Something we don't need to fix.
