Em Wed, Sep 09, 2020 at 11:02:24AM +0200, Ilya Leoshkevich escreveu: > On Tue, 2020-09-08 at 13:18 -0700, Andrii Nakryiko wrote: > > On Mon, Sep 7, 2020 at 9:02 AM Ilya Leoshkevich <iii@xxxxxxxxxxxxx> wrote: > > > On Sat, 2020-09-05 at 21:16 -0700, Tony Ambardar wrote: > > > > I'm using GCC 8.4.0, binutils 2.34 and pahole 1.17, compiling on > > > > an Ubuntu/x86_64 host and targeting both little- and big-endian > > > > mips running on malta/qemu. When cross-compiling Linux 5.4.x LTS > > > > and testing bpftool/BTF functionality on the target, I encounter > > > > errors on big-endian targets: > > > > > root@OpenWrt:/# bpftool btf dump file /sys/kernel/btf/vmlinux > > > > > libbpf: failed to get EHDR from /sys/kernel/btf/vmlinux Error: > > > > > failed to load BTF from /sys/kernel/btf/vmlinux: No error > > > > > information > > > > > After investigating, the problem appears to be that "pahole -J" > > > > running on the x86_64 little-endian host will always generate > > > > raw BTF of native endianness (based on BTF magic), which causes > > > > the error above on big-endian targets. > > > > Is this expected? Is DEBUG_INFO_BTF supported in general when > > > > cross-compiling? How does one generate BTF encoded for the > > > > target endianness with pahole? The BTF loader has support for endianness, its just the encoder that doesn't :-\ I.e. pahole can grok a big endian BTF payload on a little endian machine and vice-versa, just can't cross-build BTF payloads ATM. > > Yes, it's expected, unfortunately. Right now cross-compiling to a > > different endianness isn't supported. You can cross-compile only if > > target endianness matches host endianness. I agree that having this in libbpf is better, it should be done as part of producing the result of the deduplication phase. > > > > Thanks for any feedback or suggestions, > > > > Tony > > > > > > We have the same problem on s390, and I'm not aware of any solution > > > at > > > the moment. It would be great if we could figure out how to resolve > > > this. > > > > I'm working on extending BTF APIs in libbpf at the moment. Switching > > endianness would be rather easy once all that is done. With these new > > APIs it will be possible to switch pahole to use libbpf APIs to > > produce BTF output and support arbitrary endianness as well. Right > > now, I'd rather avoid implementing this in pahole, libbpf is a much > > better place for this (and will require ongoing updates if/when we > > introduce new types and fields to BTF). Right, we could do it right after btf_dedup() and before btf_elf__write(), doing the same process as in btf_loader.c, i.e. checking if the ELF target arch is different in endianness and doing the reverse of the loader. > > Hope this plan works for you guys. > > That sounds really good to me, thanks! - Arnaldo
