Re: [PATCH bpf-next v4 0/3] Annotate kfuncs in .BTF_ids section

[Manu Bretelle <chantr4@gmail.com>]

On Sat, Feb 03, 2024 at 03:40:24PM +0100, Jiri Olsa wrote:
> On Fri, Feb 02, 2024 at 03:09:05PM -0800, Manu Bretelle wrote:
> > On Sun, Jan 28, 2024 at 06:24:05PM -0700, Daniel Xu wrote:
> > > === Description ===
> > > 
> > > This is a bpf-treewide change that annotates all kfuncs as such inside
> > > .BTF_ids. This annotation eventually allows us to automatically generate
> > > kfunc prototypes from bpftool.
> > > 
> > > We store this metadata inside a yet-unused flags field inside struct
> > > btf_id_set8 (thanks Kumar!). pahole will be taught where to look.
> > > 
> > > More details about the full chain of events are available in commit 3's
> > > description.
> > > 
> > > The accompanying pahole and bpftool changes can be viewed
> > > here on these "frozen" branches [0][1].
> > > 
> > > [0]: https://github.com/danobi/pahole/tree/kfunc_btf-v3-mailed
> > > [1]: https://github.com/danobi/linux/tree/kfunc_bpftool-mailed
> > 
> > 
> > I hit a similar issue to [0] on master
> > 943b043aeecc ("selftests/bpf: Fix bench runner SIGSEGV")
> >  when cross-compiling on x86_64 (LE) to s390x (BE).
> > I do have CONFIG_DEBUG_INFO_BTF enable and the issue would not trigger if
> > I disabled CONFIG_DEBUG_INFO_BTF (and with the fix mentioned in [0]).
> > 
> > What seems to happen is that `tools/resolve_btfids` is ran in the context of the
> > host endianess and if I printk before the WARN_ON:
> > diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> > index ef380e546952..a9ed7a1a4936 100644
> >   --- a/kernel/bpf/btf.c
> >   +++ b/kernel/bpf/btf.c
> >   @@ -8128,6 +8128,7 @@ int register_btf_kfunc_id_set(enum bpf_prog_type prog_type,
> >            * WARN() for initcall registrations that do not check errors.
> >            */
> >           if (!(kset->set->flags & BTF_SET8_KFUNCS)) {
> >   +        printk("Flag 0x%08X, expected 0x%08X\n", kset->set->flags, BTF_SET8_KFUNCS);
> >                   WARN_ON(!kset->owner);
> >                   return -EINVAL;
> >           }
> > 
> > the boot logs would show:
> >   Flag 0x01000000, expected 0x00000001
> > 
> > The issue did not happen prior to
> > 6f3189f38a3e ("bpf: treewide: Annotate BPF kfuncs in BTF")
> > has only 0 was written before.
> > 
> > It seems [1] will be addressing cross-compilation, but it did not fix it as is
> > by just applying on top of master, so probably some of the changes will also need
> > to be ported to `tools/include/linux/btf_ids.h`?
> 
> the fix in [1] is fixing flags in set8's pairs, but not the global flags
> 
> it looks like Viktor's fix should now also swap that as well? like in the
> change below on top of Viktor's changes (untested)
> 
> jirka
> 
> 
> ---
> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
> index d01603ef6283..c44d57fec390 100644
> --- a/tools/bpf/resolve_btfids/main.c
> +++ b/tools/bpf/resolve_btfids/main.c
> @@ -706,6 +706,8 @@ static int sets_patch(struct object *obj)
>  			 * correctly translate everything.
>  			 */
>  			if (need_bswap) {
> +				set8->flags = bswap_32(set8->flags);
> +
>  				for (i = 0; i < cnt; i++) {
>  					set8->pairs[i].flags =
>  						bswap_32(set8->pairs[i].flags);
> 

That should work. Here are a few tests I ran:

$ md5sum /tmp/kbuild-s390x/vmlinux.*
eb658e51e089f3c5b2c8909a29dc9997  /tmp/kbuild-s390x/vmlinux.a
# plain vmlinux before running resolv_btfids (all 0s)
ea907cd46a1a73b8276b5f2a82af00ca  /tmp/kbuild-s390x/vmlinux.before_resolv
# x86_64 resolv_btfids on master without Viktor's patch
980a40c3a3ff563d1c2d1ebdd5071a23  /tmp/kbuild-s390x/vmlinux.resolv_native
# x86_64 resolv_btfids on master with Viktor's patch
b986d19e242719ebea41c578235da662  /tmp/kbuild-s390x/vmlinux.resolv_native_patch_viktor
# x86_64 resolv_btfids on master with Viktor's patch and your suggested patch
4edd8752ff01129945bd442689b1927b  /tmp/kbuild-s390x/vmlinux.resolv_native_patch_viktor_patched
# s390x resolv_btfids run with qemu-s390x-static
4edd8752ff01129945bd442689b1927b  /tmp/kbuild-s390x/vmlinux.resolv_s390x


and some hexdiff of those binaries:


# difference between master's native build and s390x build.... has byte swapping for set8 and others
diff -ruN <(xxd /tmp/kbuild-s390x/vmlinux.resolv_s390x) <(xxd /tmp/kbuild-s390x/vmlinux.resolv_native) > diff_s390x_native.diff
https://gist.github.com/chantra/c3d58637a08a6f7340953dc155bb18cc

# difference betwee Viktor's version and  s390x build.... squinting my eyes I only see the global set8 is missing
diff -ruN <(xxd /tmp/kbuild-s390x/vmlinux.resolv_s390x) <(xxd /tmp/kbuild-s390x/vmlinux.resolv_native_patch_viktor) > diff_s390x_native_viktor.diff
https://gist.github.com/chantra/61cfff02b456ae72d3c0161ce1897097

Have a good weekend all!

Manu