On Mon, Jun 17, 2024 at 7:29 PM Alan Maguire <alan.maguire@xxxxxxxxxx> wrote: > > On 16/06/2024 01:29, Donglin Peng wrote: > > I encountered an issue when building the test_progs using the repository[1]: > > > > $ clang --version > > Ubuntu clang version 17.0.6 (++20231208085846+6009708b4367-1~exp1~20231208085949.74) > > Target: x86_64-pc-linux-gnu > > Thread model: posix > > InstalledDir: /usr/bin > > > > $ pwd > > /work/Qemu/x86_64/linux-6.10-rc2/tools/testing/selftests/bpf/ > > > > $ make test_progs V=1 > > ... > > /work/Qemu/x86_64/linux-6.10-rc2/tools/testing/selftests/bpf/tools/sbin/bpftool > > gen object > > /work/Qemu/x86_64/linux-6.10-rc2/tools/testing/selftests/bpf/ip_check_defrag.bpf.linked2.o > > /work/Qemu/x86_64/linux-6.10-rc2/tools/testing/selftests/bpf/ip_check_defrag.bpf.linked1.o > > libbpf: failed to find symbol for variable 'bpf_dynptr_slice' in section > > '.ksyms' > > Error: failed to link > > '/work/Qemu/x86_64/linux-6.10-rc2/tools/testing/selftests/bpf/ip_check_defrag.bpf.linked1.o': > > No such file or directory (2) > > make: *** [Makefile:656: > > /work/Qemu/x86_64/linux-6.10-rc2/tools/testing/selftests/bpf/ip_check_defrag.skel.h] > > Error 254 > > > > After investigation, I found that the btf_types in the '.ksyms' section have a kind of > > BTF_KIND_FUNC instead of BTF_KIND_VAR: > > > > $ bpftool btf dump file ./ip_check_defrag.bpf.linked1.o > > ... > > [2] DATASEC '.ksyms' size=0 vlen=2 > > type_id=16 offset=0 size=0 (FUNC 'bpf_dynptr_from_skb') > > type_id=17 offset=0 size=0 (FUNC 'bpf_dynptr_slice') > > ... > > [16] FUNC 'bpf_dynptr_from_skb' type_id=82 linkage=extern > > [17] FUNC 'bpf_dynptr_slice' type_id=85 linkage=extern > > ... > > > > To fix this, we can a add check for the kind. > > > > [1] https://github.com/eddyz87/bpf/tree/binsort-btf-dedup > > Link: https://lore.kernel.org/all/4f551dc5fc792936ca364ce8324c0adea38162f1.camel@xxxxxxxxx/ > > > > The fix makes sense; what I was trying to figure out is why we're only > seeing this now with the above repo. > > So as I understand it, the reason the kfuncs end up in the .ksyms > section is due to the "__weak __ksym" tagging recently added to > vmlinux.h construction from BTF via > > 770abbb5a25a ("bpftool: Support dumping kfunc prototypes from BTF") > > We see as noted > > [112] DATASEC '.ksyms' size=0 vlen=2 > type_id=84 offset=0 size=0 (FUNC 'bpf_dynptr_from_skb') > type_id=90 offset=0 size=0 (FUNC 'bpf_dynptr_slice') > > So that makes sense, but prior to the above series, we also tagged > kfuncs in this way before via bpf_kfuncs.h. So there should be no > difference there. > > And with an upstream kernel I don't run into this problem. > > The only thing I could come up with is we were usually lucky; when we > misinterpreted the func as a var and looked its type up, we got > > int var_linkage = btf_var(vt)->linkage; > > ...and were lucky it never equalled 1 (BTF_VAR_GLOBAL_ALLOCATED): > > /* no need to patch up static or extern vars */ > if (var_linkage != BTF_VAR_GLOBAL_ALLOCATED) > continue; > > In the case of a function, the above btf_var(vt) would really be > pointing at the struct btf_type immediately after the relevant > function's struct btf_type (since unlike variables, functions don't have > metadata following them). So the only way we'd trip this bug would be if > the struct btf_type following the func was had a name_off value that > happened to equal 1 (BTF_VAR_GLOBAL_ALLOCATED). > > So maybe the sorting changes to BTF order resulted in us tripping on > this bug, but regardless the fix seems right to me. Yes, I agree. > > > Fixes: 8fd27bf69b86 ("libbpf: Add BPF static linker BTF and BTF.ext support") > > Signed-off-by: Donglin Peng <dolinux.peng@xxxxxxxxx> > > A few small things below, but > > Reviewed-by: Alan Maguire <alan.maguire@xxxxxxxxxx> Thanks. > > > --- > > tools/lib/bpf/linker.c | 11 +++++++++-- > > 1 file changed, 9 insertions(+), 2 deletions(-) > > > > diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c > > index 0d4be829551b..7f5fc9ac4ad6 100644 > > --- a/tools/lib/bpf/linker.c > > +++ b/tools/lib/bpf/linker.c > > @@ -2213,10 +2213,17 @@ static int linker_fixup_btf(struct src_obj *obj) > > vi = btf_var_secinfos(t); > > for (j = 0, m = btf_vlen(t); j < m; j++, vi++) { > > const struct btf_type *vt = btf__type_by_id(obj->btf, vi->type); > > - const char *var_name = btf__str_by_offset(obj->btf, vt->name_off); > > - int var_linkage = btf_var(vt)->linkage; > > + const char *var_name; > > + int var_linkage; > > Elf64_Sym *sym; > > > > + /* should be a variable */ > > + if (btf_kind(vt) != BTF_KIND_VAR) > > nit: could use if (!btf_is_var(vt)) here instead. It might also be worth > reworking the comment to acknowledge that we can legitimately have a > function in this section; i.e. something like > > /* could be a variable or function */ Thanks, I will update in v2. > > We handle the func case elsewhere in libbpf (see add_dummy_ksym_var()). > > > > + continue; > > + > > + var_name = btf__str_by_offset(obj->btf, vt->name_off); > > + var_linkage = btf_var(vt)->linkage; > > + > > /* no need to patch up static or extern vars */ > > if (var_linkage != BTF_VAR_GLOBAL_ALLOCATED) > > continue;
