On Wed, Nov 10, 2021 at 1:25 PM Kumar Kartikeya Dwivedi
<memxor@xxxxxxxxx> wrote:
>
> On Thu, Nov 11, 2021 at 02:24:18AM IST, Vinicius Costa Gomes wrote:
> > When CONFIG_DEBUG_INFO_BTF is enabled and CONFIG_BPF_SYSCALL is
> > disabled, the following compilation error can be seen:
> >
> > GEN .version
> > CHK include/generated/compile.h
> > UPD include/generated/compile.h
> > CC init/version.o
> > AR init/built-in.a
> > LD vmlinux.o
> > MODPOST vmlinux.symvers
> > MODINFO modules.builtin.modinfo
> > GEN modules.builtin
> > LD .tmp_vmlinux.btf
> > ld: net/ipv4/tcp_cubic.o: in function `cubictcp_unregister':
> > net/ipv4/tcp_cubic.c:545: undefined reference to `bpf_tcp_ca_kfunc_list'
> > ld: net/ipv4/tcp_cubic.c:545: undefined reference to `unregister_kfunc_btf_id_set'
> > ld: net/ipv4/tcp_cubic.o: in function `cubictcp_register':
> > net/ipv4/tcp_cubic.c:539: undefined reference to `bpf_tcp_ca_kfunc_list'
> > ld: net/ipv4/tcp_cubic.c:539: undefined reference to `register_kfunc_btf_id_set'
> > BTF .btf.vmlinux.bin.o
> > pahole: .tmp_vmlinux.btf: No such file or directory
> > LD .tmp_vmlinux.kallsyms1
> > .btf.vmlinux.bin.o: file not recognized: file format not recognized
> > make: *** [Makefile:1187: vmlinux] Error 1
> >
> > 'bpf_tcp_ca_kfunc_list', 'register_kfunc_btf_id_set()' and
> > 'unregister_kfunc_btf_id_set()' are only defined when
> > CONFIG_BPF_SYSCALL is enabled.
> >
> > Fix that by moving those definitions somewhere that doesn't depend on
> > the bpf() syscall.
> >
> > Fixes: 14f267d95fe4 ("bpf: btf: Introduce helpers for dynamic BTF set registration")
> > Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@xxxxxxxxx>
>
> Thanks for the fix.
>
> But instead of moving this to core.c, you can probably make the btf.h
> declaration conditional on CONFIG_BPF_SYSCALL, since this is not useful in
> isolation (only used by verifier for module kfunc support). For the case of
> kfunc_btf_id_list variables, just define it as an empty struct and static
> variables, since the definition is still inside btf.c. So it becomes a noop for
> !CONFIG_BPF_SYSCALL.
>
> I am also not sure whether BTF is useful without BPF support, but maybe I'm
> missing some usecase.
Unlikely. I would just disallow such config instead of sprinkling
the code with ifdefs.
