Re: [PATCH net v2] bpf: Fix build when CONFIG_BPF_SYSCALL is disabled

On Wed, Nov 10, 2021 at 1:25 PM Kumar Kartikeya Dwivedi
[off-list ref] wrote:

On Thu, Nov 11, 2021 at 02:24:18AM IST, Vinicius Costa Gomes wrote:

quoted

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@intel.com>

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.