Hi Kumar,
On Mon, 22 Nov 2021, Kumar Kartikeya Dwivedi wrote:
Vinicius Costa Gomes reported [0] that build fails when
CONFIG_DEBUG_INFO_BTF is enabled and CONFIG_BPF_SYSCALL is disabled.
This leads to btf.c not being compiled, and then no symbol being present
in vmlinux for the declarations in btf.h. Since BTF is not useful
without enabling BPF subsystem, disallow this combination.
However, theoretically disabling both now could still fail, as the
symbol for kfunc_btf_id_list variables is not available. This isn't a
problem as the compiler usually optimizes the whole register/unregister
call, but at lower optimization levels it can fail the build in linking
stage.
Fix that by adding dummy variables so that modules taking address of
them still work, but the whole thing is a noop.
[0]: https://lore.kernel.org/bpf/20211110205418.332403-1-vinicius.gomes@xxxxxxxxx
Fixes: 14f267d95fe4 ("bpf: btf: Introduce helpers for dynamic BTF set registration")
Reported-by: Vinicius Costa Gomes <vinicius.gomes@xxxxxxxxx>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
Thanks for your patch, which is now commit d9847eb8be3d895b ("bpf:
Make CONFIG_DEBUG_INFO_BTF depend upon CONFIG_BPF_SYSCALL") in v5.16.
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -316,6 +316,7 @@ config DEBUG_INFO_BTF
bool "Generate BTF typeinfo"
depends on !DEBUG_INFO_SPLIT && !DEBUG_INFO_REDUCED
depends on !GCC_PLUGIN_RANDSTRUCT || COMPILE_TEST
+ depends on BPF_SYSCALL
help
Generate deduplicated BTF type information from DWARF debug info.
Turning this on expects presence of pahole tool, which will convert
I wanted to run pahole on a kernel object file, but was greeted by an
error message:
libbpf: failed to find '.BTF' ELF section in <foo>.o
Then I discovered I could not enable CONFIG_DEBUG_INFO_BTF without also
enabling BPF_SYSCALL, which looks totally unrelated to me. So yes,
there seems to be a use case for BTF without enabling the BPF subsystem.
Thanks!
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
