Re: [PATCH bpf-next v2 2/3] bpf: export crash_kexec() as destructive kfunc

[Alexei Starovoitov]

On Tue, Aug 2, 2022 at 2:10 AM Artem Savkov <asavkov@xxxxxxxxxx> wrote:
>
> Allow properly marked bpf programs to call crash_kexec().
>
> Signed-off-by: Artem Savkov <asavkov@xxxxxxxxxx>
> ---
>  kernel/kexec_core.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index 4d34c78334ce..9259ea3bd693 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -39,6 +39,8 @@
>  #include <linux/hugetlb.h>
>  #include <linux/objtool.h>
>  #include <linux/kmsg_dump.h>
> +#include <linux/btf.h>
> +#include <linux/btf_ids.h>
>
>  #include <asm/page.h>
>  #include <asm/sections.h>
> @@ -1238,3 +1240,22 @@ void __weak arch_kexec_protect_crashkres(void)
>
>  void __weak arch_kexec_unprotect_crashkres(void)
>  {}
> +
> +#ifdef CONFIG_DEBUG_INFO_BTF_MODULES
> +BTF_SET8_START(kexec_btf_ids)
> +BTF_ID_FLAGS(func, crash_kexec, KF_DESTRUCTIVE)
> +BTF_SET8_END(kexec_btf_ids)
> +
> +static const struct btf_kfunc_id_set kexec_kfunc_set = {
> +       .owner = THIS_MODULE,
> +       .set   = &kexec_btf_ids,
> +};
> +
> +static int __init crash_kfunc_init(void)
> +{
> +       register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &kexec_kfunc_set);
> +       return 0;
> +}
> +
> +subsys_initcall(crash_kfunc_init);
> +#endif

It feels there will be a bunch of such boiler plate code
in different .c files in many places in the kernel
if we go with this approach.

Maybe we should do one call:
register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING
from kernel/bpf/helper.c
to register all tracing kfuncs ?

And gate
BTF_ID_FLAGS(func, crash_kexec, KF_DESTRUCTIVE)
with #ifdef CONFIG_KEXEC_CORE.

We have such a pattern in verifier.c already.