Re: [PATCH bpf-next RFC v1 1/8] bpf: Introduce BPF support for kernel module function calls

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Mon, Aug 30, 2021 at 11:04:17PM +0530, Kumar Kartikeya Dwivedi wrote:
> This change adds support on the kernel side to allow for BPF programs to
> call kernel module functions. Userspace will prepare an array of module
> BTF fds that is passed in during BPF_PROG_LOAD. In the kernel, the
> module BTF array is placed in the auxilliary struct for bpf_prog.
> 
> The verifier then uses insn->off to index into this table. insn->off is
> used by subtracting one from it, as userspace has to set the index of
> array in insn->off incremented by 1. This lets us denote vmlinux btf by
> insn->off == 0, and the prog->aux->kfunc_btf_tab[insn->off - 1] for
> module BTFs.
> 
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
> ---
>  include/linux/bpf.h            |  1 +
>  include/linux/filter.h         |  9 ++++
>  include/uapi/linux/bpf.h       |  3 +-
>  kernel/bpf/core.c              | 14 ++++++
>  kernel/bpf/syscall.c           | 55 +++++++++++++++++++++-
>  kernel/bpf/verifier.c          | 85 ++++++++++++++++++++++++++--------
>  tools/include/uapi/linux/bpf.h |  3 +-
>  7 files changed, 147 insertions(+), 23 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index f4c16f19f83e..39f59e5f3a26 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -874,6 +874,7 @@ struct bpf_prog_aux {
>  	void *jit_data; /* JIT specific data. arch dependent */
>  	struct bpf_jit_poke_descriptor *poke_tab;
>  	struct bpf_kfunc_desc_tab *kfunc_tab;
> +	struct bpf_kfunc_btf_tab *kfunc_btf_tab;
>  	u32 size_poke_tab;
>  	struct bpf_ksym ksym;
>  	const struct bpf_prog_ops *ops;
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 7d248941ecea..46451891633d 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -592,6 +592,15 @@ struct bpf_prog {
>  	struct bpf_insn		insnsi[];
>  };
>  
> +#define MAX_KFUNC_DESCS 256
> +/* There can only be at most MAX_KFUNC_DESCS module BTFs for kernel module
> + * function calls.
> + */
> +struct bpf_kfunc_btf_tab {
> +	u32 nr_btfs;
> +	struct btf_mod_pair btfs[];
> +};
> +
>  struct sk_filter {
>  	refcount_t	refcnt;
>  	struct rcu_head	rcu;
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 791f31dd0abe..4cbb2082a553 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1334,8 +1334,9 @@ union bpf_attr {
>  			/* or valid module BTF object fd or 0 to attach to vmlinux */
>  			__u32		attach_btf_obj_fd;
>  		};
> -		__u32		:32;		/* pad */
> +		__u32		kfunc_btf_fds_cnt; /* reuse hole for count of BTF fds below */

No need for size.

>  		__aligned_u64	fd_array;	/* array of FDs */
> +		__aligned_u64   kfunc_btf_fds;  /* array of BTF FDs for module kfunc support */

Just reuse fd_array. No need for another array of FDs.

> +		tab = prog->aux->kfunc_btf_tab;
> +		for (i = 0; i < n; i++) {
> +			struct btf_mod_pair *p;
> +			struct btf *mod_btf;
> +
> +			mod_btf = btf_get_by_fd(fds[i]);
> +			if (IS_ERR(mod_btf)) {
> +				err = PTR_ERR(mod_btf);
> +				goto free_prog;
> +			}
> +			if (!btf_is_module(mod_btf)) {
> +				err = -EINVAL;
> +				btf_put(mod_btf);
> +				goto free_prog;
> +			}

just do that dynamically like access to fd_array is handled in other places.
no need to preload.