bpf: save the start of functions in bpf_prog_aux

Introduce a new subprog_start field in bpf_prog_aux. This field may
be used by JIT compilers wanting to know the real absolute xlated
offset of the function being jitted. The func_info[func_id] may have
served this purpose, but func_info may be NULL, so JIT compilers
can't rely on it.

Signed-off-by: Anton Protopopov <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: aspsk

include/linux/bpf.h

@@ -1623,6 +1623,7 @@ struct bpf_prog_aux {
 	u32 ctx_arg_info_size;
 	u32 max_rdonly_access;
 	u32 max_rdwr_access;
+	u32 subprog_start;
 	struct btf *attach_btf;
 	struct bpf_ctx_arg_aux *ctx_arg_info;
 	void __percpu *priv_stack_ptr;

kernel/bpf/verifier.c

@@ -21607,6 +21607,7 @@ static int jit_subprogs(struct bpf_verifier_env *env)
 		func[i]->aux->func_idx = i;
 		/* Below members will be freed only at prog->aux */
 		func[i]->aux->btf = prog->aux->btf;
+		func[i]->aux->subprog_start = subprog_start;
 		func[i]->aux->func_info = prog->aux->func_info;
 		func[i]->aux->func_info_cnt = prog->aux->func_info_cnt;
 		func[i]->aux->poke_tab = prog->aux->poke_tab;