Re: [PATCH bpf-next 2/5] bpf: tracing: support to attach program to multi hooks

Jiri Olsa
2024-02-20T17:18:34Z

On Tue, Feb 20, 2024 at 11:51:02AM +0800, Menglong Dong wrote:

SNIP

> @@ -3228,7 +3260,9 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
> struct bpf_link_primer link_primer;
> struct bpf_prog *tgt_prog = NULL;
> struct bpf_trampoline *tr = NULL;
> + struct btf *attach_btf = NULL;
> struct bpf_tracing_link *link;
> + struct module *mod = NULL;
> u64 key = 0;
> int err;
>
> @@ -3258,31 +3292,50 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
> goto out_put_prog;
> }
>
> - if (!!tgt_prog_fd != !!btf_id) {
> - err = -EINVAL;
> - goto out_put_prog;
> - }
> -
> if (tgt_prog_fd) {
> - /*
> - * For now we only allow new targets for BPF_PROG_TYPE_EXT. If this
> - * part would be changed to implement the same for
> - * BPF_PROG_TYPE_TRACING, do not forget to update the way how
> - * attach_tracing_prog flag is set.
> - */
> - if (prog->type != BPF_PROG_TYPE_EXT) {
> + if (!btf_id) {
> err = -EINVAL;
> goto out_put_prog;
> }
> -
> tgt_prog = bpf_prog_get(tgt_prog_fd);
> if (IS_ERR(tgt_prog)) {
> - err = PTR_ERR(tgt_prog);
> tgt_prog = NULL;
> - goto out_put_prog;
> + /* tgt_prog_fd is the fd of the kernel module BTF */
> + attach_btf = btf_get_by_fd(tgt_prog_fd);

I think we should pass the btf_fd through attr, like add
link_create.tracing_btf_fd instead, this seems confusing

> + if (IS_ERR(attach_btf)) {
> + attach_btf = NULL;
> + err = -EINVAL;
> + goto out_put_prog;
> + }
> + if (!btf_is_kernel(attach_btf)) {
> + btf_put(attach_btf);
> + err = -EOPNOTSUPP;
> + goto out_put_prog;
> + }
> + } else if (prog->type == BPF_PROG_TYPE_TRACING &&
> + tgt_prog->type == BPF_PROG_TYPE_TRACING) {
> + prog->aux->attach_tracing_prog = true;
> }

could you please add comment on why this check is in here?

> -
> - key = bpf_trampoline_compute_key(tgt_prog, NULL, btf_id);
> + key = bpf_trampoline_compute_key(tgt_prog, attach_btf,
> + btf_id);
> + } else if (btf_id) {
> + attach_btf = bpf_get_btf_vmlinux();
> + if (IS_ERR(attach_btf)) {
> + attach_btf = NULL;
> + err = PTR_ERR(attach_btf);
> + goto out_unlock;
> + }
> + if (!attach_btf) {
> + err = -EINVAL;
> + goto out_unlock;
> + }
> + btf_get(attach_btf);
> + key = bpf_trampoline_compute_key(NULL, attach_btf, btf_id);
> + } else {
> + attach_btf = prog->aux->attach_btf;
> + /* get the reference of the btf for bpf link */
> + if (attach_btf)
> + btf_get(attach_btf);
> }
>
> link = kzalloc(sizeof(*link), GFP_USER);
> @@ -3319,7 +3372,7 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
> * are NULL, then program was already attached and user did not provide
> * tgt_prog_fd so we have no way to find out or create trampoline
> */
> - if (!prog->aux->dst_trampoline && !tgt_prog) {
> + if (!prog->aux->dst_trampoline && !tgt_prog && !btf_id) {
> /*
> * Allow re-attach for TRACING and LSM programs. If it's
> * currently linked, bpf_trampoline_link_prog will fail.
> @@ -3346,17 +3399,27 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
> * different from the destination specified at load time, we
> * need a new trampoline and a check for compatibility
> */
> + struct btf *origin_btf = prog->aux->attach_btf;
> struct bpf_attach_target_info tgt_info = {};
>
> + /* use the new attach_btf to check the target */
> + prog->aux->attach_btf = attach_btf;
> err = bpf_check_attach_target(NULL, prog, tgt_prog, btf_id,
> &tgt_info);
> + prog->aux->attach_btf = origin_btf;

could we pass the attach_btf as argument then?

jirka

> if (err)
> goto out_unlock;
>
> - if (tgt_info.tgt_mod) {
> - module_put(prog->aux->mod);
> - prog->aux->mod = tgt_info.tgt_mod;
> - }
> + mod = tgt_info.tgt_mod;
> + /* the new target and the previous target are in the same
> + * module, release the reference once.
> + */
> + if (mod && mod == prog->aux->mod)
> + module_put(mod);
> + err = bpf_tracing_check_multi(prog, tgt_prog, attach_btf,
> + tgt_info.tgt_type);
> + if (err)
> + goto out_unlock;
>
> tr = bpf_trampoline_get(key, &tgt_info);
> if (!tr) {
> @@ -3373,6 +3436,7 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
> */
> tr = prog->aux->dst_trampoline;
> tgt_prog = prog->aux->dst_prog;
> + mod = prog->aux->mod;
> }
>
> err = bpf_link_prime(&link->link.link, &link_primer);
> @@ -3388,6 +3452,8 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
>
> link->tgt_prog = tgt_prog;
> link->trampoline = tr;
> + link->attach_btf = attach_btf;
> + link->mod = mod;
>
> /* Always clear the trampoline and target prog from prog->aux to make
> * sure the original attach destination is not kept alive after a
> @@ -3400,20 +3466,27 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
> if (prog->aux->dst_trampoline && tr != prog->aux->dst_trampoline)
> /* we allocated a new trampoline, so free the old one */
> bpf_trampoline_put(prog->aux->dst_trampoline);
> + if (prog->aux->mod && mod != prog->aux->mod)
> + /* the mod in prog is not used anywhere, move it to link */
> + module_put(prog->aux->mod);
>
> prog->aux->dst_prog = NULL;
> prog->aux->dst_trampoline = NULL;
> + prog->aux->mod = NULL;
> mutex_unlock(&prog->aux->dst_mutex);
>
> return bpf_link_settle(&link_primer);
> out_unlock:
> if (tr && tr != prog->aux->dst_trampoline)
> bpf_trampoline_put(tr);
> + if (mod && mod != prog->aux->mod)
> + module_put(mod);
> mutex_unlock(&prog->aux->dst_mutex);
> kfree(link);
> out_put_prog:
> if (tgt_prog_fd && tgt_prog)
> bpf_prog_put(tgt_prog);
> + btf_put(attach_btf);
> return err;
> }
>
> --
> 2.39.2
>