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

[Jiri Olsa]

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
>