netdev - Re: [PATCH bpf-next 4/6] libbpf: add kernel module BTF support for CO-RE relocations

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Message-ID: <20201120230549.37k4zsjsrxbyjin3@kafai-mbp.dhcp.thefacebook.com>
Date:   Fri, 20 Nov 2020 15:05:49 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Andrii Nakryiko <andrii@...nel.org>
CC:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>, <ast@...com>,
        <daniel@...earbox.net>, <kernel-team@...com>
Subject: Re: [PATCH bpf-next 4/6] libbpf: add kernel module BTF support for
 CO-RE relocations

On Thu, Nov 19, 2020 at 03:22:42PM -0800, Andrii Nakryiko wrote:
[ ... ]

> +static int load_module_btfs(struct bpf_object *obj)
> +{
> +	struct bpf_btf_info info;
> +	struct module_btf *mod_btf;
> +	struct btf *btf;
> +	char name[64];
> +	__u32 id, len;
> +	int err, fd;
> +
> +	if (obj->btf_modules_loaded)
> +		return 0;
> +
> +	/* don't do this again, even if we find no module BTFs */
> +	obj->btf_modules_loaded = true;
> +
> +	/* kernel too old to support module BTFs */
> +	if (!kernel_supports(FEAT_MODULE_BTF))
> +		return 0;
> +
> +	while (true) {
> +		err = bpf_btf_get_next_id(id, &id);
> +		if (err && errno == ENOENT)
> +			return 0;
> +		if (err) {
> +			err = -errno;
> +			pr_warn("failed to iterate BTF objects: %d\n", err);
> +			return err;
> +		}
> +
> +		fd = bpf_btf_get_fd_by_id(id);
> +		if (fd < 0) {
> +			if (errno == ENOENT)
> +				continue; /* expected race: BTF was unloaded */
> +			err = -errno;
> +			pr_warn("failed to get BTF object #%d FD: %d\n", id, err);
> +			return err;
> +		}
> +
> +		len = sizeof(info);
> +		memset(&info, 0, sizeof(info));
> +		info.name = ptr_to_u64(name);
> +		info.name_len = sizeof(name);
> +
> +		err = bpf_obj_get_info_by_fd(fd, &info, &len);
> +		if (err) {
> +			err = -errno;
> +			pr_warn("failed to get BTF object #%d info: %d\n", id, err);

			close(fd);

> +			return err;
> +		}
> +
> +		/* ignore non-module BTFs */
> +		if (!info.kernel_btf || strcmp(name, "vmlinux") == 0) {
> +			close(fd);
> +			continue;
> +		}
> +

[ ... ]

> @@ -8656,9 +8815,6 @@ static inline int __find_vmlinux_btf_id(struct btf *btf, const char *name,
>  	else
>  		err = btf__find_by_name_kind(btf, name, BTF_KIND_FUNC);
>  
> -	if (err <= 0)
> -		pr_warn("%s is not found in vmlinux BTF\n", name);
> -
>  	return err;
>  }
>  
> @@ -8675,6 +8831,9 @@ int libbpf_find_vmlinux_btf_id(const char *name,
>  	}
>  
>  	err = __find_vmlinux_btf_id(btf, name, attach_type);
> +	if (err <= 0)
> +		pr_warn("%s is not found in vmlinux BTF\n", name);
> +
Please explain this move in the commit message.

>  	btf__free(btf);
>  	return err;
>  }
> -- 
> 2.24.1
>