netdev - Re: [PATCH v2 bpf-next 6/7] libbpf: support kernel module ksym externs

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Date:   Sun, 10 Jan 2021 20:15:12 -0800
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andrii@...nel.org>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>, <ast@...com>, <daniel@...earbox.net>
CC:     <kernel-team@...com>, Hao Luo <haoluo@...gle.com>
Subject: Re: [PATCH v2 bpf-next 6/7] libbpf: support kernel module ksym
 externs



On 1/8/21 2:09 PM, Andrii Nakryiko wrote:
> Add support for searching for ksym externs not just in vmlinux BTF, but across
> all module BTFs, similarly to how it's done for CO-RE relocations. Kernels
> that expose module BTFs through sysfs are assumed to support new ldimm64
> instruction extension with BTF FD provided in insn[1].imm field, so no extra
> feature detection is performed.
> 
> Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> ---
>   tools/lib/bpf/libbpf.c | 47 +++++++++++++++++++++++++++---------------
>   1 file changed, 30 insertions(+), 17 deletions(-)
> 
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 6ae748f6ea11..57559a71e4de 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -395,7 +395,8 @@ struct extern_desc {
>   			unsigned long long addr;
>   
>   			/* target btf_id of the corresponding kernel var. */
> -			int vmlinux_btf_id;
> +			int kernel_btf_obj_fd;
> +			int kernel_btf_id;
>   
>   			/* local btf_id of the ksym extern's type. */
>   			__u32 type_id;
> @@ -6162,7 +6163,8 @@ bpf_object__relocate_data(struct bpf_object *obj, struct bpf_program *prog)
>   			} else /* EXT_KSYM */ {
>   				if (ext->ksym.type_id) { /* typed ksyms */
>   					insn[0].src_reg = BPF_PSEUDO_BTF_ID;
> -					insn[0].imm = ext->ksym.vmlinux_btf_id;
> +					insn[0].imm = ext->ksym.kernel_btf_id;
> +					insn[1].imm = ext->ksym.kernel_btf_obj_fd;
>   				} else { /* typeless ksyms */
>   					insn[0].imm = (__u32)ext->ksym.addr;
>   					insn[1].imm = ext->ksym.addr >> 32;
> @@ -7319,7 +7321,8 @@ static int bpf_object__read_kallsyms_file(struct bpf_object *obj)
>   static int bpf_object__resolve_ksyms_btf_id(struct bpf_object *obj)
>   {
>   	struct extern_desc *ext;
> -	int i, id;
> +	struct btf *btf;
> +	int i, j, id, btf_fd, err;
>   
>   	for (i = 0; i < obj->nr_extern; i++) {
>   		const struct btf_type *targ_var, *targ_type;
> @@ -7331,8 +7334,22 @@ static int bpf_object__resolve_ksyms_btf_id(struct bpf_object *obj)
>   		if (ext->type != EXT_KSYM || !ext->ksym.type_id)
>   			continue;
>   
> -		id = btf__find_by_name_kind(obj->btf_vmlinux, ext->name,
> -					    BTF_KIND_VAR);
> +		btf = obj->btf_vmlinux;
> +		btf_fd = 0;
> +		id = btf__find_by_name_kind(btf, ext->name, BTF_KIND_VAR);
> +		if (id == -ENOENT) {
> +			err = load_module_btfs(obj);
> +			if (err)
> +				return err;
> +
> +			for (j = 0; j < obj->btf_module_cnt; j++) {
> +				btf = obj->btf_modules[j].btf;
> +				btf_fd = obj->btf_modules[j].fd;

Do we have possibility btf_fd == 0 here?

> +				id = btf__find_by_name_kind(btf, ext->name, BTF_KIND_VAR);
> +				if (id != -ENOENT)
> +					break;
> +			}
> +		}
>   		if (id <= 0) {
>   			pr_warn("extern (ksym) '%s': failed to find BTF ID in vmlinux BTF.\n",
>   				ext->name);
> @@ -7343,24 +7360,19 @@ static int bpf_object__resolve_ksyms_btf_id(struct bpf_object *obj)
>   		local_type_id = ext->ksym.type_id;
>   
>   		/* find target type_id */
> -		targ_var = btf__type_by_id(obj->btf_vmlinux, id);
> -		targ_var_name = btf__name_by_offset(obj->btf_vmlinux,
> -						    targ_var->name_off);
> -		targ_type = skip_mods_and_typedefs(obj->btf_vmlinux,
> -						   targ_var->type,
> -						   &targ_type_id);
> +		targ_var = btf__type_by_id(btf, id);
> +		targ_var_name = btf__name_by_offset(btf, targ_var->name_off);
> +		targ_type = skip_mods_and_typedefs(btf, targ_var->type, &targ_type_id);
>   
>   		ret = bpf_core_types_are_compat(obj->btf, local_type_id,
> -						obj->btf_vmlinux, targ_type_id);
> +						btf, targ_type_id);
>   		if (ret <= 0) {
>   			const struct btf_type *local_type;
>   			const char *targ_name, *local_name;
>   
>   			local_type = btf__type_by_id(obj->btf, local_type_id);
> -			local_name = btf__name_by_offset(obj->btf,
> -							 local_type->name_off);
> -			targ_name = btf__name_by_offset(obj->btf_vmlinux,
> -							targ_type->name_off);
> +			local_name = btf__name_by_offset(obj->btf, local_type->name_off);
> +			targ_name = btf__name_by_offset(btf, targ_type->name_off);
>   
>   			pr_warn("extern (ksym) '%s': incompatible types, expected [%d] %s %s, but kernel has [%d] %s %s\n",
>   				ext->name, local_type_id,
> @@ -7370,7 +7382,8 @@ static int bpf_object__resolve_ksyms_btf_id(struct bpf_object *obj)
>   		}
>   
>   		ext->is_set = true;
> -		ext->ksym.vmlinux_btf_id = id;
> +		ext->ksym.kernel_btf_obj_fd = btf_fd;
> +		ext->ksym.kernel_btf_id = id;
>   		pr_debug("extern (ksym) '%s': resolved to [%d] %s %s\n",
>   			 ext->name, id, btf_kind_str(targ_var), targ_var_name);
>   	}
>