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Date: Mon, 11 Jan 2021 22:45:25 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Yonghong Song <yhs@...com>
Cc: Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
Networking <netdev@...r.kernel.org>,
Alexei Starovoitov <ast@...com>,
Daniel Borkmann <daniel@...earbox.net>,
Kernel Team <kernel-team@...com>, Hao Luo <haoluo@...gle.com>
Subject: Re: [PATCH v2 bpf-next 6/7] libbpf: support kernel module ksym externs
On Mon, Jan 11, 2021 at 5:34 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 1/11/21 1:37 PM, Andrii Nakryiko wrote:
> > On Sun, Jan 10, 2021 at 8:15 PM Yonghong Song <yhs@...com> wrote:
> >>
> >>
> >>
> >> 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?
> >
> > Extremely unlikely. But if we are really worried about 0 fd, we should
> > handle that in a centralized fashion in libbpf. I.e., for any
> > operation that can return FD, check if that FD is 0, and if yes, dup()
> > it. And then make everything call that helper. So in the context of
> > this patch I'm just ignoring such possibility.
> Maybe at least add some comments here to document such a possibility?
sure, will add
>
> >
> >>
> >>> + 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);
> >>> }
> >>>
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