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Date: Thu, 8 Jul 2021 21:11:56 +0800
From: Shuyi Cheng <chengshuyi@...ux.alibaba.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
john fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>,
Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
open list <linux-kernel@...r.kernel.org>,
kernel-janitors@...r.kernel.org
Subject: Re: [PATCH bpf-next] libbpf: Introduce 'custom_btf_path' to
'bpf_obj_open_opts'.
On 7/8/21 4:52 AM, Andrii Nakryiko wrote:
> On Wed, Jun 23, 2021 at 9:04 PM Shuyi Cheng
> <chengshuyi@...ux.alibaba.com> wrote:
>>
>> In order to enable the older kernel to use the CO-RE feature, load the
>> vmlinux btf of the specified path.
>>
>> Learn from Andrii's comments in [0], add the custom_btf_path parameter
>> to bpf_obj_open_opts, you can directly use the skeleton's
>> <objname>_bpf__open_opts function to pass in the custom_btf_path
>> parameter.
>>
>> Prior to this, there was also a developer who provided a patch with
>> similar functions. It is a pity that the follow-up did not continue to
>> advance. See [1].
>>
>> [0]https://lore.kernel.org/bpf/CAEf4BzbJZLjNoiK8_VfeVg_Vrg=9iYFv+po-38SMe=UzwDKJ=Q@mail.gmail.com/#t
>> [1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@mail.gmail.com/
>>
>> Signed-off-by: Shuyi Cheng <chengshuyi@...ux.alibaba.com>
>> ---
>> tools/lib/bpf/libbpf.c | 23 ++++++++++++++++++++---
>> tools/lib/bpf/libbpf.h | 6 +++++-
>> 2 files changed, 25 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 1e04ce7..518b19f 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -509,6 +509,8 @@ struct bpf_object {
>> void *priv;
>> bpf_object_clear_priv_t clear_priv;
>>
>> + char *custom_btf_path;
>> +
>> char path[];
>> };
>> #define obj_elf_valid(o) ((o)->efile.elf)
>> @@ -2679,8 +2681,15 @@ static int bpf_object__load_vmlinux_btf(struct bpf_object *obj, bool force)
>> if (!force && !obj_needs_vmlinux_btf(obj))
>> return 0;
>>
>> - obj->btf_vmlinux = libbpf_find_kernel_btf();
>> - err = libbpf_get_error(obj->btf_vmlinux);
>> + if (obj->custom_btf_path) {
>> + obj->btf_vmlinux = btf__parse(obj->custom_btf_path, NULL);
>> + err = libbpf_get_error(obj->btf_vmlinux);
>> + pr_debug("loading custom vmlinux BTF '%s': %d\n", obj->custom_btf_path, err);
>> + } else {
>> + obj->btf_vmlinux = libbpf_find_kernel_btf();
>> + err = libbpf_get_error(obj->btf_vmlinux);
>> + }
>
>
> I think it will be more flexible to treat custom_btf as an vmlinux BTF
> override, just like [1] did. I can see how in some situations users
> might want to treat this custom BTF as either a replacement of vmlinux
> BTF or as an augmentation of vmlinux BTF for the purpose of extra
> custom CO-RE relocations (e.g., something along the XDP hints that
> were discussed recently). For now it's probably enough to implement
> "custom BTF is a replacement for vmlinux BTF" policy and, if
> necessary, add "custom BTF is in addition to vmlinux BTF" later with
> extra opts flag/field.
>
Thank you very much for your reply.
So this part of the code remains the same, right?
> Keep in mind that this custom BTF is only useful for BPF CO-RE
> relocation. Any other kernel feature relying on vmlinux BTF (e.g.,
> fentry) won't work with custom BTF because it expects correct BTF type
> IDs.
>
>
>> +
>> if (err) {
>> pr_warn("Error loading vmlinux BTF: %d\n", err);
>> obj->btf_vmlinux = NULL;
>> @@ -7554,7 +7563,7 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
>> __bpf_object__open(const char *path, const void *obj_buf, size_t obj_buf_sz,
>> const struct bpf_object_open_opts *opts)
>> {
>> - const char *obj_name, *kconfig;
>> + const char *obj_name, *kconfig, *tmp_btf_path;
>> struct bpf_program *prog;
>> struct bpf_object *obj;
>> char tmp_name[64];
>> @@ -7584,6 +7593,13 @@ int bpf_program__load(struct bpf_program *prog, char *license, __u32 kern_ver)
>> obj = bpf_object__new(path, obj_buf, obj_buf_sz, obj_name);
>> if (IS_ERR(obj))
>> return obj;
>> +
>> + tmp_btf_path = OPTS_GET(opts, custom_btf_path, NULL);
>> + if (tmp_btf_path && strlen(tmp_btf_path) < PATH_MAX) {
>
> if strlen() is >= PATH_MAX you'll just silently ignore it? We should
> either truncate silently (because PATH_MAX is totally reasonable
> assumption) or error out.
Agree. I will send the second version of the patch.
Regards,
Shuyi
>
>> + obj->custom_btf_path = strdup(tmp_btf_path);
>> + if (!obj->custom_btf_path)
>> + return ERR_PTR(-ENOMEM);
>> + }
>>
>> kconfig = OPTS_GET(opts, kconfig, NULL);
>> if (kconfig) {
>> @@ -8702,6 +8718,7 @@ void bpf_object__close(struct bpf_object *obj)
>> for (i = 0; i < obj->nr_maps; i++)
>> bpf_map__destroy(&obj->maps[i]);
>>
>> + zfree(&obj->custom_btf_path);
>> zfree(&obj->kconfig);
>> zfree(&obj->externs);
>> obj->nr_extern = 0;
>> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
>> index 6e61342..16e0f01 100644
>> --- a/tools/lib/bpf/libbpf.h
>> +++ b/tools/lib/bpf/libbpf.h
>> @@ -94,8 +94,12 @@ struct bpf_object_open_opts {
>> * system Kconfig for CONFIG_xxx externs.
>> */
>> const char *kconfig;
>> + /* Specify the path of vmlinux btf to facilitate the use of CO-RE features
>> + * in the old kernel.
>> + */
>> + char *custom_btf_path;
>> };
>> -#define bpf_object_open_opts__last_field kconfig
>> +#define bpf_object_open_opts__last_field custom_btf_path
>>
>> LIBBPF_API struct bpf_object *bpf_object__open(const char *path);
>> LIBBPF_API struct bpf_object *
>> --
>> 1.8.3.1
>>
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