On Wed, Jun 23, 2021 at 9:04 PM Shuyi Cheng <chengshuyi@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx/#t > [1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@xxxxxxxxxxxxxx/ > > Signed-off-by: Shuyi Cheng <chengshuyi@xxxxxxxxxxxxxxxxx> > --- > 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. 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. > + 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 >
