On Thu, Jul 8, 2021 at 6:07 AM 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 btf_custom_path parameter > to bpf_obj_open_opts, you can directly use the skeleton's > <objname>_bpf__open_opts function to pass in the btf_custom_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/ > > Cc: Andrii Nakryiko <andrii@xxxxxxxxxx> > Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx> > Signed-off-by: Shuyi Cheng <chengshuyi@xxxxxxxxxxxxxxxxx> > --- > v1: https://lore.kernel.org/bpf/CAEf4BzaGjEC4t1OefDo11pj2-HfNy0BLhs_G2UREjRNTmb2u=A@xxxxxxxxxxxxxx/t/#m4d9f7c6761fbd2b436b5dfe491cd864b70225804 > v1->v2: > -- Change custom_btf_path to btf_custom_path. > -- If the length of btf_custom_path of bpf_obj_open_opts is too long, > return ERR_PTR(-ENAMETOOLONG). > -- Add `custom BTF is in addition to vmlinux BTF` > with btf_custom_path field. > > tools/lib/bpf/libbpf.c | 27 ++++++++++++++++++++++++--- > tools/lib/bpf/libbpf.h | 6 +++++- > 2 files changed, 29 insertions(+), 4 deletions(-) > > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c > index 1e04ce7..aed156c 100644 > --- a/tools/lib/bpf/libbpf.c > +++ b/tools/lib/bpf/libbpf.c > @@ -494,6 +494,10 @@ struct bpf_object { > struct btf *btf; > struct btf_ext *btf_ext; > > + /* custom BTF is in addition to vmlinux BTF (i.e., Use the CO-RE > + * feature in the old kernel). > + */ > + char *btf_custom_path; > /* Parse and load BTF vmlinux if any of the programs in the object need > * it at load time. > */ > @@ -2679,8 +2683,15 @@ 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->btf_custom_path) { > + obj->btf_vmlinux = btf__parse(obj->btf_custom_path, NULL); I don't think this is right. Keep in mind the general case where vmlinux BTF might be available, but user wants to perform CO-RE using custom BTF (like we do in our selftests in core_reloc.c). In such cases all the other features that rely on vmlinux BTF would use real vmlinux BTF, but CO-RE relocations would use custom BTF. See the original patch you are referencing, it loaded btf_override separately from obj->btf_vmlinux. > + err = libbpf_get_error(obj->btf_vmlinux); > + pr_debug("loading custom vmlinux BTF '%s': %d\n", obj->btf_custom_path, err); > + } else { > + obj->btf_vmlinux = libbpf_find_kernel_btf(); > + err = libbpf_get_error(obj->btf_vmlinux); > + } > + > if (err) { > pr_warn("Error loading vmlinux BTF: %d\n", err); > obj->btf_vmlinux = NULL; > @@ -7554,7 +7565,7 @@ 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, *btf_tmp_path; > struct bpf_program *prog; > struct bpf_object *obj; > char tmp_name[64]; > @@ -7584,6 +7595,15 @@ 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; > + > + btf_tmp_path = OPTS_GET(opts, btf_custom_path, NULL); > + if (btf_tmp_path) { > + if (strlen(btf_tmp_path) >= PATH_MAX) > + return ERR_PTR(-ENAMETOOLONG); we are leaking obj here > + obj->btf_custom_path = strdup(btf_tmp_path); > + if (!obj->btf_custom_path) > + return ERR_PTR(-ENOMEM); and here > + } > > kconfig = OPTS_GET(opts, kconfig, NULL); > if (kconfig) { And a few lines below. You didn't introduce this bug, I just spotted it while reviewing your patch, but it would be nice to fix it as well. > @@ -8702,6 +8722,7 @@ void bpf_object__close(struct bpf_object *obj) > for (i = 0; i < obj->nr_maps; i++) > bpf_map__destroy(&obj->maps[i]); > > + zfree(&obj->btf_custom_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..5002d1f 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; > + /* custom BTF is in addition to vmlinux BTF (i.e., Use the CO-RE > + * feature in the old kernel). > + */ > + char *btf_custom_path; > }; > -#define bpf_object_open_opts__last_field kconfig > +#define bpf_object_open_opts__last_field btf_custom_path > > LIBBPF_API struct bpf_object *bpf_object__open(const char *path); > LIBBPF_API struct bpf_object * > -- > 1.8.3.1 >
