Re: [PATCH bpf-next] libbpf: Introduce 'custom_btf_path' to 'bpf_obj_open_opts'.
From: Andrii Nakryiko <hidden>
Date: 2021-07-07 20:52:48
Also in:
bpf, kernel-janitors, lkml
On Wed, Jun 23, 2021 at 9:04 PM Shuyi Cheng [off-list ref] wrote:
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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 (local) [1]https://yhbt.net/lore/all/CAEf4Bzbgw49w2PtowsrzKQNcxD4fZRE6AKByX-5-dMo-+oWHHA@mail.gmail.com/ Signed-off-by: Shuyi Cheng <redacted> --- 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.
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+ 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.
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+ 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