Re: [PATCH bpf-next] libbpf: Introduce 'custom_btf_path' to 'bpf_obj_open_opts'.

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 





On 7/8/21 4:52 AM, Andrii Nakryiko wrote:
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.


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




[Index of Archives]     [Kernel Development]     [Kernel Announce]     [Kernel Newbies]     [Linux Networking Development]     [Share Photos]     [IDE]     [Security]     [Git]     [Netfilter]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Device Mapper]

  Powered by Linux