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

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On 6/24/21 11:06 PM, Daniel Borkmann wrote:
On 6/24/21 6:03 AM, Shuyi Cheng 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;
+

nit: This should rather go to the 'Parse and load BTF vmlinux if any of [...]' section of struct bpf_object, and for consistency, I'd keep the btf_ prefix,
like: char *btf_custom_path


Thank you very much for your reply.

Agree.


      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);
+    }

Couldn't we do something like (only compile-tested):


Your approach is very inspiring to me. But I did it for two reasons.

1. When the developer specifies btf_custom_path, btf should only be loaded from btf_custom_path; 2. Now pahole supports saving vmlinux's btf in raw format, so the old kernel can provide btf in elf format or raw format. see [0].

[0] https://git.kernel.org/pub/scm/devel/pahole/pahole.git/tree/pahole.c#n1157

What do you think?
	
Regards,
Shuyi

diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
index b46760b93bb4..5b88ce3e483c 100644
--- a/tools/lib/bpf/btf.c
+++ b/tools/lib/bpf/btf.c
@@ -4394,7 +4394,7 @@ static int btf_dedup_remap_types(struct btf_dedup *d)
  * Probe few well-known locations for vmlinux kernel image and try to load BTF
   * data out of it to use for target BTF.
   */
-struct btf *libbpf_find_kernel_btf(void)
+static struct btf *__libbpf_find_kernel_btf(char *btf_custom_path)
  {
      struct {
          const char *path_fmt;
@@ -4402,6 +4402,8 @@ struct btf *libbpf_find_kernel_btf(void)
      } locations[] = {
          /* try canonical vmlinux BTF through sysfs first */
          { "/sys/kernel/btf/vmlinux", true /* raw BTF */ },
+        /* try user defined vmlinux ELF if a path was specified */
+        { btf_custom_path },
          /* fall back to trying to find vmlinux ELF on disk otherwise */
          { "/boot/vmlinux-%1$s" },
          { "/lib/modules/%1$s/vmlinux-%1$s" },
@@ -4419,11 +4421,11 @@ struct btf *libbpf_find_kernel_btf(void)
      uname(&buf);

      for (i = 0; i < ARRAY_SIZE(locations); i++) {
+        if (!locations[i].path_fmt)
+            continue;
          snprintf(path, PATH_MAX, locations[i].path_fmt, buf.release);
-
          if (access(path, R_OK))
              continue;
-
          if (locations[i].raw_btf)
              btf = btf__parse_raw(path);
          else
@@ -4440,6 +4442,11 @@ struct btf *libbpf_find_kernel_btf(void)
      return libbpf_err_ptr(-ESRCH);
  }

+struct btf *libbpf_find_kernel_btf(void)
+{
+    return __libbpf_find_kernel_btf(NULL);
+}
+
 int btf_type_visit_type_ids(struct btf_type *t, type_id_visit_fn visit, void *ctx)
  {
      int i, n, err;

And then you just call it as:

     obj->btf_vmlinux = __libbpf_find_kernel_btf(obj->btf_custom_path);
     err = libbpf_get_error(obj->btf_vmlinux);

      if (err) {
          pr_warn("Error loading vmlinux BTF: %d\n", err);
          obj->btf_vmlinux = NULL;




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