Re: [PATCH bpf-next 4/6] libbpf: add kernel module BTF support for CO-RE relocations

On Thu, Nov 19, 2020 at 4:55 PM Maciej Fijalkowski
[off-list ref] wrote:

On Thu, Nov 19, 2020 at 03:22:42PM -0800, Andrii Nakryiko wrote:

quoted

Teach libbpf to search for candidate types for CO-RE relocations across kernel
modules BTFs, in addition to vmlinux BTF. If at least one candidate type is
found in vmlinux BTF, kernel module BTFs are not iterated. If vmlinux BTF has
no matching candidates, then find all kernel module BTFs and search for all
matching candidates across all of them.

Kernel's support for module BTFs are inferred from the support for BTF name
pointer in BPF UAPI.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 tools/lib/bpf/libbpf.c | 185 ++++++++++++++++++++++++++++++++++++++---
 1 file changed, 172 insertions(+), 13 deletions(-)

[...]

quoted

+static int probe_module_btf(void)
+{
+     static const char strs[] = "\0int";
+     __u32 types[] = {
+             /* int */
+             BTF_TYPE_INT_ENC(1, BTF_INT_SIGNED, 0, 32, 4),
+     };
+     struct bpf_btf_info info;
+     __u32 len = sizeof(info);
+     char name[16];
+     int fd, err;
+
+     fd = libbpf__load_raw_btf((char *)types, sizeof(types), strs, sizeof(strs));
+     if (fd < 0)
+             return 0; /* BTF not supported at all */
+
+     len = sizeof(info);

nit: reinit of len

oops, right, I'll remove it

quoted

+     memset(&info, 0, sizeof(info));

use len in memset

why?

quoted

+     info.name = ptr_to_u64(name);
+     info.name_len = sizeof(name);
+
+     /* check that BPF_OBJ_GET_INFO_BY_FD supports specifying name pointer;
+      * kernel's module BTF support coincides with support for
+      * name/name_len fields in struct bpf_btf_info.
+      */
+     err = bpf_obj_get_info_by_fd(fd, &info, &len);
+     close(fd);
+     return !err;
+}

[...]