bpf: pass attached BTF to the bpf_struct_ops subsystem

Pass the fd of a btf from the userspace to the bpf() syscall, and then
convert the fd into a btf. The btf is generated from the module that
defines the target BPF struct_ops type.

In order to inform the kernel about the module that defines the target
struct_ops type, the userspace program needs to provide a btf fd for the
respective module's btf. This btf contains essential information on the
types defined within the module, including the target struct_ops type.

A btf fd must be provided to the kernel for struct_ops maps and for the bpf
programs attached to those maps.

In the case of the bpf programs, the attach_btf_obj_fd parameter is passed
as part of the bpf_attr and is converted into a btf. This btf is then
stored in the prog->aux->attach_btf field. Here, it just let the verifier
access attach_btf directly.

In the case of struct_ops maps, a btf fd is passed as value_type_btf_obj_fd
of bpf_attr. The bpf_struct_ops_map_alloc() function converts the fd to a
btf and stores it as st_map->btf. A flag BPF_F_VTYPE_BTF_OBJ_FD is added
for map_flags to indicate that the value of value_type_btf_obj_fd is set.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Link: https://lore.kernel.org/r/20240119225005.668602-9-thinker.li@gmail.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

1 files changed, 8 insertions, 0 deletions

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index a380047c86af..1fef6d5a1330 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -1330,6 +1330,9 @@ enum {
 
 /* Get path from provided FD in BPF_OBJ_PIN/BPF_OBJ_GET commands */
 	BPF_F_PATH_FD		= (1U << 14),
+
+/* Flag for value_type_btf_obj_fd, the fd is available */
+	BPF_F_VTYPE_BTF_OBJ_FD	= (1U << 15),
 };
 
 /* Flags for BPF_PROG_QUERY. */
@@ -1403,6 +1406,11 @@ union bpf_attr {
 		 * to using 5 hash functions).
 		 */
 		__u64	map_extra;
+
+		__s32   value_type_btf_obj_fd;	/* fd pointing to a BTF
+						 * type data for
+						 * btf_vmlinux_value_type_id.
+						 */
 	};
 
 	struct { /* anonymous struct used by BPF_MAP_*_ELEM commands */