libbpf: Fix relocation of kfunc ksym in ld_imm64 insn.

void *p = kfunc; -> generates ld_imm64 insn.
kfunc() -> generates bpf_call insn.

libbpf patches bpf_call insn correctly while only btf_id part of ld_imm64 is
set in the former case. Which means that pointers to kfuncs in modules are not
patched correctly and the verifier rejects load of such programs due to btf_id
being out of range. Fix libbpf to patch ld_imm64 for kfunc.

Signed-off-by: Alexei Starovoitov <[email protected]>
Signed-off-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

Author: Alexei Starovoitov

tools/lib/bpf/libbpf.c

@@ -7533,6 +7533,12 @@ static int bpf_object__resolve_ksym_func_btf_id(struct bpf_object *obj,
 	ext->is_set = true;
 	ext->ksym.kernel_btf_id = kfunc_id;
 	ext->ksym.btf_fd_idx = mod_btf ? mod_btf->fd_array_idx : 0;
+	/* Also set kernel_btf_obj_fd to make sure that bpf_object__relocate_data()
+	 * populates FD into ld_imm64 insn when it's used to point to kfunc.
+	 * {kernel_btf_id, btf_fd_idx} -> fixup bpf_call.
+	 * {kernel_btf_id, kernel_btf_obj_fd} -> fixup ld_imm64.
+	 */
+	ext->ksym.kernel_btf_obj_fd = mod_btf ? mod_btf->fd : 0;
 	pr_debug("extern (func ksym) '%s': resolved to kernel [%d]\n",
 		 ext->name, kfunc_id);