bpftool: add kernel.kptr_restrict hint for no instructions

from bpftool github repo issue [0], when Linux distribution
kernel.kptr_restrict is set to 2, bpftool prog dump jited returns "no
instructions returned", this message can be puzzling to bpftool users
who is not familiar with kernel BPF internal, so add small hint for
bpftool users to check kernel.kptr_restrict setting. Set
kernel.kptr_restrict to expose kernel address to allow bpftool prog
dump jited to dump the jited bpf program instructions.

[0]: libbpf/bpftool#184

Signed-off-by: Vincent Li <[email protected].

Author: vincentmli

tools/bpf/bpftool/prog.c

@@ -714,7 +714,7 @@ prog_dump(struct bpf_prog_info *info, enum dump_mode mode,
 
 	if (mode == DUMP_JITED) {
 		if (info->jited_prog_len == 0 || !info->jited_prog_insns) {
-			p_info("no instructions returned");
+			p_info("no instructions returned: set kernel.kptr_restrict to expose kernel addresses");
 			return -1;
 		}
 		buf = u64_to_ptr(info->jited_prog_insns);