bpftool: Add kernel.kptr_restrict hint for no instructions

From bpftool's github repository issue [0]: When a Linux distribution
has the kernel.kptr_restrict set to 2, bpftool prog dump jited returns
"no instructions returned". This message can be puzzling to bpftool
users who are not familiar with kernel BPF internals, so add a small
hint for bpftool users to check the kernel.kptr_restrict setting
similar to the DUMP_XLATED case. Outside of kernel.kptr_restrict, no
instructions could also be returned in case the JIT was disabled.

Signed-off-by: Vincent Li <vincent.mc.li@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: #184 [0]
Link: https://lore.kernel.org/bpf/20250818165113.15982-1-vincent.mc.li@gmail.com

Author: vincentmli

src/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_err("error retrieving jit dump: no instructions returned or kernel.kptr_restrict set?");
 			return -1;
 		}
 		buf = u64_to_ptr(info->jited_prog_insns);