bpftool: Probe for bounded loop support

This patch introduces a new probe to check whether the verifier supports
bounded loops as introduced in commit 2589726d ("bpf: introduce
bounded loops"). This patch will allow BPF users such as Cilium to probe
for loop support on startup and only unconditionally unroll loops on
older kernels.

The results are displayed as part of the miscellaneous section, as shown
below.

  $ bpftool feature probe | grep loops
  Bounded loop support is available
  $ bpftool feature probe macro | grep LOOPS
  #define HAVE_BOUNDED_LOOPS
  $ bpftool feature probe -j | jq .misc
  {
    "have_large_insn_limit": true,
    "have_bounded_loops": true
  }
Signed-off-by: Paul Chaignon <paul@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/f7807c0b27d79f48e71de7b5a99c680ca4bd0151.1641314075.git.paul@isovalent.com

tools/bpf/bpftool/feature.c

@@ -687,6 +687,27 @@ static void probe_large_insn_limit(const char *define_prefix, __u32 ifindex)
 			   "LARGE_INSN_LIMIT");
 }
 
+/*
+ * Probe for bounded loop support introduced in commit 2589726d12a1
+ * ("bpf: introduce bounded loops").
+ */
+static void
+probe_bounded_loops(const char *define_prefix, __u32 ifindex)
+{
+	struct bpf_insn insns[4] = {
+		BPF_MOV64_IMM(BPF_REG_0, 10),
+		BPF_ALU64_IMM(BPF_SUB, BPF_REG_0, 1),
+		BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, -2),
+		BPF_EXIT_INSN()
+	};
+
+	probe_misc_feature(insns, ARRAY_SIZE(insns),
+			   define_prefix, ifindex,
+			   "have_bounded_loops",
+			   "Bounded loop support",
+			   "BOUNDED_LOOPS");
+}
+
 static void
 section_system_config(enum probe_component target, const char *define_prefix)
 {
@@ -801,6 +822,7 @@ static void section_misc(const char *define_prefix, __u32 ifindex)
 			    "/*** eBPF misc features ***/",
 			    define_prefix);
 	probe_large_insn_limit(define_prefix, ifindex);
+	probe_bounded_loops(define_prefix, ifindex);
 	print_end_section();
 }