[PATCH bpf-next v2 1/2] libbpf: Probe for bounded loop support

[Paul Chaignon <paul@xxxxxxxxxxxxx>]

This patch introduces a new probe to check whether the verifier supports
bounded loops as introduced in commit 2589726d12a1 ("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.

Reviewed-by: Quentin Monnet <quentin@xxxxxxxxxxxxx>
Signed-off-by: Paul Chaignon <paul@xxxxxxxxxxxxx>
---
Changes in v2:
  - Resending to fix a format error. I had the good idea of saving the
    patch in Gmail as a draft before sending with mutt, which caused
    Gmail to remove the trailing space in the Git version signature.
    That then caused pathwork to generate an incorrect mbox file.

 tools/lib/bpf/libbpf.h        |  1 +
 tools/lib/bpf/libbpf.map      |  1 +
 tools/lib/bpf/libbpf_probes.c | 20 ++++++++++++++++++++
 3 files changed, 22 insertions(+)

diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
index 42b2f36fd9f0..3621aaaff67c 100644
--- a/tools/lib/bpf/libbpf.h
+++ b/tools/lib/bpf/libbpf.h
@@ -1058,6 +1058,7 @@ LIBBPF_API bool bpf_probe_map_type(enum bpf_map_type map_type, __u32 ifindex);
 LIBBPF_API bool bpf_probe_helper(enum bpf_func_id id,
 				 enum bpf_prog_type prog_type, __u32 ifindex);
 LIBBPF_API bool bpf_probe_large_insn_limit(__u32 ifindex);
+LIBBPF_API bool bpf_probe_bounded_loops(__u32 ifindex);
 
 /*
  * Get bpf_prog_info in continuous memory
diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
index b3938b3f8fc9..059d168452d7 100644
--- a/tools/lib/bpf/libbpf.map
+++ b/tools/lib/bpf/libbpf.map
@@ -423,6 +423,7 @@ LIBBPF_0.6.0 {
 LIBBPF_0.7.0 {
 	global:
 		bpf_btf_load;
+		bpf_probe_bounded_loops;
 		bpf_program__log_buf;
 		bpf_program__log_level;
 		bpf_program__set_log_buf;
diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
index 4bdec69523a7..e5bd691059e4 100644
--- a/tools/lib/bpf/libbpf_probes.c
+++ b/tools/lib/bpf/libbpf_probes.c
@@ -356,3 +356,23 @@ bool bpf_probe_large_insn_limit(__u32 ifindex)
 
 	return errno != E2BIG && errno != EINVAL;
 }
+
+/*
+ * Probe for bounded loop support introduced in commit 2589726d12a1
+ * ("bpf: introduce bounded loops").
+ */
+bool bpf_probe_bounded_loops(__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()
+	};
+
+	errno = 0;
+	probe_load(BPF_PROG_TYPE_SOCKET_FILTER, insns, ARRAY_SIZE(insns), NULL,
+		   0, ifindex);
+
+	return !errno;
+}
-- 
2.25.1