[DTrace-devel] [PATCH 2/2] bpf: fix have_attach_type() detection

[Kris Van Hees]

There are kernel versions that support the BPF_TRACE_FENTRY attach type
at program load, but do not support opening the attachment point (a
kernel symbol by BTF id) as a raw tracepoint.  The cause is that the
support for fentry as a raw tracepoint was initially only implemented
on x86_64.

We now test both program load *and* opening the raw tracepoint to know
if BPF_TRACE_FENTRY as attach type is supported.

Signed-off-by: Kris Van Hees <kris.van.hees at oracle.com>
---
 libdtrace/dt_bpf.c      | 29 ++++++++++++++++++++++-------
 libdtrace/dt_prov_fbt.c |  2 ++
 2 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/libdtrace/dt_bpf.c b/libdtrace/dt_bpf.c
index e50bb536..9ee32e8b 100644
--- a/libdtrace/dt_bpf.c
+++ b/libdtrace/dt_bpf.c
@@ -486,19 +486,34 @@ have_attach_type(enum bpf_prog_type ptype, enum bpf_attach_type atype,
 				BPF_RETURN()
 			};
 	dtrace_difo_t	dp;
-	int		fd;
+	int		pfd, tfd = -1;
 
 	dp.dtdo_buf = insns;
 	dp.dtdo_len = ARRAY_SIZE(insns);
 
-	fd = dt_bpf_prog_attach(ptype, atype, 0, btf_id, &dp, 0, NULL, 0);
-	/* If the program loads, we can use the attach type. */
-	if (fd > 0) {
-		close(fd);
-		return 1;
-	}
+	pfd = dt_bpf_prog_attach(ptype, atype, 0, btf_id, &dp, 0, NULL, 0);
+	/* If the program load fails, we cannot iuse the attach type. */
+	if (pfd < 0)
+		goto fail;
 
+	/*
+	 * If the program loads, we still need to verify that probe can be
+	 * opened as a raw tracepoint.  Some kernels allow the program load
+	 * but return -ENOTSUPP when you try to open the raw tracepoint.
+	 */
+	tfd = dt_bpf_raw_tracepoint_open(NULL, pfd);
+	if (tfd < 0)
+		goto fail;
+
+	close(tfd);
+	close(pfd);
+	return 1;
+
+fail:
 	/* Failed -> attach type not available to us */
+	if (pfd >= 0)
+		close(pfd);
+
 	return 0;
 }
 
diff --git a/libdtrace/dt_prov_fbt.c b/libdtrace/dt_prov_fbt.c
index 1489275a..00f9174c 100644
--- a/libdtrace/dt_prov_fbt.c
+++ b/libdtrace/dt_prov_fbt.c
@@ -74,6 +74,8 @@ dt_provimpl_t			dt_rawfbt;
 static int populate(dtrace_hdl_t *dtp)
 {
 	dt_fbt = BPF_HAS(dtp, BPF_FEAT_FENTRY) ? dt_fbt_fprobe : dt_fbt_kprobe;
+	dt_dprintf("fbt: Using %s implementation\n",
+		   BPF_HAS(dtp, BPF_FEAT_FENTRY) ? "fentry/fexit" : "kprobe");
 
 	if (dt_provider_create(dtp, dt_fbt.name, &dt_fbt, &pattr,
 			       NULL) == NULL ||
-- 
2.43.5