linux-kernel - [PATCH bpf v2] libbpf: Return btf_fd in libbpf__probe_raw

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Date:   Wed, 29 May 2019 15:20:00 +0200
From:   Michal Rostecki <mrostecki@...nsuse.org>
To:     unlisted-recipients:; (no To-header on input)
Cc:     Michal Rostecki <mrostecki@...nsuse.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        netdev@...r.kernel.org (open list:BPF (Safe dynamic programs and tools)),
        bpf@...r.kernel.org (open list:BPF (Safe dynamic programs and tools)),
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH bpf v2] libbpf: Return btf_fd in libbpf__probe_raw_btf

Function load_sk_storage_btf expects that libbpf__probe_raw_btf is
returning a BTF descriptor, but before this change it was returning
an information about whether the probe was successful (0 or 1).
load_sk_storage_btf was using that value as an argument of the close
function, which was resulting in closing stdout and thus terminating the
process which called that function.

That bug was visible in bpftool. `bpftool feature` subcommand was always
exiting too early (because of closed stdout) and it didn't display all
requested probes. `bpftool -j feature` or `bpftool -p feature` were not
returning a valid json object.

v2:
- Fix typo in the commit message.

Fixes: d7c4b3980c18 ("libbpf: detect supported kernel BTF features and sanitize BTF")
Signed-off-by: Michal Rostecki <mrostecki@...nsuse.org>
---
 tools/lib/bpf/libbpf.c        | 36 +++++++++++++++++++++--------------
 tools/lib/bpf/libbpf_probes.c |  7 +------
 2 files changed, 23 insertions(+), 20 deletions(-)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 197b574406b3..bc2dca36bced 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -1645,15 +1645,19 @@ static int bpf_object__probe_btf_func(struct bpf_object *obj)
 		/* FUNC x */                                    /* [3] */
 		BTF_TYPE_ENC(5, BTF_INFO_ENC(BTF_KIND_FUNC, 0, 0), 2),
 	};
-	int res;
+	int btf_fd;
+	int ret;
 
-	res = libbpf__probe_raw_btf((char *)types, sizeof(types),
-				    strs, sizeof(strs));
-	if (res < 0)
-		return res;
-	if (res > 0)
+	btf_fd = libbpf__probe_raw_btf((char *)types, sizeof(types),
+				       strs, sizeof(strs));
+	if (btf_fd < 0)
+		ret = 0;
+	else {
+		ret = 1;
 		obj->caps.btf_func = 1;
-	return 0;
+	}
+	close(btf_fd);
+	return ret;
 }
 
 static int bpf_object__probe_btf_datasec(struct bpf_object *obj)
@@ -1670,15 +1674,19 @@ static int bpf_object__probe_btf_datasec(struct bpf_object *obj)
 		BTF_TYPE_ENC(3, BTF_INFO_ENC(BTF_KIND_DATASEC, 0, 1), 4),
 		BTF_VAR_SECINFO_ENC(2, 0, 4),
 	};
-	int res;
+	int btf_fd;
+	int ret;
 
-	res = libbpf__probe_raw_btf((char *)types, sizeof(types),
-				    strs, sizeof(strs));
-	if (res < 0)
-		return res;
-	if (res > 0)
+	btf_fd = libbpf__probe_raw_btf((char *)types, sizeof(types),
+				       strs, sizeof(strs));
+	if (btf_fd < 0)
+		ret = 0;
+	else {
+		ret = 1;
 		obj->caps.btf_datasec = 1;
-	return 0;
+	}
+	close(btf_fd);
+	return ret;
 }
 
 static int
diff --git a/tools/lib/bpf/libbpf_probes.c b/tools/lib/bpf/libbpf_probes.c
index 5e2aa83f637a..2c2828345514 100644
--- a/tools/lib/bpf/libbpf_probes.c
+++ b/tools/lib/bpf/libbpf_probes.c
@@ -157,14 +157,9 @@ int libbpf__probe_raw_btf(const char *raw_types, size_t types_len,
 	memcpy(raw_btf + hdr.hdr_len + hdr.type_len, str_sec, hdr.str_len);
 
 	btf_fd = bpf_load_btf(raw_btf, btf_len, NULL, 0, false);
-	if (btf_fd < 0) {
-		free(raw_btf);
-		return 0;
-	}
 
-	close(btf_fd);
 	free(raw_btf);
-	return 1;
+	return btf_fd;
 }
 
 static int load_sk_storage_btf(void)
-- 
2.21.0