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libbpf: fix reuse of pinned map on older kernel

PATCH In review Martynas Pumputis 2 participants 2 messages Jul 12, 2021

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Martynas Pumputis

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#0 lore raw

PATCH v2 1/1 libbpf: fix reuse of pinned map on older kernel

When loading a BPF program with a pinned map, the loader checks whether the pinned map can be reused, i.e. their properties match. To derive such of the pinned map, the loader invokes BPF_OBJ_GET_INFO_BY_FD and then does the comparison.

Unfortunately, on < 4.12 kernels the BPF_OBJ_GET_INFO_BY_FD is not available, so loading the program fails with the following error:

libbpf: failed to get map info for map FD 5: Invalid argument
libbpf: couldn't reuse pinned map at
'/sys/fs/bpf/tc/globals/cilium_call_policy': parameter mismatch"
libbpf: map 'cilium_call_policy': error reusing pinned map
libbpf: map 'cilium_call_policy': failed to create:
Invalid argument(-22)
libbpf: failed to load object 'bpf_overlay.o'

To fix this, fallback to derivation of the map properties via /proc/$PID/fdinfo/$MAP_FD if BPF_OBJ_GET_INFO_BY_FD fails with EINVAL, which can be used as an indicator that the kernel doesn't support the latter.

Signed-off-by Martynas Pumputis <[email protected]>
---
 tools/lib/bpf/libbpf.c | 48 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 45 insertions(+), 3 deletions(-)
tools/lib/bpf/libbpf.c +45 -3
index 1e04ce724240..a952acd6cd77 100644
@@ -3894,6 +3894,42 @@ static int bpf_map_find_btf_info(struct bpf_object *obj, struct bpf_map *map)
 	return 0;
 }
 
+static int bpf_get_map_info_from_fdinfo(int fd, struct bpf_map_info *info)
+{
+	char file[PATH_MAX], buff[4096];
+	FILE *fp;
+	__u32 val;
+	int err;
+
+	snprintf(file, sizeof(file), "/proc/%d/fdinfo/%d", getpid(), fd);
+	memset(info, 0, sizeof(*info));
+
+	fp = fopen(file, "r");
+	if (!fp) {
+		err = -errno;
+		pr_warn("failed to open %s: %d. No procfs support?\n", file,
+			err);
+		return err;
+	}
+
+	while (fgets(buff, sizeof(buff), fp)) {
+		if (sscanf(buff, "map_type:\t%u", &val) == 1)
+			info->type = val;
+		else if (sscanf(buff, "key_size:\t%u", &val) == 1)
+			info->key_size = val;
+		else if (sscanf(buff, "value_size:\t%u", &val) == 1)
+			info->value_size = val;
+		else if (sscanf(buff, "max_entries:\t%u", &val) == 1)
+			info->max_entries = val;
+		else if (sscanf(buff, "map_flags:\t%i", &val) == 1)
+			info->map_flags = val;
+	}
+
+	fclose(fp);
+
+	return 0;
+}
+
 int bpf_map__reuse_fd(struct bpf_map *map, int fd)
 {
 	struct bpf_map_info info = {};

@@ -3902,6 +3938,8 @@ int bpf_map__reuse_fd(struct bpf_map *map, int fd)
 	char *new_name;
 
 	err = bpf_obj_get_info_by_fd(fd, &info, &len);
+	if (err && errno == EINVAL)
+		err = bpf_get_map_info_from_fdinfo(fd, &info);
 	if (err)
 		return libbpf_err(err);
 

@@ -4381,12 +4419,16 @@ static bool map_is_reuse_compat(const struct bpf_map *map, int map_fd)
 	struct bpf_map_info map_info = {};
 	char msg[STRERR_BUFSIZE];
 	__u32 map_info_len;
+	int err;
 
 	map_info_len = sizeof(map_info);
 
-	if (bpf_obj_get_info_by_fd(map_fd, &map_info, &map_info_len)) {
-		pr_warn("failed to get map info for map FD %d: %s\n",
-			map_fd, libbpf_strerror_r(errno, msg, sizeof(msg)));
+	err = bpf_obj_get_info_by_fd(map_fd, &map_info, &map_info_len);
+	if (err && errno == EINVAL)
+		err = bpf_get_map_info_from_fdinfo(map_fd, &map_info);
+	if (err) {
+		pr_warn("failed to get map info for map FD %d: %s\n", map_fd,
+			libbpf_strerror_r(errno, msg, sizeof(msg)));
 		return false;
 	}
Signature · 1 line
2.32.0
Acked-by on v2 1/1
#1 lore raw
Quoted 17 lines · Martynas Pumputis

When loading a BPF program with a pinned map, the loader checks whether the pinned map can be reused, i.e. their properties match. To derive such of the pinned map, the loader invokes BPF_OBJ_GET_INFO_BY_FD and then does the comparison.

Unfortunately, on < 4.12 kernels the BPF_OBJ_GET_INFO_BY_FD is not available, so loading the program fails with the following error:

libbpf: failed to get map info for map FD 5: Invalid argument
libbpf: couldn't reuse pinned map at
'/sys/fs/bpf/tc/globals/cilium_call_policy': parameter mismatch"
libbpf: map 'cilium_call_policy': error reusing pinned map
libbpf: map 'cilium_call_policy': failed to create:
Invalid argument(-22)
libbpf: failed to load object 'bpf_overlay.o'

Quoted 7 lines

To fix this, fallback to derivation of the map properties via /proc/$PID/fdinfo/$MAP_FD if BPF_OBJ_GET_INFO_BY_FD fails with EINVAL, which can be used as an indicator that the kernel doesn't support the latter.

Signed-off-by Martynas Pumputis <[email protected]>

---

LGTM.

Acked-by John Fastabend <[email protected]>