Commit 97eb3138 authored by Martynas Pumputis's avatar Martynas Pumputis Committed by Andrii Nakryiko
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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: default avatarMartynas Pumputis <m@lambda.lt>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Acked-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20210712125552.58705-1-m@lambda.lt
parent eff94154

tools/lib/bpf/libbpf.c

+45 −3
Original line number Diff line number Diff line
@@ -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;

	}