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 <m@lambda.lt>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20210712125552.58705-1-m@lambda.lt

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 1e04ce724240f54a9e19d1c50135536d7a6c6901..88b99401040c0de3c40e13f6db832d45e9f394b2 100644 (file)
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -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;
        }