Tests:
```
% stat --file-system --format="%T" /root/bpf/trivial/
bpf_fs
% systemd-nspawn -D/ --volatile=yes \
--property=BPFProgram=egress:/root/bpf/trivial/cgroup_skb_egress \
--quiet -- ping -c 5 -W 1 ::1
PING ::1(::1) 56 data bytes
--- ::1 ping statistics ---
5 packets transmitted, 0 received, 100% packet loss, time 4110ms
```
```
% stat --file-system --format='%T' /root/meh
btrfs
% systemd-nspawn -D/ --volatile=yes --property=BPFProgram=egress:/root/meh
--quiet -- ping -c 5 -W 1 ::1
```
sudo ./build/systemd-nspawn \
-D/ --volatile=yes --property=BPFProgram=egress:/home/hex --quiet -- \
ping -c 1 -W 1 ::1
PING ::1(::1) 56 data bytes
64 bytes from ::1: icmp_seq=1 ttl=64 time=0.017 ms
--- ::1 ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
#include "bpf-program.h"
#include "cgroup.h"
#include "memory-util.h"
+#include "missing_magic.h"
#include "mountpoint-util.h"
#include "set.h"
+#include "stat-util.h"
typedef struct BPFForeignKey BPFForeignKey;
struct BPFForeignKey {
assert(u);
assert(bpffs_path);
+ r = path_is_fs_type(bpffs_path, BPF_FS_MAGIC);
+ if (r < 0)
+ return log_unit_error_errno(u, r,
+ "Failed to determine filesystem type of %s: %m", bpffs_path);
+ if (r == 0)
+ return log_unit_error_errno(u, SYNTHETIC_ERRNO(EINVAL),
+ "Path in BPF filesystem is expected.");
+
r = bpf_program_new_from_bpffs_path(bpffs_path, &prog);
if (r < 0)
return log_unit_error_errno(u, r, "Failed to create foreign BPFProgram: %m");