bpf-firewall: give a name to maps used

Running systemd with IP accounting enabled generates many bpf maps (two
per unit for accounting, another two if IPAddressAllow/Deny are used).

Systemd itself knows which maps belong to what unit and commands like
`systemctl status <unit>` can be used to query what service has which
map, but monitoring these values all the time costs 4 dbus requests
(calling the .IP{E,I}gress{Bytes,Packets} method for each unit) and
makes services like the prometheus systemd_exporter[1] somewhat slow
when doing that for every units, while less precise information could
quickly be obtained by looking directly at the maps.

Unfortunately, bpf map names are rather limited:
- only 15 characters in length (16, but last byte must be 0)
- only allows isalnum(), _ and . characters

If it wasn't for the length limit we could use the normal unit escape
functions but I've opted to just make any forbidden character into
underscores for maximum brievty -- the map prefix is also rather short:
This isn't meant as a precise mapping, but as a hint for admins who want
to look at these.

(Note there is no problem if multiple maps have the same name)

Link: https://github.com/povilasv/systemd_exporter [1]

1 files changed, 16 insertions, 1 deletions

diff --git a/src/shared/bpf-program.c b/src/shared/bpf-program.c
index 531ae9b680..d5eb6f4ccb 100644
--- a/src/shared/bpf-program.c
+++ b/src/shared/bpf-program.c
@@ -300,8 +300,16 @@ int bpf_program_cgroup_detach(BPFProgram *p) {
         return 0;
 }
 
-int bpf_map_new(enum bpf_map_type type, size_t key_size, size_t value_size, size_t max_entries, uint32_t flags) {
+int bpf_map_new(
+                const char *name,
+                enum bpf_map_type type,
+                size_t key_size,
+                size_t value_size,
+                size_t max_entries,
+                uint32_t flags) {
+
         union bpf_attr attr;
+        const char *n = name;
 
         zero(attr);
         attr.map_type = type;
@@ -310,6 +318,13 @@ int bpf_map_new(enum bpf_map_type type, size_t key_size, size_t value_size, size
         attr.max_entries = max_entries;
         attr.map_flags = flags;
 
+        /* The map name is primarily informational for debugging purposes, and typically too short
+         * to carry the full unit name, hence we employ a trivial lossy escaping to make it fit
+         * (truncation + only alphanumerical, "." and "_" are allowed as per
+         * https://www.kernel.org/doc/html/next/bpf/maps.html#usage-notes) */
+        for (size_t i = 0; i < sizeof(attr.map_name) - 1 && *n; i++, n++)
+                attr.map_name[i] = strchr(ALPHANUMERICAL ".", *n) ? *n : '_';
+
         return RET_NERRNO(bpf(BPF_MAP_CREATE, &attr, sizeof(attr)));
 }