cgroup, systemd: validate ebpf is loaded

systemd ignores errors with BPFProgram so make sure the ebpf program
is installed when requested.

Alternative to #1854

Signed-off-by: Giuseppe Scrivano <[email protected]>

Author: giuseppe

src/libcrun/cgroup-systemd.c

@@ -1668,6 +1668,28 @@ reset_failed_unit (sd_bus *bus, const char *unit)
   return sd_err;
 }
 
+static int
+verify_ebpf_device_filter_installed (const char *cgroup_path, libcrun_error_t *err)
+{
+  cleanup_free uint32_t *progs = NULL;
+  cleanup_free char *full_path = NULL;
+  size_t n_progs = 0;
+  int ret;
+
+  ret = append_paths (&full_path, err, CGROUP_ROOT, cgroup_path, NULL);
+  if (UNLIKELY (ret < 0))
+    return ret;
+
+  ret = libcrun_ebpf_query_cgroup_progs (full_path, &progs, &n_progs, err);
+  if (UNLIKELY (ret < 0))
+    return ret;
+
+  if (n_progs == 0)
+    return crun_make_error (err, 0, "systemd failed to install eBPF device filter on cgroup `%s`", full_path);
+
+  return 0;
+}
+
 static int
 enter_systemd_cgroup_scope (runtime_spec_schema_config_linux_resources *resources,
                             int cgroup_mode,
@@ -2023,6 +2045,30 @@ libcrun_cgroup_enter_systemd (struct libcrun_cgroup_args *args,
   if (UNLIKELY (ret < 0))
     return ret;
 
+  /* Verify that systemd has actually installed the eBPF device filter if one was requested. */
+  if (out->bpf_dev_set)
+    {
+      cleanup_free char *scope_path = NULL;
+
+      /* eBPF programs are attached to the systemd scope, not the subgroup.
+         Remove the suffix that was added by systemd_finalize. */
+      if (is_empty_string (suffix))
+        scope_path = xstrdup (path);
+      else
+        {
+          size_t path_len = strlen (path);
+          size_t suffix_len = strlen (suffix);
+
+          scope_path = strndup (path, path_len - suffix_len - 1);
+          if (UNLIKELY (scope_path == NULL))
+            OOM ();
+        }
+
+      ret = verify_ebpf_device_filter_installed (scope_path, err);
+      if (UNLIKELY (ret < 0))
+        return ret;
+    }
+
   out->path = path;
   path = NULL;
 

src/libcrun/ebpf.c

@@ -607,6 +607,25 @@ libcrun_ebpf_read_program (struct bpf_program **program_ret, const char *path, l
 #endif
 }
 
+int
+libcrun_ebpf_query_cgroup_progs (const char *cgroup_path, uint32_t **progs_out, size_t *n_progs_out, libcrun_error_t *err)
+{
+#ifndef HAVE_EBPF
+  (void) cgroup_path;
+  *progs_out = NULL;
+  *n_progs_out = 0;
+  return crun_make_error (err, 0, "eBPF not supported");
+#else
+  cleanup_close int cgroup_fd = -1;
+
+  cgroup_fd = open (cgroup_path, O_RDONLY | O_CLOEXEC);
+  if (UNLIKELY (cgroup_fd < 0))
+    return crun_make_error (err, errno, "open cgroup path `%s`", cgroup_path);
+
+  return read_all_progs (cgroup_fd, progs_out, n_progs_out, err);
+#endif
+}
+
 bool
 libcrun_ebpf_cmp_programs (struct bpf_program *program1, struct bpf_program *program2)
 {

src/libcrun/ebpf.h

@@ -42,6 +42,7 @@ struct bpf_program *bpf_program_complete_dev (struct bpf_program *program, libcr
 
 int libcrun_ebpf_load (struct bpf_program *program, int dirfd, const char *pin, libcrun_error_t *err);
 int libcrun_ebpf_read_program (struct bpf_program **program, const char *path, libcrun_error_t *err);
+int libcrun_ebpf_query_cgroup_progs (const char *cgroup_path, uint32_t **progs_out, size_t *n_progs_out, libcrun_error_t *err);
 bool libcrun_ebpf_cmp_programs (struct bpf_program *program1, struct bpf_program *program2);
 
 #endif