seccomp: also detect if seccomp filtering is enabled

In https://github.com/systemd/systemd/pull/4004 , a runtime detection
method for seccomp was added. However, it does not detect the case
where CONFIG_SECCOMP=y but CONFIG_SECCOMP_FILTER=n. This is possible
if the architecture does not support filtering yet.
Add a check for that case too.

While at it, change get_proc_field usage to use PR_GET_SECCOMP prctl,
as that should save a few system calls and (unnecessary) allocations.
Previously, reading of /proc/self/stat was done as recommended by
prctl(2) as safer. However, given that we need to do the prctl call
anyway, lets skip opening, reading and parsing the file.

Code for checking inspired by
https://outflux.net/teach-seccomp/autodetect.html

src/core/execute.c

@@ -1077,7 +1077,7 @@ static void rename_process_from_path(const char *path) {
 static bool skip_seccomp_unavailable(const Unit* u, const char* msg) {
         if (!is_seccomp_available()) {
                 log_open();
-                log_unit_debug(u, "SECCOMP not detected in the kernel, skipping %s", msg);
+                log_unit_debug(u, "SECCOMP features not detected in the kernel, skipping %s", msg);
                 log_close();
                 return true;
         }

src/shared/seccomp-util.c

@@ -20,9 +20,9 @@
 #include <errno.h>
 #include <seccomp.h>
 #include <stddef.h>
+#include <sys/prctl.h>
+#include <linux/seccomp.h>
 
-#include "alloc-util.h"
-#include "fileio.h"
 #include "macro.h"
 #include "seccomp-util.h"
 #include "string-util.h"
@@ -91,11 +91,22 @@ int seccomp_add_secondary_archs(scmp_filter_ctx *c) {
 
 }
 
+static bool is_basic_seccomp_available(void) {
+        int r;
+        r = prctl(PR_GET_SECCOMP, 0, 0, 0, 0);
+        return r >= 0;
+}
+
+static bool is_seccomp_filter_available(void) {
+        int r;
+        r = prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, NULL, 0, 0);
+        return r < 0 && errno == EFAULT;
+}
+
 bool is_seccomp_available(void) {
-        _cleanup_free_ char* field = NULL;
         static int cached_enabled = -1;
         if (cached_enabled < 0)
-                cached_enabled = get_proc_field("/proc/self/status", "Seccomp", "\n", &field) == 0;
+                cached_enabled = is_basic_seccomp_available() && is_seccomp_filter_available();
         return cached_enabled;
 }