nspawn: don't hard fail when setting capabilities

The OCI changes in #9762 broke a use case in which we use nspawn from
inside a container that has dropped capabilities from the bounding set
that nspawn expected to retain. In an attempt to keep OCI compliance
and support our use case, I made hard failing on setting capabilities
not in the bounding set optional (hard fail if using OCI and log only
if using nspawn cmdline).

Fixes #12539

src/basic/capability-util.c

@@ -10,6 +10,7 @@
 
 #include "alloc-util.h"
 #include "capability-util.h"
+#include "cap-list.h"
 #include "fileio.h"
 #include "log.h"
 #include "macro.h"
@@ -364,6 +365,43 @@ bool ambient_capabilities_supported(void) {
         return cache;
 }
 
+bool capability_quintet_mangle(CapabilityQuintet *q) {
+        unsigned long i;
+        uint64_t combined, drop = 0;
+        bool ambient_supported;
+
+        assert(q);
+
+        combined = q->effective | q->bounding | q->inheritable | q->permitted;
+
+        ambient_supported = q->ambient != (uint64_t) -1;
+        if (ambient_supported)
+                combined |= q->ambient;
+
+        for (i = 0; i <= cap_last_cap(); i++) {
+                unsigned long bit = UINT64_C(1) << i;
+                if (!FLAGS_SET(combined, bit))
+                        continue;
+
+                if (prctl(PR_CAPBSET_READ, i) > 0)
+                        continue;
+
+                drop |= bit;
+
+                log_debug("Not in the current bounding set: %s", capability_to_name(i));
+        }
+
+        q->effective &= ~drop;
+        q->bounding &= ~drop;
+        q->inheritable &= ~drop;
+        q->permitted &= ~drop;
+
+        if (ambient_supported)
+                q->ambient &= ~drop;
+
+        return drop != 0; /* Let the caller know we changed something */
+}
+
 int capability_quintet_enforce(const CapabilityQuintet *q) {
         _cleanup_cap_free_ cap_t c = NULL, modified = NULL;
         int r;

src/basic/capability-util.h

@@ -69,4 +69,9 @@ static inline bool capability_quintet_is_set(const CapabilityQuintet *q) {
                 q->ambient != (uint64_t) -1;
 }
 
+/* Mangles the specified caps quintet taking the current bounding set into account:
+ * drops all caps from all five sets if our bounding set doesn't allow them.
+ * Returns true if the quintet was modified. */
+bool capability_quintet_mangle(CapabilityQuintet *q);
+
 int capability_quintet_enforce(const CapabilityQuintet *q);