pam-util: include PID in PAM data field id

Let's systematically avoid sharing cached busses between processes (i.e.
from parent and child after fork()), by including the PID in the field
name.

With that we're never tempted to use a bus object the parent created in
the child.

(Note this is about *use*, not about *destruction*. Destruction needs to
be checked by other means.)

src/shared/pam-util.c

@@ -6,8 +6,10 @@
 
 #include "alloc-util.h"
 #include "errno-util.h"
+#include "format-util.h"
 #include "macro.h"
 #include "pam-util.h"
+#include "process-util.h"
 #include "stdio-util.h"
 #include "string-util.h"
 
@@ -64,6 +66,21 @@ static void cleanup_system_bus(pam_handle_t *handle, void *data, int error_statu
         sd_bus_flush_close_unref(data);
 }
 
+static char* pam_make_bus_cache_id(const char *module_name) {
+        char *id;
+
+        /* We want to cache bus connections between hooks. But we don't want to allow them to be reused in
+         * child processes (because sd-bus doesn't support that). We also don't want them to be reused
+         * between our own PAM modules, because they might be linked against different versions of our
+         * utility functions and share different state. Hence include both a module ID and a PID in the data
+         * field ID. */
+
+        if (asprintf(&id, "system-bus-%s-" PID_FMT, ASSERT_PTR(module_name), getpid_cached()) < 0)
+                return NULL;
+
+        return id;
+}
+
 int pam_acquire_bus_connection(pam_handle_t *handle, const char *module_name, sd_bus **ret) {
         _cleanup_(sd_bus_unrefp) sd_bus *bus = NULL;
         _cleanup_free_ char *cache_id = NULL;
@@ -73,7 +90,7 @@ int pam_acquire_bus_connection(pam_handle_t *handle, const char *module_name, sd
         assert(module_name);
         assert(ret);
 
-        cache_id = strjoin("system-bus-", module_name);
+        cache_id = pam_make_bus_cache_id(module_name);
         if (!cache_id)
                 return pam_log_oom(handle);
 
@@ -106,7 +123,7 @@ int pam_release_bus_connection(pam_handle_t *handle, const char *module_name) {
 
         assert(module_name);
 
-        cache_id = strjoin("system-bus-", module_name);
+        cache_id = pam_make_bus_cache_id(module_name);
         if (!cache_id)
                 return pam_log_oom(handle);