launch/config: use AT_RANDOM for XML hash salt

Forward the entropy from AT_RANDOM to the hash-salt used by expat. Use
XML_SetHashSalt() for this (available and fixed since expat-2.1).

This fixes an issue where libexpat might read from `/dev/urandom` and
thus block until the entropy pool is initialized. This hidden
dependency is very hard to debug. Instead, we require the service
launcher to delay startup until suitable entropy is available. This
explicit dependency is much easier to manage, debug, and control.

Reported-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: David Rheinsberg <david@readahead.eu>

src/launch/config.c

@@ -6,6 +6,7 @@
 #include <c-stdaux.h>
 #include <expat.h>
 #include <stdlib.h>
+#include <sys/auxv.h>
 #include "dbus/protocol.h"
 #include "launch/config.h"
 #include "launch/nss-cache.h"
@@ -1227,10 +1228,25 @@ static void config_parser_blob_fn(void *userdata, const XML_Char *data, int n_da
  * config_parser_init() - XXX
  */
 void config_parser_init(ConfigParser *parser) {
+        void *random;
+
         *parser = (ConfigParser)CONFIG_PARSER_NULL(*parser);
 
         parser->xml = XML_ParserCreate(NULL);
         c_assert(parser->xml);
+
+        /*
+         * The hash-tables of libexpat require a reliable random seed.
+         * Depending on libexpat compilation flags, this might end up using
+         * `/dev/urandom` and thus block until random-initialization is
+         * finished. We avoid this hidden dependency and instead use the
+         * entropy provided via `AT_RANDOM`. Hence, entropy availability is
+         * tightly coupled to process startup, and it is the job of the
+         * service manager to order processes accordingly.
+         */
+        random = (void *)getauxval(AT_RANDOM);
+        c_assert(random);
+        c_memcpy(&parser->salt, random, sizeof(parser->salt));
 }
 
 /**
@@ -1274,6 +1290,7 @@ static int config_parser_include(ConfigParser *parser, ConfigRoot *root, ConfigN
         }
 
         XML_ParserReset(parser->xml, NULL);
+        XML_SetHashSalt(parser->xml, parser->salt);
         XML_SetUserData(parser->xml, &parser->state);
         XML_SetElementHandler(parser->xml, config_parser_begin_fn, config_parser_end_fn);
         XML_SetCharacterDataHandler(parser->xml, config_parser_blob_fn);

src/launch/config.h

@@ -220,6 +220,7 @@ struct ConfigRoot {
 
 struct ConfigParser {
         struct XML_ParserStruct *xml;
+        unsigned long salt;
 
         struct ConfigState {
                 NSSCache *nss;