From 01b195e0c674d7de5c1320866b2cadce86085c0f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marco=20Trevisan=20=28Trevi=C3=B1o=29?= <mail@3v1n0.net>
Date: Thu, 30 Jan 2025 18:42:06 +0100
Subject: [PATCH] gvariant: Do not byteswap empty GVariant values

If a value has no size, we'd end up to use g_variant_store() with a NULL
value, and this is not allowed by the API, leading to errors:

stderr:
 ../glib/gvariant-core.c:1429:9: runtime error: null pointer passed as argument
    1, which is declared to never be null
    #0 0x7f9c139ce559 in g_variant_store ../glib/gvariant-core.c:732
    #1 0x7f9c13c60b01 in g_variant_byteswap ../glib/gvariant.c:6211
    #2 0x564ae412e9b9 in test_byteswap ../glib/tests/gvariant.c:2321
    #3 0x564ae412e9b9 in test_byteswaps ../glib/tests/gvariant.c:2374
    #4 0x7f9c13bc1000 in test_case_run ../glib/gtestutils.c:3115
    #5 0x7f9c13bc1000 in g_test_run_suite_internal ../glib/gtestutils.c:3210
    #6 0x7f9c13bc0d7b in g_test_run_suite_internal ../glib/gtestutils.c:3229
    #7 0x7f9c13bc0d7b in g_test_run_suite_internal ../glib/gtestutils.c:3229
    #8 0x7f9c13bc2019 in g_test_run_suite ../glib/gtestutils.c:3310
    #9 0x7f9c13bc216f in g_test_run ../glib/gtestutils.c:2379
    #10 0x564ae410f326 in main ../glib/tests/gvariant.c:6045
---
 glib/gvariant.c       |  6 ++++--
 glib/tests/gvariant.c | 14 ++++++++++++++
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/glib/gvariant.c b/glib/gvariant.c
index bdcbe412b..432c0a57d 100644
--- a/glib/gvariant.c
+++ b/glib/gvariant.c
@@ -6187,12 +6187,14 @@ g_variant_byteswap (GVariant *value)
   GVariantTypeInfo *type_info;
   guint alignment;
   GVariant *new;
+  gsize size;
 
   type_info = g_variant_get_type_info (value);
+  size = g_variant_get_size (value);
 
   g_variant_type_info_query (type_info, &alignment, NULL);
 
-  if (alignment && g_variant_is_normal_form (value))
+  if (alignment && size > 0 && g_variant_is_normal_form (value))
     {
       /* (potentially) contains multi-byte numeric data, but is also already in
        * normal form so we can use a faster byteswapping codepath on the
@@ -6201,7 +6203,7 @@ g_variant_byteswap (GVariant *value)
       GBytes *bytes;
 
       serialised.type_info = g_variant_get_type_info (value);
-      serialised.size = g_variant_get_size (value);
+      serialised.size = size;
       serialised.data = g_malloc (serialised.size);
       serialised.depth = g_variant_get_depth (value);
       serialised.ordered_offsets_up_to = G_MAXSIZE;  /* operating on the normal form */
diff --git a/glib/tests/gvariant.c b/glib/tests/gvariant.c
index 12a35dad9..e215e9861 100644
--- a/glib/tests/gvariant.c
+++ b/glib/tests/gvariant.c
@@ -2367,6 +2367,19 @@ test_byteswaps (void)
   g_variant_type_info_assert_no_infos ();
 }
 
+static void
+test_byteswap_zero_sized (void)
+{
+  GVariant *variant;
+  GVariant *swapped;
+
+  variant = g_variant_new_from_data (G_VARIANT_TYPE_STRING, NULL, 0, TRUE, NULL, NULL);
+  swapped = g_variant_byteswap (variant);
+
+  g_variant_unref (variant);
+  g_variant_unref (swapped);
+}
+
 static void
 test_serialiser_children (void)
 {
@@ -5939,6 +5952,7 @@ main (int argc, char **argv)
   g_test_add_func ("/gvariant/serialiser/variant", test_variants);
   g_test_add_func ("/gvariant/serialiser/strings", test_strings);
   g_test_add_func ("/gvariant/serialiser/byteswap", test_byteswaps);
+  g_test_add_func ("/gvariant/serialiser/byteswap/zero-sized", test_byteswap_zero_sized);
   g_test_add_func ("/gvariant/serialiser/children", test_serialiser_children);
 
   for (i = 1; i <= 20; i += 4)