Add OtVariantBuilder

This is similar to GVariantBuilder in that it constructs variant
containers, but it writes it directly to a file descriptor rather
than keep the entier thing in memory. This is useful to create large
variants without using a lot of memory.

Closes: #1309
Approved by: cgwalters

4 files changed, 1283 insertions, 0 deletions

diff --git a/Makefile-otutil.am b/Makefile-otutil.am
index c4c0338d..b4ee1c4b 100644
--- a/Makefile-otutil.am
+++ b/Makefile-otutil.am
@@ -34,6 +34,8 @@ libotutil_la_SOURCES = \
 	src/libotutil/ot-unix-utils.h \
 	src/libotutil/ot-variant-utils.c \
 	src/libotutil/ot-variant-utils.h \
+	src/libotutil/ot-variant-builder.c \
+	src/libotutil/ot-variant-builder.h \
 	src/libotutil/ot-gio-utils.c \
 	src/libotutil/ot-gio-utils.h \
 	src/libotutil/ot-gpg-utils.c \
diff --git a/src/libotutil/ot-variant-builder.c b/src/libotutil/ot-variant-builder.c
new file mode 100644
index 00000000..8bf7544a
--- /dev/null
+++ b/src/libotutil/ot-variant-builder.c
@@ -0,0 +1,1219 @@
+/*
+ * Copyright (C) 2017 Alexander Larsson <alexl@redhat.com>.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ *
+ * Author: Alexander Larsson <alexl@redhat.com>.
+ */
+
+#include "config.h"
+
+#include "ot-variant-builder.h"
+#include "libglnx/libglnx.h"
+
+/*****************************************************************************************
+ * This code is copied from gvariant in glib. With the following copyright:
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ *
+ * Copyright © 2007, 2008 Ryan Lortie
+ * Copyright © 2010 Codethink Limited
+ *****************************************************************************************/
+
+typedef struct _GVariantTypeInfo GVariantTypeInfo;
+
+#define G_VARIANT_TYPE_INFO_CHAR_MAYBE      'm'
+#define G_VARIANT_TYPE_INFO_CHAR_ARRAY      'a'
+#define G_VARIANT_TYPE_INFO_CHAR_TUPLE      '('
+#define G_VARIANT_TYPE_INFO_CHAR_DICT_ENTRY '{'
+#define G_VARIANT_TYPE_INFO_CHAR_VARIANT    'v'
+#define g_variant_type_info_get_type_char(info) \
+  (g_variant_type_info_get_type_string(info)[0])
+
+struct _GVariantTypeInfo
+{
+  gsize fixed_size;
+  guchar alignment;
+  guchar container_class;
+};
+
+typedef struct
+{
+  GVariantTypeInfo *type_info;
+
+  gsize i, a;
+  gint8 b, c;
+
+  guint8 ending_type;
+} GVariantMemberInfo;
+
+#define G_VARIANT_MEMBER_ENDING_FIXED   0
+#define G_VARIANT_MEMBER_ENDING_LAST    1
+#define G_VARIANT_MEMBER_ENDING_OFFSET  2
+
+typedef struct
+{
+  GVariantTypeInfo info;
+
+  gchar *type_string;
+  gint ref_count;
+} ContainerInfo;
+
+typedef struct
+{
+  ContainerInfo container;
+
+  GVariantTypeInfo *element;
+} ArrayInfo;
+
+typedef struct
+{
+  ContainerInfo container;
+
+  GVariantMemberInfo *members;
+  gsize n_members;
+} TupleInfo;
+
+/* Hard-code the base types in a constant array */
+static const GVariantTypeInfo g_variant_type_info_basic_table[24] = {
+#define fixed_aligned(x)  x, x - 1
+#define not_a_type             0,
+#define unaligned         0, 0
+#define aligned(x)        0, x - 1
+  /* 'b' */ { fixed_aligned(1) },   /* boolean */
+  /* 'c' */ { not_a_type },
+  /* 'd' */ { fixed_aligned(8) },   /* double */
+  /* 'e' */ { not_a_type },
+  /* 'f' */ { not_a_type },
+  /* 'g' */ { unaligned        },   /* signature string */
+  /* 'h' */ { fixed_aligned(4) },   /* file handle (int32) */
+  /* 'i' */ { fixed_aligned(4) },   /* int32 */
+  /* 'j' */ { not_a_type },
+  /* 'k' */ { not_a_type },
+  /* 'l' */ { not_a_type },
+  /* 'm' */ { not_a_type },
+  /* 'n' */ { fixed_aligned(2) },   /* int16 */
+  /* 'o' */ { unaligned        },   /* object path string */
+  /* 'p' */ { not_a_type },
+  /* 'q' */ { fixed_aligned(2) },   /* uint16 */
+  /* 'r' */ { not_a_type },
+  /* 's' */ { unaligned        },   /* string */
+  /* 't' */ { fixed_aligned(8) },   /* uint64 */
+  /* 'u' */ { fixed_aligned(4) },   /* uint32 */
+  /* 'v' */ { aligned(8)       },   /* variant */
+  /* 'w' */ { not_a_type },
+  /* 'x' */ { fixed_aligned(8) },   /* int64 */
+  /* 'y' */ { fixed_aligned(1) },   /* byte */
+#undef fixed_aligned
+#undef not_a_type
+#undef unaligned
+#undef aligned
+};
+
+static GRecMutex g_variant_type_info_lock;
+static GHashTable *g_variant_type_info_table;
+
+static GVariantTypeInfo * g_variant_type_info_ref (GVariantTypeInfo *info);
+static void g_variant_type_info_unref (GVariantTypeInfo *info);
+static GVariantTypeInfo * g_variant_type_info_get (const GVariantType *type);
+
+#define GV_ARRAY_INFO_CLASS 'a'
+static ArrayInfo *
+GV_ARRAY_INFO (GVariantTypeInfo *info)
+{
+  return (ArrayInfo *) info;
+}
+
+static void
+array_info_free (GVariantTypeInfo *info)
+{
+  ArrayInfo *array_info;
+
+  g_assert (info->container_class == GV_ARRAY_INFO_CLASS);
+  array_info = (ArrayInfo *) info;
+
+  g_variant_type_info_unref (array_info->element);
+  g_slice_free (ArrayInfo, array_info);
+}
+
+static ContainerInfo *
+array_info_new (const GVariantType *type)
+{
+  ArrayInfo *info;
+
+  info = g_slice_new (ArrayInfo);
+  info->container.info.container_class = GV_ARRAY_INFO_CLASS;
+
+  info->element = g_variant_type_info_get (g_variant_type_element (type));
+  info->container.info.alignment = info->element->alignment;
+  info->container.info.fixed_size = 0;
+
+  return (ContainerInfo *) info;
+}
+
+/* == tuple == */
+#define GV_TUPLE_INFO_CLASS 'r'
+static TupleInfo *
+GV_TUPLE_INFO (GVariantTypeInfo *info)
+{
+  return (TupleInfo *) info;
+}
+
+static void
+tuple_info_free (GVariantTypeInfo *info)
+{
+  TupleInfo *tuple_info;
+  gint i;
+
+  g_assert (info->container_class == GV_TUPLE_INFO_CLASS);
+  tuple_info = (TupleInfo *) info;
+
+  for (i = 0; i < tuple_info->n_members; i++)
+    g_variant_type_info_unref (tuple_info->members[i].type_info);
+
+  g_slice_free1 (sizeof (GVariantMemberInfo) * tuple_info->n_members,
+                 tuple_info->members);
+  g_slice_free (TupleInfo, tuple_info);
+}
+
+static void
+tuple_allocate_members (const GVariantType  *type,
+                        GVariantMemberInfo **members,
+                        gsize               *n_members)
+{
+  const GVariantType *item_type;
+  gsize i = 0;
+
+  *n_members = g_variant_type_n_items (type);
+  *members = g_slice_alloc (sizeof (GVariantMemberInfo) * *n_members);
+
+  item_type = g_variant_type_first (type);
+  while (item_type)
+    {
+      GVariantMemberInfo *member = &(*members)[i++];
+
+      member->type_info = g_variant_type_info_get (item_type);
+      item_type = g_variant_type_next (item_type);
+
+      if (member->type_info->fixed_size)
+        member->ending_type = G_VARIANT_MEMBER_ENDING_FIXED;
+      else if (item_type == NULL)
+        member->ending_type = G_VARIANT_MEMBER_ENDING_LAST;
+      else
+        member->ending_type = G_VARIANT_MEMBER_ENDING_OFFSET;
+    }
+
+  g_assert (i == *n_members);
+}
+
+/* this is g_variant_type_info_query for a given member of the tuple.
+ * before the access is done, it is ensured that the item is within
+ * range and %FALSE is returned if not.
+ */
+static gboolean
+tuple_get_item (TupleInfo          *info,
+                GVariantMemberInfo *item,
+                gsize              *d,
+                gsize              *e)
+{
+  if (&info->members[info->n_members] == item)
+    return FALSE;
+
+  *d = item->type_info->alignment;
+  *e = item->type_info->fixed_size;
+  return TRUE;
+}
+
+/* Read the documentation for #GVariantMemberInfo in gvarianttype.h
+ * before attempting to understand this.
+ *
+ * This function adds one set of "magic constant" values (for one item
+ * in the tuple) to the table.
+ *
+ * The algorithm in tuple_generate_table() calculates values of 'a', 'b'
+ * and 'c' for each item, such that the procedure for finding the item
+ * is to start at the end of the previous variable-sized item, add 'a',
+ * then round up to the nearest multiple of 'b', then add 'c'.
+ * Note that 'b' is stored in the usual "one less than" form.  ie:
+ *
+ *   start = ROUND_UP(prev_end + a, (b + 1)) + c;
+ *
+ * We tweak these values a little to allow for a slightly easier
+ * computation and more compact storage.
+ */
+static void
+tuple_table_append (GVariantMemberInfo **items,
+                    gsize                i,
+                    gsize                a,
+                    gsize                b,
+                    gsize                c)
+{
+  GVariantMemberInfo *item = (*items)++;
+
+  /* We can shift multiples of the alignment size from 'c' into 'a'.
+   * As long as we're shifting whole multiples, it won't affect the
+   * result.  This means that we can take the "aligned" portion off of
+   * 'c' and add it into 'a'.
+   *
+   *  Imagine (for sake of clarity) that ROUND_10 rounds up to the
+   *  nearest 10.  It is clear that:
+   *
+   *   ROUND_10(a) + c == ROUND_10(a + 10*(c / 10)) + (c % 10)
+   *
+   * ie: remove the 10s portion of 'c' and add it onto 'a'.
+   *
+   * To put some numbers on it, imagine we start with a = 34 and c = 27:
+   *
+   *  ROUND_10(34) + 27 = 40 + 27 = 67
+   *
+   * but also, we can split 27 up into 20 and 7 and do this:
+   *
+   *  ROUND_10(34 + 20) + 7 = ROUND_10(54) + 7 = 60 + 7 = 67
+   *                ^^    ^
+   * without affecting the result.  We do that here.
+   *
+   * This reduction in the size of 'c' means that we can store it in a
+   * gchar instead of a gsize.  Due to how the structure is packed, this
+   * ends up saving us 'two pointer sizes' per item in each tuple when
+   * allocating using GSlice.
+   */
+  a += ~b & c;    /* take the "aligned" part of 'c' and add to 'a' */
+  c &= b;         /* chop 'c' to contain only the unaligned part */
+
+
+  /* Finally, we made one last adjustment.  Recall:
+   *
+   *   start = ROUND_UP(prev_end + a, (b + 1)) + c;
+   *
+   * Forgetting the '+ c' for the moment:
+   *
+   *   ROUND_UP(prev_end + a, (b + 1));
+   *
+   * we can do a "round up" operation by adding 1 less than the amount
+   * to round up to, then rounding down.  ie:
+   *
+   *   #define ROUND_UP(x, y)    ROUND_DOWN(x + (y-1), y)
+   *
+   * Of course, for rounding down to a power of two, we can just mask
+   * out the appropriate number of low order bits:
+   *
+   *   #define ROUND_DOWN(x, y)  (x & ~(y - 1))
+   *
+   * Which gives us
+   *
+   *   #define ROUND_UP(x, y)    (x + (y - 1) & ~(y - 1))
+   *
+   * but recall that our alignment value 'b' is already "one less".
+   * This means that to round 'prev_end + a' up to 'b' we can just do:
+   *
+   *   ((prev_end + a) + b) & ~b
+   *
+   * Associativity, and putting the 'c' back on:
+   *
+   *   (prev_end + (a + b)) & ~b + c
+   *
+   * Now, since (a + b) is constant, we can just add 'b' to 'a' now and
+   * store that as the number to add to prev_end.  Then we use ~b as the
+   * number to take a bitwise 'and' with.  Finally, 'c' is added on.
+   *
+   * Note, however, that all the low order bits of the 'aligned' value
+   * are masked out and that all of the high order bits of 'c' have been
+   * "moved" to 'a' (in the previous step).  This means that there are
+   * no overlapping bits in the addition -- so we can do a bitwise 'or'
+   * equivalently.
+   *
+   * This means that we can now compute the start address of a given
+   * item in the tuple using the algorithm given in the documentation
+   * for #GVariantMemberInfo:
+   *
+   *   item_start = ((prev_end + a) & b) | c;
+   */
+
+  item->i = i;
+  item->a = a + b;
+  item->b = ~b;
+  item->c = c;
+}
+
+static gsize
+tuple_align (gsize offset,
+             guint alignment)
+{
+  return offset + ((-offset) & alignment);
+}
+
+/* This function is the heart of the algorithm for calculating 'i', 'a',
+ * 'b' and 'c' for each item in the tuple.
+ *
+ * Imagine we want to find the start of the "i" in the type "(su(qx)ni)".
+ * That's a string followed by a uint32, then a tuple containing a
+ * uint16 and a int64, then an int16, then our "i".  In order to get to
+ * our "i" we:
+ *
+ * Start at the end of the string, align to 4 (for the uint32), add 4.
+ * Align to 8, add 16 (for the tuple).  Align to 2, add 2 (for the
+ * int16).  Then we're there.  It turns out that, given 3 simple rules,
+ * we can flatten this iteration into one addition, one alignment, then
+ * one more addition.
+ *
+ * The loop below plays through each item in the tuple, querying its
+ * alignment and fixed_size into 'd' and 'e', respectively.  At all
+ * times the variables 'a', 'b', and 'c' are maintained such that in
+ * order to get to the current point, you add 'a', align to 'b' then add
+ * 'c'.  'b' is kept in "one less than" form.  For each item, the proper
+ * alignment is applied to find the values of 'a', 'b' and 'c' to get to
+ * the start of that item.  Those values are recorded into the table.
+ * The fixed size of the item (if applicable) is then added on.
+ *
+ * These 3 rules are how 'a', 'b' and 'c' are modified for alignment and
+ * addition of fixed size.  They have been proven correct but are
+ * presented here, without proof:
+ *
+ *  1) in order to "align to 'd'" where 'd' is less than or equal to the
+ *     largest level of alignment seen so far ('b'), you align 'c' to
+ *     'd'.
+ *  2) in order to "align to 'd'" where 'd' is greater than the largest
+ *     level of alignment seen so far, you add 'c' aligned to 'b' to the
+ *     value of 'a', set 'b' to 'd' (ie: increase the 'largest alignment
+ *     seen') and reset 'c' to 0.
+ *  3) in order to "add 'e'", just add 'e' to 'c'.
+ */
+static void
+tuple_generate_table (TupleInfo *info)
+{
+  GVariantMemberInfo *items = info->members;
+  gsize i = -1, a = 0, b = 0, c = 0, d, e;
+
+  /* iterate over each item in the tuple.
+   *   'd' will be the alignment of the item (in one-less form)
+   *   'e' will be the fixed size (or 0 for variable-size items)
+   */
+  while (tuple_get_item (info, items, &d, &e))
+    {
+      /* align to 'd' */
+      if (d <= b)
+        c = tuple_align (c, d);                   /* rule 1 */
+      else
+        a += tuple_align (c, b), b = d, c = 0;    /* rule 2 */
+
+      /* the start of the item is at this point (ie: right after we
+       * have aligned for it).  store this information in the table.
+       */
+      tuple_table_append (&items, i, a, b, c);
+
+      /* "move past" the item by adding in its size. */
+      if (e == 0)
+        /* variable size:
+         *
+         * we'll have an offset stored to mark the end of this item, so
+         * just bump the offset index to give us a new starting point
+         * and reset all the counters.
+         */
+        i++, a = b = c = 0;
+      else
+        /* fixed size */
+        c += e;                                   /* rule 3 */
+    }
+}
+
+static void
+tuple_set_base_info (TupleInfo *info)
+{
+  GVariantTypeInfo *base = &info->container.info;
+
+  if (info->n_members > 0)
+    {
+      GVariantMemberInfo *m;
+
+      /* the alignment requirement of the tuple is the alignment
+       * requirement of its largest item.
+       */
+      base->alignment = 0;
+      for (m = info->members; m < &info->members[info->n_members]; m++)
+        /* can find the max of a list of "one less than" powers of two
+         * by 'or'ing them
+         */
+        base->alignment |= m->type_info->alignment;
+
+      m--; /* take 'm' back to the last item */
+
+      /* the structure only has a fixed size if no variable-size
+       * offsets are stored and the last item is fixed-sized too (since
+       * an offset is never stored for the last item).
+       */
+      if (m->i == -1 && m->type_info->fixed_size)
+        /* in that case, the fixed size can be found by finding the
+         * start of the last item (in the usual way) and adding its
+         * fixed size.
+         *
+         * if a tuple has a fixed size then it is always a multiple of
+         * the alignment requirement (to make packing into arrays
+         * easier) so we round up to that here.
+         */
+        base->fixed_size =
+          tuple_align (((m->a & m->b) | m->c) + m->type_info->fixed_size,
+                       base->alignment);
+      else
+        /* else, the tuple is not fixed size */
+        base->fixed_size = 0;
+    }
+  else
+    {
+      /* the empty tuple: '()'.
+       *
+       * has a size of 1 and an no alignment requirement.
+       *
+       * It has a size of 1 (not 0) for two practical reasons:
+       *
+       *  1) So we can determine how many of them are in an array
+       *     without dividing by zero or without other tricks.
+       *
+       *  2) Even if we had some trick to know the number of items in
+       *     the array (as GVariant did at one time) this would open a
+       *     potential denial of service attack: an attacker could send
+       *     you an extremely small array (in terms of number of bytes)
+       *     containing trillions of zero-sized items.  If you iterated
+       *     over this array you would effectively infinite-loop your
+       *     program.  By forcing a size of at least one, we bound the
+       *     amount of computation done in response to a message to a
+       *     reasonable function of the size of that message.
+       */
+      base->alignment = 0;
+      base->fixed_size = 1;
+    }
+}
+
+static ContainerInfo *
+tuple_info_new (const GVariantType *type)
+{
+  TupleInfo *info;
+
+  info = g_slice_new (TupleInfo);
+  info->container.info.container_class = GV_TUPLE_INFO_CLASS;
+
+  tuple_allocate_members (type, &info->members, &info->n_members);
+  tuple_generate_table (info);
+  tuple_set_base_info (info);
+
+  return (ContainerInfo *) info;
+}
+
+static const GVariantMemberInfo *
+g_variant_type_info_member_info (GVariantTypeInfo *info,
+                                 gsize             index)
+{
+  TupleInfo *tuple_info = GV_TUPLE_INFO (info);
+
+  if (index < tuple_info->n_members)
+    return &tuple_info->members[index];
+
+  return NULL;
+}
+
+static GVariantTypeInfo *
+g_variant_type_info_element (GVariantTypeInfo *info)
+{
+  return GV_ARRAY_INFO (info)->element;
+}
+
+static GVariantTypeInfo *
+g_variant_type_info_ref (GVariantTypeInfo *info)
+{
+  if (info->container_class)
+    {
+      ContainerInfo *container = (ContainerInfo *) info;
+
+      g_assert_cmpint (container->ref_count, >, 0);
+      g_atomic_int_inc (&container->ref_count);
+    }
+
+  return info;
+}
+
+static void
+g_variant_type_info_unref (GVariantTypeInfo *info)
+{
+  if (info->container_class)
+    {
+      ContainerInfo *container = (ContainerInfo *) info;
+
+      g_rec_mutex_lock (&g_variant_type_info_lock);
+      if (g_atomic_int_dec_and_test (&container->ref_count))
+        {
+          g_hash_table_remove (g_variant_type_info_table,
+                               container->type_string);
+          if (g_hash_table_size (g_variant_type_info_table) == 0)
+            {
+              g_hash_table_unref (g_variant_type_info_table);
+              g_variant_type_info_table = NULL;
+            }
+          g_rec_mutex_unlock (&g_variant_type_info_lock);
+
+          g_free (container->type_string);
+
+          if (info->container_class == GV_ARRAY_INFO_CLASS)
+            array_info_free (info);
+
+          else if (info->container_class == GV_TUPLE_INFO_CLASS)
+            tuple_info_free (info);
+
+          else
+            g_assert_not_reached ();
+        }
+      else
+        g_rec_mutex_unlock (&g_variant_type_info_lock);
+    }
+}
+
+static GVariantTypeInfo *
+g_variant_type_info_get (const GVariantType *type)
+{
+  char type_char;
+
+  type_char = g_variant_type_peek_string (type)[0];
+
+  if (type_char == G_VARIANT_TYPE_INFO_CHAR_MAYBE ||
+      type_char == G_VARIANT_TYPE_INFO_CHAR_ARRAY ||
+      type_char == G_VARIANT_TYPE_INFO_CHAR_TUPLE ||
+      type_char == G_VARIANT_TYPE_INFO_CHAR_DICT_ENTRY)
+    {
+      GVariantTypeInfo *info;
+      gchar *type_string;
+
+      type_string = g_variant_type_dup_string (type);
+
+      g_rec_mutex_lock (&g_variant_type_info_lock);
+
+      if (g_variant_type_info_table == NULL)
+        g_variant_type_info_table = g_hash_table_new (g_str_hash,
+                                                      g_str_equal);
+      info = g_hash_table_lookup (g_variant_type_info_table, type_string);
+
+      if (info == NULL)
+        {
+          ContainerInfo *container;
+
+          if (type_char == G_VARIANT_TYPE_INFO_CHAR_MAYBE ||
+              type_char == G_VARIANT_TYPE_INFO_CHAR_ARRAY)
+            {
+              container = array_info_new (type);
+            }
+          else /* tuple or dict entry */
+            {
+              container = tuple_info_new (type);
+            }
+
+          info = (GVariantTypeInfo *) container;
+          container->type_string = type_string;
+          container->ref_count = 1;
+
+          g_hash_table_insert (g_variant_type_info_table, type_string, info);
+          type_string = NULL;
+        }
+      else
+        g_variant_type_info_ref (info);
+
+      g_rec_mutex_unlock (&g_variant_type_info_lock);
+      g_free (type_string);
+
+      return info;
+    }
+  else
+    {
+      const GVariantTypeInfo *info;
+      int index;
+
+      index = type_char - 'b';
+      g_assert (G_N_ELEMENTS (g_variant_type_info_basic_table) == 24);
+      g_assert_cmpint (0, <=, index);
+      g_assert_cmpint (index, <, 24);
+
+      info = g_variant_type_info_basic_table + index;
+
+      return (GVariantTypeInfo *) info;
+    }
+}
+
+static inline void
+gvs_write_unaligned_le (guchar *bytes,
+                        gsize   value,
+                        guint   size)
+{
+  union
+  {
+    guchar bytes[GLIB_SIZEOF_SIZE_T];
+    gsize integer;
+  } tmpvalue;
+
+  tmpvalue.integer = GSIZE_TO_LE (value);
+  memcpy (bytes, &tmpvalue.bytes, size);
+}
+
+static guint
+gvs_get_offset_size (gsize size)
+{
+  if (size > G_MAXUINT32)
+    return 8;
+
+  else if (size > G_MAXUINT16)
+    return 4;
+
+  else if (size > G_MAXUINT8)
+    return 2;
+
+  else if (size > 0)
+    return 1;
+
+  return 0;
+}
+
+static gsize
+gvs_calculate_total_size (gsize body_size,
+                          gsize offsets)
+{
+  if (body_size + 1 * offsets <= G_MAXUINT8)
+    return body_size + 1 * offsets;
+
+  if (body_size + 2 * offsets <= G_MAXUINT16)
+    return body_size + 2 * offsets;
+
+  if (body_size + 4 * offsets <= G_MAXUINT32)
+    return body_size + 4 * offsets;
+
+  return body_size + 8 * offsets;
+}
+
+
+/*****************************************************************************************
+ *  End of glib code
+ *****************************************************************************************/
+
+typedef struct _OtVariantBuilderInfo OtVariantBuilderInfo;
+
+struct _OtVariantBuilderInfo {
+  OtVariantBuilderInfo *parent;
+  OtVariantBuilder *builder;
+  GVariantType *type;
+  GVariantTypeInfo *type_info;
+  guint64 offset;
+  int n_children;
+  GArray *child_ends;
+
+  /* type constraint explicitly specified by 'type'.
+   * for tuple types, this moves along as we add more items.
+   */
+  const GVariantType *expected_type;
+
+  /* type constraint implied by previous array item.
+   */
+  const GVariantType *prev_item_type;
+  GVariantType *prev_item_type_base;
+
+  /* constraints on the number of children.  max = -1 for unlimited. */
+  gsize min_items;
+  gsize max_items;
+
+  /* set to '1' if all items in the container will have the same type
+   * (ie: maybe, array, variant) '0' if not (ie: tuple, dict entry)
+   */
+  guint uniform_item_types : 1;
+} ;
+
+struct _OtVariantBuilder {
+  gint ref_count;
+  int fd;
+
+  /* This is only useful for the topmost builder and points to the top
+   * of the builder stack. Public APIs take the topmost builder reference
+   * and use this to find the currently active builder */
+  OtVariantBuilderInfo *head;
+};
+
+static OtVariantBuilderInfo *
+ot_variant_builder_info_new (OtVariantBuilder *builder, const GVariantType *type)
+{
+  OtVariantBuilderInfo *info;
+
+  info = (OtVariantBuilderInfo *) g_slice_new0 (OtVariantBuilderInfo);
+
+  g_return_val_if_fail (g_variant_type_is_container (type), NULL);
+
+  info->builder = builder;
+  info->type = g_variant_type_copy (type);
+  info->type_info = g_variant_type_info_get (type);
+  info->offset = 0;
+  info->n_children = 0;
+  info->child_ends = g_array_new (FALSE, TRUE, sizeof (guint64));
+
+  switch (*(const gchar *) type)
+    {
+    case G_VARIANT_CLASS_VARIANT:
+      info->uniform_item_types = TRUE;
+      info->expected_type = NULL;
+      info->min_items = 1;
+      info->max_items = 1;
+      break;
+
+    case G_VARIANT_CLASS_ARRAY:
+      info->uniform_item_types = TRUE;
+      info->expected_type =
+        g_variant_type_element (info->type);
+      info->min_items = 0;
+      info->max_items = -1;
+      break;
+
+    case G_VARIANT_CLASS_MAYBE:
+      info->uniform_item_types = TRUE;
+      info->expected_type =
+        g_variant_type_element (info->type);
+      info->min_items = 0;
+      info->max_items = 1;
+      break;
+
+    case G_VARIANT_CLASS_DICT_ENTRY:
+      info->uniform_item_types = FALSE;
+      info->expected_type =
+        g_variant_type_key (info->type);
+      info->min_items = 2;
+      info->max_items = 2;
+      break;
+
+    case 'r': /* G_VARIANT_TYPE_TUPLE was given */
+      info->uniform_item_types = FALSE;
+      info->expected_type = NULL;
+      info->min_items = 0;
+      info->max_items = -1;
+      break;
+
+    case G_VARIANT_CLASS_TUPLE: /* a definite tuple type was given */
+      info->expected_type =
+        g_variant_type_first (info->type);
+      info->min_items = g_variant_type_n_items (type);
+      info->max_items = info->min_items;
+      info->uniform_item_types = FALSE;
+      break;
+
+    default:
+      g_assert_not_reached ();
+   }
+
+  return info;
+}
+
+static void
+ot_variant_builder_info_free (OtVariantBuilderInfo *info)
+{
+  if (info->parent)
+    ot_variant_builder_info_free (info);
+
+  g_variant_type_free (info->type);
+  g_array_unref (info->child_ends);
+  g_free (info->prev_item_type_base);
+
+  g_slice_free (OtVariantBuilderInfo, info);
+}
+
+OtVariantBuilder *
+ot_variant_builder_new (const GVariantType *type,
+                        int fd)
+{
+  OtVariantBuilder *builder;
+
+  builder = (OtVariantBuilder *) g_slice_new0 (OtVariantBuilder);
+
+  g_return_val_if_fail (g_variant_type_is_container (type), NULL);
+
+  builder->head = ot_variant_builder_info_new (builder, type);
+  builder->ref_count = 1;
+  builder->fd = fd;
+
+  return builder;
+}
+
+void
+ot_variant_builder_unref (OtVariantBuilder *builder)
+{
+  if (--builder->ref_count)
+    return;
+
+  ot_variant_builder_info_free (builder->head);
+
+  g_slice_free (OtVariantBuilder, builder);
+}
+
+OtVariantBuilder *
+ot_variant_builder_ref (OtVariantBuilder *builder)
+{
+  builder->ref_count++;
+  return builder;
+}
+
+/* This is called before adding a child to the container.  It updates
+   the internal state and does the needed alignment */
+static gboolean
+ot_variant_builder_pre_add (OtVariantBuilderInfo *info,
+                            const GVariantType *type,
+                            GError         **error)
+{
+  guint alignment = 0;
+
+  if (!info->uniform_item_types)
+    {
+      /* advance our expected type pointers */
+      if (info->expected_type)
+        info->expected_type =
+          g_variant_type_next (info->expected_type);
+
+      if (info->prev_item_type)
+        info->prev_item_type =
+          g_variant_type_next (info->prev_item_type);
+    }
+  else
+    {
+      g_free (info->prev_item_type_base);
+      info->prev_item_type_base = (GVariantType *)g_strdup ((char *)type);
+      info->prev_item_type = info->prev_item_type_base;
+    }
+
+  if (g_variant_type_is_tuple (info->type) ||
+      g_variant_type_is_dict_entry (info->type))
+    {
+      const GVariantMemberInfo *member_info;
+
+      member_info = g_variant_type_info_member_info (info->type_info, info->n_children);
+      alignment = member_info->type_info->alignment;
+    }
+  else if (g_variant_type_is_array (info->type))
+    {
+      GVariantTypeInfo *element_info = g_variant_type_info_element (info->type_info);
+
+      alignment = element_info->alignment;
+    }
+  else if (g_variant_type_is_variant (info->type))
+    {
+      alignment = info->type_info->alignment;
+    }
+  else
+    return glnx_throw (error, "adding to type %s not supported", (char *)info->type);
+
+  while (info->offset & alignment)
+    {
+      if (glnx_loop_write (info->builder->fd, "\0", 1) < 0)
+        return glnx_throw_errno (error);
+      info->offset++;
+    }
+
+  return TRUE;
+}
+
+static void
+ot_variant_builder_add_child_end (OtVariantBuilderInfo *info)
+{
+  guint64 v = info->offset;
+  g_array_append_val (info->child_ends, v);
+}
+
+/* This is called after adding a child to the container. It
+   updates offset, n_children and keeps track of an offset
+   table if needed */
+
+static gboolean
+ot_variant_builder_post_add (OtVariantBuilderInfo *info,
+                             const GVariantType *type,
+                             guint64 bytes_added,
+                             GError         **error)
+{
+  info->offset += bytes_added;
+
+  if (g_variant_type_is_tuple (info->type) ||
+      g_variant_type_is_dict_entry (info->type))
+    {
+      const GVariantMemberInfo *member_info;
+
+      member_info = g_variant_type_info_member_info (info->type_info, info->n_children);
+      if (member_info->ending_type == G_VARIANT_MEMBER_ENDING_OFFSET)
+        ot_variant_builder_add_child_end (info);
+    }
+  else if (g_variant_type_is_array (info->type))
+    {
+      GVariantTypeInfo *element_info = g_variant_type_info_element (info->type_info);
+
+      if (!element_info->fixed_size)
+        ot_variant_builder_add_child_end (info);
+    }
+  else if (g_variant_type_is_variant (info->type))
+    {
+      /* Zero separate */
+      if (glnx_loop_write (info->builder->fd, "\0", 1) < 0)
+        return glnx_throw_errno (error);
+
+      if (glnx_loop_write (info->builder->fd, (char *)type, strlen ((char *)type)) < 0)
+        return glnx_throw_errno (error);
+
+      info->offset += 1 + strlen ((char *)type);
+    }
+  else
+    return glnx_throw (error, "adding to type %s not supported", (char *)info->type);
+
+  info->n_children++;
+
+  return TRUE;
+}
+
+gboolean
+ot_variant_builder_add_from_fd (OtVariantBuilder    *builder,
+                                const GVariantType  *type,
+                                int                  fd,
+                                guint64              size,
+                                GError             **error)
+{
+  OtVariantBuilderInfo *info = builder->head;
+
+  g_return_val_if_fail (info->n_children < info->max_items,
+                        FALSE);
+  g_return_val_if_fail (!info->expected_type ||
+                        g_variant_type_is_subtype_of (type,
+                                                      info->expected_type),
+                        FALSE);
+  g_return_val_if_fail (!info->prev_item_type ||
+                        g_variant_type_is_subtype_of (info->prev_item_type,
+                                                      type),
+                        FALSE);
+
+  if (!ot_variant_builder_pre_add (info, type, error))
+    return FALSE;
+
+  if (glnx_regfile_copy_bytes (fd, builder->fd, size) < 0)
+    return glnx_throw_errno (error);
+
+  if (!ot_variant_builder_post_add (info, type, size, error))
+    return FALSE;
+
+  return TRUE;
+}
+
+gboolean
+ot_variant_builder_add_value (OtVariantBuilder *builder,
+                              GVariant        *value,
+                              GError         **error)
+{
+  OtVariantBuilderInfo *info = builder->head;
+  gconstpointer data;
+  gsize data_size;
+  /* We ref-sink value, just like g_variant_builder_add_value does */
+  g_autoptr(GVariant) keep_around_until_return G_GNUC_UNUSED = g_variant_ref_sink (value);
+
+  g_return_val_if_fail (info->n_children < info->max_items,
+                        FALSE);
+  g_return_val_if_fail (!info->expected_type ||
+                        g_variant_is_of_type (value,
+                                              info->expected_type),
+                        FALSE);
+  g_return_val_if_fail (!info->prev_item_type ||
+                        g_variant_is_of_type (value,
+                                              info->prev_item_type),
+                        FALSE);
+
+  if (!ot_variant_builder_pre_add (info, g_variant_get_type (value), error))
+    return FALSE;
+
+  data = g_variant_get_data (value);
+  data_size = g_variant_get_size (value);
+
+  if (data)
+    {
+      if (glnx_loop_write (builder->fd, data, data_size) < 0)
+        return glnx_throw_errno (error);
+    }
+
+  if (!ot_variant_builder_post_add (info, g_variant_get_type (value), data_size, error))
+    return FALSE;
+
+  return TRUE;
+}
+
+gboolean
+ot_variant_builder_add (OtVariantBuilder *builder,
+                        GError          **error,
+                        const gchar     *format_string,
+                        ...)
+{
+  GVariant *variant;
+  va_list ap;
+
+  va_start (ap, format_string);
+  variant = g_variant_new_va (format_string, NULL, &ap);
+  va_end (ap);
+
+  return ot_variant_builder_add_value (builder, variant, error);
+}
+
+
+gboolean
+ot_variant_builder_open (OtVariantBuilder *builder,
+                         const GVariantType *type,
+                         GError **error)
+{
+  OtVariantBuilderInfo *info = builder->head;
+  OtVariantBuilderInfo *new_info;
+
+  g_return_val_if_fail (info->n_children < info->max_items,
+                        FALSE);
+  g_return_val_if_fail (!info->expected_type ||
+                        g_variant_type_is_subtype_of (type,
+                                                      info->expected_type),
+                        FALSE);
+  g_return_val_if_fail (!info->prev_item_type ||
+                        g_variant_type_is_subtype_of (info->prev_item_type,
+                                                      type),
+                        FALSE);
+
+  if (!ot_variant_builder_pre_add (info, type, error))
+    return FALSE;
+
+  new_info = ot_variant_builder_info_new (builder, type);
+  new_info->parent = info;
+
+  /* push the prev_item_type down into the subcontainer */
+  if (info->prev_item_type)
+    {
+      if (!new_info->uniform_item_types)
+        /* tuples and dict entries */
+        new_info->prev_item_type =
+          g_variant_type_first (info->prev_item_type);
+
+      else if (!g_variant_type_is_variant (new_info->type))
+        /* maybes and arrays */
+        new_info->prev_item_type =
+          g_variant_type_element (info->prev_item_type);
+    }
+
+  builder->head = new_info;
+  return TRUE;
+}
+
+gboolean
+ot_variant_builder_close (OtVariantBuilder *builder,
+                          GError **error)
+{
+  OtVariantBuilderInfo *info = builder->head;
+  OtVariantBuilderInfo *parent;
+
+  g_return_val_if_fail (info->parent != NULL, FALSE);
+
+  if (!ot_variant_builder_end (builder, error))
+    return FALSE;
+
+  parent = info->parent;
+
+  if (!ot_variant_builder_post_add (parent, info->type, info->offset, error))
+    return FALSE;
+
+  builder->head = parent;
+
+  info->parent = NULL;
+  ot_variant_builder_info_free (info);
+
+  return TRUE;
+}
+
+gboolean
+ot_variant_builder_end (OtVariantBuilder *builder,
+                        GError **error)
+{
+  OtVariantBuilderInfo *info = builder->head;
+  gsize total_size;
+  gsize offset_size;
+  int i;
+  g_autofree guchar *offset_table = NULL;
+  gsize offset_table_size;
+  gboolean add_offset_table = FALSE;
+  gboolean reverse_offset_table = FALSE;
+  guchar *p;
+
+  g_return_val_if_fail (info->n_children >= info->min_items,
+                        FALSE);
+  g_return_val_if_fail (!info->uniform_item_types ||
+                        info->prev_item_type != NULL ||
+                        g_variant_type_is_definite (info->type),
+                        FALSE);
+
+  if (g_variant_type_is_tuple (info->type) ||
+      g_variant_type_is_dict_entry (info->type))
+    {
+      add_offset_table = TRUE;
+      reverse_offset_table = TRUE;
+    }
+  else if (g_variant_type_is_array (info->type))
+    {
+      GVariantTypeInfo *element_info = g_variant_type_info_element (info->type_info);
+
+      if (!element_info->fixed_size)
+        add_offset_table = TRUE;
+    }
+  else if (g_variant_type_is_variant (info->type))
+    {
+      /* noop */
+    }
+  else
+    return glnx_throw (error, "closing type %s not supported", (char *)info->type);
+
+  if (add_offset_table)
+    {
+      total_size = gvs_calculate_total_size (info->offset, info->child_ends->len);
+      offset_size = gvs_get_offset_size (total_size);
+
+      offset_table_size = total_size - info->offset;
+      offset_table = g_malloc (offset_table_size);
+      p = offset_table;
+      if (reverse_offset_table)
+        {
+          for (i = info->child_ends->len - 1; i >= 0; i--)
+            {
+              gvs_write_unaligned_le (p, g_array_index (info->child_ends, guint64, i), offset_size);
+              p += offset_size;
+            }
+        }
+      else
+        {
+          for (i = 0; i < info->child_ends->len; i++)
+            {
+              gvs_write_unaligned_le (p, g_array_index (info->child_ends, guint64, i), offset_size);
+              p += offset_size;
+            }
+        }
+
+      if (glnx_loop_write (builder->fd, offset_table, offset_table_size) < 0)
+        return glnx_throw_errno (error);
+
+      info->offset += offset_table_size;
+    }
+  else
+    g_assert (info->child_ends->len == 0);
+
+  return TRUE;
+}
diff --git a/src/libotutil/ot-variant-builder.h b/src/libotutil/ot-variant-builder.h
new file mode 100644
index 00000000..684bd8c4
--- /dev/null
+++ b/src/libotutil/ot-variant-builder.h
@@ -0,0 +1,61 @@
+/*
+ * Copyright (C) 2017 Alexander Larsson <alexl@redhat.com>.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ *
+ * Author: Alexander Larsson <alexl@redhat.com>.
+ */
+
+#pragma once
+
+#include <gio/gio.h>
+
+#include "libglnx.h"
+
+G_BEGIN_DECLS
+
+typedef struct _OtVariantBuilder OtVariantBuilder;
+
+OtVariantBuilder *ot_variant_builder_new         (const GVariantType  *type,
+                                                  int                  fd);
+void              ot_variant_builder_unref       (OtVariantBuilder    *builder);
+OtVariantBuilder *ot_variant_builder_ref         (OtVariantBuilder    *builder);
+gboolean          ot_variant_builder_end         (OtVariantBuilder    *builder,
+                                                  GError             **error);
+gboolean          ot_variant_builder_open        (OtVariantBuilder    *builder,
+                                                  const GVariantType  *type,
+                                                  GError             **error);
+gboolean          ot_variant_builder_close       (OtVariantBuilder    *builder,
+                                                  GError             **error);
+gboolean          ot_variant_builder_add_from_fd (OtVariantBuilder    *builder,
+                                                  const GVariantType  *type,
+                                                  int                  fd,
+                                                  guint64              size,
+                                                  GError             **error);
+gboolean          ot_variant_builder_add_value   (OtVariantBuilder    *builder,
+                                                  GVariant            *value,
+                                                  GError             **error);
+gboolean          ot_variant_builder_add         (OtVariantBuilder    *builder,
+                                                  GError             **error,
+                                                  const gchar         *format_string,
+                                                  ...);
+void              ot_variant_builder_add_parsed  (OtVariantBuilder    *builder,
+                                                  const gchar         *format,
+                                                  ...);
+
+G_DEFINE_AUTOPTR_CLEANUP_FUNC (OtVariantBuilder, ot_variant_builder_unref)
+
+G_END_DECLS
diff --git a/src/libotutil/otutil.h b/src/libotutil/otutil.h
index 80acb475..05666d2c 100644
--- a/src/libotutil/otutil.h
+++ b/src/libotutil/otutil.h
@@ -48,6 +48,7 @@
 #include <ot-opt-utils.h>
 #include <ot-unix-utils.h>
 #include <ot-variant-utils.h>
+#include <ot-variant-builder.h>
 #include <ot-checksum-utils.h>
 #include <ot-gpg-utils.h>
 #include <ot-checksum-instream.h>