qemu_opts_append: Play nicely with QemuOptsList's head

When running a libvirt test suite I've noticed the qemu-img is
crashing occasionally. Tracing the problem down led me to the
following valgrind output:

qemu.git $ valgrind -q ./qemu-img create -f qed -obacking_file=/dev/null,backing_fmt=raw qed
==14881== Invalid write of size 8
==14881==    at 0x1D263F: qemu_opts_create (qemu-option.c:692)
==14881==    by 0x130782: bdrv_img_create (block.c:5531)
==14881==    by 0x118DE0: img_create (qemu-img.c:462)
==14881==    by 0x11E7E4: main (qemu-img.c:2830)
==14881==  Address 0x11fedd38 is 24 bytes inside a block of size 232 free'd
==14881==    at 0x4C2CA5E: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==14881==    by 0x592D35E: g_realloc (in /usr/lib64/libglib-2.0.so.0.3800.2)
==14881==    by 0x1D38D8: qemu_opts_append (qemu-option.c:1129)
==14881==    by 0x13075E: bdrv_img_create (block.c:5528)
==14881==    by 0x118DE0: img_create (qemu-img.c:462)
==14881==    by 0x11E7E4: main (qemu-img.c:2830)
==14881==
Formatting 'qed', fmt=qed size=0 backing_file='/dev/null' backing_fmt='raw' cluster_size=65536
==14881== Invalid write of size 8
==14881==    at 0x1D28BE: qemu_opts_del (qemu-option.c:750)
==14881==    by 0x130BF3: bdrv_img_create (block.c:5638)
==14881==    by 0x118DE0: img_create (qemu-img.c:462)
==14881==    by 0x11E7E4: main (qemu-img.c:2830)
==14881==  Address 0x11fedd38 is 24 bytes inside a block of size 232 free'd
==14881==    at 0x4C2CA5E: realloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==14881==    by 0x592D35E: g_realloc (in /usr/lib64/libglib-2.0.so.0.3800.2)
==14881==    by 0x1D38D8: qemu_opts_append (qemu-option.c:1129)
==14881==    by 0x13075E: bdrv_img_create (block.c:5528)
==14881==    by 0x118DE0: img_create (qemu-img.c:462)
==14881==    by 0x11E7E4: main (qemu-img.c:2830)
==14881==

The problem is apparently in the qemu_opts_append(). Well, if it
gets called twice or more. On the first call, when @dst is NULL
some initialization is done during which @dst->head list gets
initialized. The list is initialized in a way, so that the list
tail points at the list head. However, the next time
qemu_opts_append() is called for new options to be added,
g_realloc() may move @dst to a new address making the old list tail
point at an invalid address. If that's the case, we must update the
list pointers.

Signed-off-by: Michal Privoznik <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>

util/qemu-option.c

@@ -1111,6 +1111,7 @@ QemuOptsList *qemu_opts_append(QemuOptsList *dst,
     size_t num_opts, num_dst_opts;
     QemuOptDesc *desc;
     bool need_init = false;
+    bool need_head_update;
 
     if (!list) {
         return dst;
@@ -1121,6 +1122,12 @@ QemuOptsList *qemu_opts_append(QemuOptsList *dst,
      */
     if (!dst) {
         need_init = true;
+        need_head_update = true;
+    } else {
+        /* Moreover, even if dst is not NULL, the realloc may move it to a
+         * different address in which case we may get a stale tail pointer
+         * in dst->head. */
+        need_head_update = QTAILQ_EMPTY(&dst->head);
     }
 
     num_opts = count_opts_list(dst);
@@ -1131,9 +1138,11 @@ QemuOptsList *qemu_opts_append(QemuOptsList *dst,
     if (need_init) {
         dst->name = NULL;
         dst->implied_opt_name = NULL;
-        QTAILQ_INIT(&dst->head);
         dst->merge_lists = false;
     }
+    if (need_head_update) {
+        QTAILQ_INIT(&dst->head);
+    }
     dst->desc[num_dst_opts].name = NULL;
 
     /* append list->desc to dst->desc */