Xen devel - Sep 2010 - [PATCH]: libxl: don''t ''leak'' gc pointers to caller''s structs thereby preventing a double free

libxl_build_device_model uses a pointer in a caller supplied data
structure to synthesize a vif-name if one is not supplied. This is bad
juju because the caller may want to free this pointer but by the time it
get''s a chance the gc has already done so. Switch to using a local
variable for this pointer and avoid a double-free in the domain create
path.

Gianni Tedesco <gianni.tedesco@citrix.com>

diff -r ef2d0a9b2036 tools/libxl/libxl.c
--- a/tools/libxl/libxl.c	Fri Sep 10 14:59:29 2010 +0100
+++ b/tools/libxl/libxl.c	Fri Sep 10 15:16:41 2010 +0100
@@ -1190,14 +1190,17 @@ static char ** libxl_build_device_model_
                 char *smac = libxl__sprintf(gc,
"%02x:%02x:%02x:%02x:%02x:%02x",
                                            vifs[i].mac[0], vifs[i].mac[1],
vifs[i].mac[2],
                                            vifs[i].mac[3], vifs[i].mac[4],
vifs[i].mac[5]);
+                char *ifname;
                 if (!vifs[i].ifname)
-                    vifs[i].ifname = libxl__sprintf(gc, "tap%d.%d",
info->domid, vifs[i].devid);
+                    ifname = libxl__sprintf(gc, "tap%d.%d",
info->domid, vifs[i].devid);
+                else
+                    ifname = vifs[i].ifname;
                 flexarray_set(dm_args, num++, "-net");
                 flexarray_set(dm_args, num++, libxl__sprintf(gc,
"nic,vlan=%d,macaddr=%s,model=%s",
                             vifs[i].devid, smac, vifs[i].model));
                 flexarray_set(dm_args, num++, "-net");
                 flexarray_set(dm_args, num++, libxl__sprintf(gc,
"tap,vlan=%d,ifname=%s,bridge=%s,script=no",
-                            vifs[i].devid, vifs[i].ifname, vifs[i].bridge));
+                            vifs[i].devid, ifname, vifs[i].bridge));
                 ioemu_vifs++;
             }
         }



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