[PATCH]: xl: don't free string literals

[PATCH]: xl: don't free string literals

[Gianni Tedesco]

The function init_dm_info() is initialising some strings from literals.
This is bad juju because when the destructor is called we cannot know if
the string literal was overridden with a strdup()'d value. Therefore
strdup the initialisers in init_dm_info() and unconditionally free them
before assigning non-default values to prevent their leakage.

Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>

diff -r 5f53805b349e -r e954e6b6d311 tools/libxl/xl_cmdimpl.c
--- a/tools/libxl/xl_cmdimpl.c	Fri Sep 03 18:44:49 2010 +0100
+++ b/tools/libxl/xl_cmdimpl.c	Mon Sep 06 13:13:46 2010 +0100
@@ -294,7 +294,7 @@ static void init_dm_info(libxl_device_mo
     libxl_uuid_generate(&dm_info->uuid);
 
     dm_info->dom_name = c_info->name;
-    dm_info->device_model = "qemu-dm";
+    dm_info->device_model = strdup("qemu-dm");
     dm_info->videoram = b_info->video_memkb / 1024;
     dm_info->apic = b_info->u.hvm.apic;
     dm_info->vcpus = b_info->max_vcpus;
@@ -302,7 +302,7 @@ static void init_dm_info(libxl_device_mo
 
     dm_info->stdvga = 0;
     dm_info->vnc = 1;
-    dm_info->vnclisten = "127.0.0.1";
+    dm_info->vnclisten = strdup("127.0.0.1");
     dm_info->vncdisplay = 0;
     dm_info->vncunused = 1;
     dm_info->keymap = NULL;
@@ -310,7 +310,7 @@ static void init_dm_info(libxl_device_mo
     dm_info->opengl = 0;
     dm_info->nographic = 0;
     dm_info->serial = NULL;
-    dm_info->boot = "cda";
+    dm_info->boot = strdup("cda");
     dm_info->usb = 0;
     dm_info->usbdevice = NULL;
     dm_info->xen_platform_pci = 1;
@@ -1019,38 +1019,54 @@ skip_vfb:
         init_dm_info(dm_info, c_info, b_info);
 
         /* then process config related to dm */
-        if (!xlu_cfg_get_string (config, "device_model", &buf))
+        if (!xlu_cfg_get_string (config, "device_model", &buf)) {
+            free(dm_info->device_model);
             dm_info->device_model = strdup(buf);
+        }
         if (!xlu_cfg_get_long (config, "stdvga", &l))
             dm_info->stdvga = l;
         if (!xlu_cfg_get_long (config, "vnc", &l))
             dm_info->vnc = l;
-        if (!xlu_cfg_get_string (config, "vnclisten", &buf))
+        if (!xlu_cfg_get_string (config, "vnclisten", &buf)) {
+            free(dm_info->vnclisten);
             dm_info->vnclisten = strdup(buf);
-        if (!xlu_cfg_get_string (config, "vncpasswd", &buf))
+        }
+        if (!xlu_cfg_get_string (config, "vncpasswd", &buf)) {
+            free(dm_info->vncpasswd);
             dm_info->vncpasswd = strdup(buf);
+        }
         if (!xlu_cfg_get_long (config, "vncdisplay", &l))
             dm_info->vncdisplay = l;
         if (!xlu_cfg_get_long (config, "vncunused", &l))
             dm_info->vncunused = l;
-        if (!xlu_cfg_get_string (config, "keymap", &buf))
+        if (!xlu_cfg_get_string (config, "keymap", &buf)) {
+            free(dm_info->keymap);
             dm_info->keymap = strdup(buf);
+        }
         if (!xlu_cfg_get_long (config, "sdl", &l))
             dm_info->sdl = l;
         if (!xlu_cfg_get_long (config, "opengl", &l))
             dm_info->opengl = l;
         if (!xlu_cfg_get_long (config, "nographic", &l))
             dm_info->nographic = l;
-        if (!xlu_cfg_get_string (config, "serial", &buf))
+        if (!xlu_cfg_get_string (config, "serial", &buf)) {
+            free(dm_info->serial);
             dm_info->serial = strdup(buf);
-        if (!xlu_cfg_get_string (config, "boot", &buf))
+        }
+        if (!xlu_cfg_get_string (config, "boot", &buf)) {
+            free(dm_info->boot);
             dm_info->boot = strdup(buf);
+        }
         if (!xlu_cfg_get_long (config, "usb", &l))
             dm_info->usb = l;
-        if (!xlu_cfg_get_string (config, "usbdevice", &buf))
+        if (!xlu_cfg_get_string (config, "usbdevice", &buf)) {
+            free(dm_info->usbdevice);
             dm_info->usbdevice = strdup(buf);
-        if (!xlu_cfg_get_string (config, "soundhw", &buf))
+        }
+        if (!xlu_cfg_get_string (config, "soundhw", &buf)) {
+            free(dm_info->soundhw);
             dm_info->soundhw = strdup(buf);
+        }
         if (!xlu_cfg_get_long (config, "xen_platform_pci", &l))
             dm_info->xen_platform_pci = l;
     }

Re: [PATCH]: xl: don't free string literals

[Ian Jackson]

Gianni Tedesco writes ("[PATCH]: xl: don't free string literals"):
> The function init_dm_info() is initialising some strings from literals.
> This is bad juju because when the destructor is called we cannot know if
> the string literal was overridden with a strdup()'d value. Therefore
> strdup the initialisers in init_dm_info() and unconditionally free them
> before assigning non-default values to prevent their leakage.

Can't we replace the dozen copies of this

> -        if (!xlu_cfg_get_string (config, "device_model", &buf))
> +        if (!xlu_cfg_get_string (config, "device_model", &buf)) {
> +            free(dm_info->device_model);
>              dm_info->device_model = strdup(buf);
> +        }

with something like

> -        if (!xlu_cfg_get_string (config, "device_model", &buf))
> +        if (!xlu_cfg_get_string_mallocd (config, "device_model", &buf)) {

or whatever you want to call the function ?  Or just change the
definition of xlu_cfg_get_string and declare that it always frees its
argument if it's not 0 to start with.

Ian.

Re: [PATCH]: xl: don't free string literals

[Gianni Tedesco]

On Tue, 2010-09-07 at 19:02 +0100, Ian Jackson wrote:
> Gianni Tedesco writes ("[PATCH]: xl: don't free string literals"):
> > The function init_dm_info() is initialising some strings from literals.
> > This is bad juju because when the destructor is called we cannot know if
> > the string literal was overridden with a strdup()'d value. Therefore
> > strdup the initialisers in init_dm_info() and unconditionally free them
> > before assigning non-default values to prevent their leakage.
> 
> Can't we replace the dozen copies of this
> 
> > -        if (!xlu_cfg_get_string (config, "device_model", &buf))
> > +        if (!xlu_cfg_get_string (config, "device_model", &buf)) {
> > +            free(dm_info->device_model);
> >              dm_info->device_model = strdup(buf);
> > +        }
> 
> with something like
> 
> > -        if (!xlu_cfg_get_string (config, "device_model", &buf))
> > +        if (!xlu_cfg_get_string_mallocd (config, "device_model", &buf)) {
> 
> or whatever you want to call the function ?  Or just change the
> definition of xlu_cfg_get_string and declare that it always frees its
> argument if it's not 0 to start with.
> 
> Ian.

Absolutely, I think I'll just change xlu_cfg_get_string() everywhere.
Don't see a need to have two versions of a function and a non-obvious
rule about which to use where. Will hack that up and re-send later.

Gianni