[PATCH v2] libxl: don't segfault when creating domain with invalid pvusb device

[Juergen Gross <jgross@suse.com>]

Creating a domain with an invalid controller specification for a pvusb
device will currently segfault.

Avoid this by bailing out early in case of a mandatory xenstore path
not existing.

Signed-of-by: Juergen Gross <jgross@suse.com>
---
This patch is a backport candidate for 4.8

V2: introduce libxl__xs_read_mandatory() as suggested by Ian Jackson
---
 tools/libxl/libxl_internal.h |  6 ++++++
 tools/libxl/libxl_usb.c      |  6 +++---
 tools/libxl/libxl_xshelp.c   | 12 ++++++++++++
 3 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/tools/libxl/libxl_internal.h b/tools/libxl/libxl_internal.h
index 5f46578..d591b79 100644
--- a/tools/libxl/libxl_internal.h
+++ b/tools/libxl/libxl_internal.h
@@ -738,6 +738,12 @@ int libxl__xs_mknod(libxl__gc *gc, xs_transaction_t t,
 
 /* On success, *result_out came from the gc.
  * On error, *result_out is undefined.
+ */
+int libxl__xs_read_mandatory(libxl__gc *gc, xs_transaction_t t,
+                             const char *path, const char **result_out);
+
+/* On success, *result_out came from the gc.
+ * On error, *result_out is undefined.
  * ENOENT counts as success but sets *result_out=0
  */
 int libxl__xs_read_checked(libxl__gc *gc, xs_transaction_t t,
diff --git a/tools/libxl/libxl_usb.c b/tools/libxl/libxl_usb.c
index ea7a2ab..d8948d5 100644
--- a/tools/libxl/libxl_usb.c
+++ b/tools/libxl/libxl_usb.c
@@ -652,9 +652,9 @@ int libxl_device_usbctrl_getinfo(libxl_ctx *ctx, uint32_t domid,
     usbctrlinfo->devid = usbctrl->devid;
 
 #define READ_SUBPATH(path, subpath) ({                                  \
-        rc = libxl__xs_read_checked(gc, XBT_NULL,                       \
-                                    GCSPRINTF("%s/" subpath, path),     \
-                                    &tmp);                              \
+        rc = libxl__xs_read_mandatory(gc, XBT_NULL,                     \
+                                      GCSPRINTF("%s/" subpath, path),   \
+                                      &tmp);                            \
         if (rc) goto out;                                               \
         (char *)tmp;                                                    \
     })
diff --git a/tools/libxl/libxl_xshelp.c b/tools/libxl/libxl_xshelp.c
index a50805f..c4a18df 100644
--- a/tools/libxl/libxl_xshelp.c
+++ b/tools/libxl/libxl_xshelp.c
@@ -193,6 +193,18 @@ char *libxl__xs_libxl_path(libxl__gc *gc, uint32_t domid)
     return s;
 }
 
+int libxl__xs_read_mandatory(libxl__gc *gc, xs_transaction_t t,
+                             const char *path, const char **result_out)
+{
+    char *result = libxl__xs_read(gc, t, path);
+    if (!result) {
+        LOGE(ERROR, "xenstore read failed: `%s'", path);
+        return ERROR_FAIL;
+    }
+    *result_out = result;
+    return 0;
+}
+
 int libxl__xs_read_checked(libxl__gc *gc, xs_transaction_t t,
                            const char *path, const char **result_out)
 {
-- 
2.10.2


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