[PATCH 08/13 v4] libxl: don't leak ptr in libxl_list_vm error case

[Matthew Daley <mattd@bugfuzz.com>]

While at it, tidy up the function; there's no point in allocating more
than the amount of domains actually returned by xc_domain_getinfolist
(unless 0 domains are returned, in which case we should still allocate
one libxl_vminfo struct so we can return a non-NULL result and not
appear to have failed from the caller's perspective.)

Coverity-ID: 1055888
Signed-off-by: Matthew Daley <mattd@bugfuzz.com>
---
v4: Add a comment describing the calloc malarkey

 tools/libxl/libxl.c |   19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
index b112294..7308d44 100644
--- a/tools/libxl/libxl.c
+++ b/tools/libxl/libxl.c
@@ -674,17 +674,22 @@ libxl_vminfo * libxl_list_vm(libxl_ctx *ctx, int *nb_vm_out)
     libxl_vminfo *ptr;
     int idx, i, ret;
     xc_domaininfo_t info[1024];
-    int size = 1024;
 
-    ptr = calloc(size, sizeof(libxl_vminfo));
-    if (!ptr)
+    ret = xc_domain_getinfolist(ctx->xch, 1, ARRAY_SIZE(info), info);
+    if (ret < 0) {
+        LIBXL__LOG_ERRNO(ctx, LIBXL__LOG_ERROR, "getting domain info list");
         return NULL;
+    }
 
-    ret = xc_domain_getinfolist(ctx->xch, 1, 1024, info);
-    if (ret<0) {
-        LIBXL__LOG_ERRNO(ctx, LIBXL__LOG_ERROR, "geting domain info list");
+    /*
+     * Always make sure to allocate at least one element; if we don't and we
+     * request zero, we (might) get back a null pointer, which if returned
+     * to our caller will make them think we've failed
+     */
+    ptr = calloc(ret ? ret : 1, sizeof(libxl_vminfo));
+    if (!ptr)
         return NULL;
-    }
+
     for (idx = i = 0; i < ret; i++) {
         if (libxl_is_stubdom(ctx, info[i].domain, NULL))
             continue;
-- 
1.7.10.4