[Xen-devel] Re: [PATCH 2/2] libxc: Check there's enough memory for segments

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

# HG changeset patch
# User Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
# Date 1265131163 0
# Node ID aeb8a70f42a5cc2cc7b076d128fa446201a79103
# Parent  5fc5ee3e3530d38a21d00b4ec2d559b47a23cf07
libxc: Check there's enough memory for segments we're creating

Previously, xc_dom_alloc_segment would go ahead even if the segment
we're trying to create is too big for the domain's RAM (or the
requested addr is out of range).  It would pass invalid parameters to
xc_dom_seg_to_ptr giving undefined behaviour.

Fixing xc_dom_seg_to_ptr to fail is not sufficient because we want to
provide a comprehensible explanation to the caller - which may
ultimately be the user.

In particular, with this change attempting "xl create" with a ramdisk
image bigger than the guest's specified RAM will provide a useful
error message mentioning the ramdisk.

Signed-off-by: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>

diff -r 5fc5ee3e3530 -r aeb8a70f42a5 tools/libxc/xc_dom_core.c
--- a/tools/libxc/xc_dom_core.c Tue Feb 02 17:16:07 2010 +0000
+++ b/tools/libxc/xc_dom_core.c Tue Feb 02 17:19:23 2010 +0000
@@ -409,8 +409,19 @@
     }
 
     seg->vstart = start;
+    seg->pfn = (seg->vstart - dom->parms.virt_base) / page_size;
+
+    if ( pages > dom->total_pages || /* double test avoids overflow probs */
+         pages > dom->total_pages - seg->pfn)
+    {
+        xc_dom_panic(XC_OUT_OF_MEMORY,
+                     "%s: segment %s too large (0x%"PRIpfn" > "
+                     "0x%"PRIpfn" - 0x%"PRIpfn" pages)\n",
+                     __FUNCTION__, name, pages, dom->total_pages, seg->pfn);
+        return -1;
+    }
+
     seg->vend = start + pages * page_size;
-    seg->pfn = (seg->vstart - dom->parms.virt_base) / page_size;
     dom->virt_alloc_end = seg->vend;
     if (dom->allocate)
         dom->allocate(dom, dom->virt_alloc_end);

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