[Xen-devel] [PATCH] xentrace: fix off-by-one in calculate_tbuf_size

[Olaf Hering <olaf@xxxxxxxxx>]

# HG changeset patch
# User Olaf Hering <olaf@xxxxxxxxx>
# Date 1362151425 -3600
# Node ID 13c43170cd5f653d9590916df82f60597d1d5326
# Parent  3eb62c576a1a315b9f8655a186ead470abe69b31
xentrace: fix off-by-one in calculate_tbuf_size

Commit "xentrace: reduce trace buffer size to something mfn_offset can
reach" contains an off-by-one bug. max_mfn_offset needs to be reduced by
exactly the value of t_info_first_offset.

If the system has two cpus and the number of requested trace pages is
very large, the final number of trace pages + the offset will not fit
into a short. As a result the variable offset in alloc_trace_bufs() will
wrap while allocating buffers for the second cpu. Later
share_xen_page_with_privileged_guests() will be called with a wrong page
and the ASSERT in this function triggers. If the ASSERT is ignored by
running a non-dbg hypervisor the asserts in xentrace itself trigger
because "cons" is not aligned because the very last trace page for the
second cpu is a random mfn.

Thanks to Jan for the quick analysis.


Signed-off-by: Olaf Hering <olaf@xxxxxxxxx>

diff -r 3eb62c576a1a -r 13c43170cd5f xen/common/trace.c
--- a/xen/common/trace.c
+++ b/xen/common/trace.c
@@ -133,7 +133,7 @@ static int calculate_tbuf_size(unsigned 
      * The array of mfns for the highest cpu can start at the maximum value
      * mfn_offset can hold. So reduce the number of cpus and also the 
mfn_offset.
      */
-    max_mfn_offset -= t_info_first_offset - 1;
+    max_mfn_offset -= t_info_first_offset;
     max_cpus--;
     if ( max_cpus )
         max_mfn_offset /= max_cpus;

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