[PATCH 2 of 3] xentrace: use tbuf_size for overflow check

[Olaf Hering <olaf@aepfle.de>]

# HG changeset patch
# User Olaf Hering <olaf@aepfle.de>
# Date 1301424075 -7200
# Node ID 3e95e737bc51c2295926e4f23389b1cb161d6d7b
# Parent  8a2ce5e49b2c5f2e013734b5d53eae37572f4101
xentrace: use tbuf_size for overflow check

The calculated number of per-cpu trace pages is stored in t_info and
shared with tools like xentrace. Since its an u16 the value may overflow
because the current check is based on u32.
Using the u16 means each cpu could in theory use up to 256MB as trace
buffer. However such a large allocation will currently fail on x86 due
to the MAX_ORDER limit.

Signed-off-by: Olaf Hering <olaf@aepfle.de>

diff -r 8a2ce5e49b2c -r 3e95e737bc51 xen/common/trace.c
--- a/xen/common/trace.c	Tue Mar 29 20:37:20 2011 +0200
+++ b/xen/common/trace.c	Tue Mar 29 20:41:15 2011 +0200
@@ -104,25 +104,26 @@
  * calculate_tbuf_size - check to make sure that the proposed size will fit
  * in the currently sized struct t_info and allows prod and cons to
  * reach double the value without overflow.
+ * tbuf_size is u16, while t_buf prod/cons are u32, so using tbuf_size for
+ * overflow check is good.
+ * The t_info layout is fixed and cant be changed without breaking xentrace.
  * Initialize t_info_pages based on number of trace pages.
  */
 static int calculate_tbuf_size(unsigned int pages)
 {
-    struct t_buf dummy;
-    typeof(dummy.prod) size;
+    struct t_info dummy;
+    typeof(dummy.tbuf_size) max_pages;
     unsigned int t_info_words;
 
     /* force maximum value for an unsigned type */
-    size = -1;
+    max_pages = -1;
 
-    /* max size holds up to n pages */
-    size /= PAGE_SIZE;
-    if ( pages > size )
+    if ( pages > max_pages )
     {
         printk(XENLOG_INFO "xentrace: requested number of %u pages "
                "reduced to %u\n",
-               pages, (unsigned int)size);
-        pages = size;
+               pages, max_pages);
+        pages = max_pages;
     }
 
     t_info_words = num_online_cpus() * pages * sizeof(uint32_t);