xentrace: correct overflow check for number of per-cpu trace pages

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.  Check both max theoretical number of pages
per cpu and max number of pages reachable by struct t_buf->prod/cons
variables with requested number of pages.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>

diff --git a/xen/common/trace.c b/xen/common/trace.c
index 373a3dc022078d8ff5fc9f33d4816d7e8b41a772..3ed6f171ddb0dffe5168ae4d7df2b14b22b1641a 100644 (file)
--- a/xen/common/trace.c
+++ b/xen/common/trace.c
@@ -104,25 +104,33 @@ static void calc_tinfo_first_offset(void)
  * 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.
+ * 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_buf dummy_size;
+    typeof(dummy_size.prod) max_size;
+    struct t_info dummy_pages;
+    typeof(dummy_pages.tbuf_size) max_pages;
     unsigned int t_info_words;
 
     /* force maximum value for an unsigned type */
-    size = -1;
+    max_size = -1;
+    max_pages = -1;
 
     /* max size holds up to n pages */
-    size /= PAGE_SIZE;
-    if ( pages > size )
+    max_size /= PAGE_SIZE;
+
+    if ( max_size < max_pages )
+        max_pages = max_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);