[Xen-devel] Re: [PATCH 2 of 3] xentrace: use tbuf_size for overflow chec
On Fri, Apr 01, George Dunlap wrote:
> On Wed, 2011-03-30 at 19:04 +0100, Olaf Hering wrote:
> > # HG changeset patch
> > # User Olaf Hering <olaf@xxxxxxxxx>
> > # 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.
> Hmm -- while this is true, it's possible this may change in the future.
> If we ever changed t_info.tbuf_size to be u32, then t_buf.prod/cons
> would again be the limiting factor.
> Should we perhaps add both checks?
I will test the change below.
diff -r 2b66b83b19b6 xen/common/trace.c
--- a/xen/common/trace.c Thu Apr 07 12:13:58 2011 +0100
+++ b/xen/common/trace.c Thu Apr 07 15:45:21 2011 +0200
@@ -104,25 +104,33 @@
* 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);
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