[Xen-changelog] [xen master] xen: Add RING_COPY_REQUEST()

[patchbot@xxxxxxx]

commit 3f20b8def0f4c0d912f1ffc1893199b6e8820477
Author:     David Vrabel <david.vrabel@xxxxxxxxxx>
AuthorDate: Fri Nov 20 11:59:05 2015 -0500
Commit:     Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
CommitDate: Fri Dec 18 09:49:07 2015 -0500

    xen: Add RING_COPY_REQUEST()
    
    Using RING_GET_REQUEST() on a shared ring is easy to use incorrectly
    (i.e., by not considering that the other end may alter the data in the
    shared ring while it is being inspected).  Safe usage of a request
    generally requires taking a local copy.
    
    Provide a RING_COPY_REQUEST() macro to use instead of
    RING_GET_REQUEST() and an open-coded memcpy().  This takes care of
    ensuring that the copy is done correctly regardless of any possible
    compiler optimizations.
    
    Use a volatile source to prevent the compiler from reordering or
    omitting the copy.
    
    This is part of XSA155.
    
    Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
    Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
---
 xen/include/public/io/ring.h |   14 ++++++++++++++
 1 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/xen/include/public/io/ring.h b/xen/include/public/io/ring.h
index ba9401b..801c0da 100644
--- a/xen/include/public/io/ring.h
+++ b/xen/include/public/io/ring.h
@@ -212,6 +212,20 @@ typedef struct __name##_back_ring __name##_back_ring_t
 #define RING_GET_REQUEST(_r, _idx)                                      \
     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))
 
+/*
+ * Get a local copy of a request.
+ *
+ * Use this in preference to RING_GET_REQUEST() so all processing is
+ * done on a local copy that cannot be modified by the other end.
+ *
+ * Note that https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 may cause this
+ * to be ineffective where _req is a struct which consists of only bitfields.
+ */
+#define RING_COPY_REQUEST(_r, _idx, _req) do {                         \
+       /* Use volatile to force the copy into _req. */                 \
+       *(_req) = *(volatile typeof(_req))RING_GET_REQUEST(_r, _idx);   \
+} while (0)
+
 #define RING_GET_RESPONSE(_r, _idx)                                     \
     (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].rsp))
 
--
generated by git-patchbot for /home/xen/git/xen.git#master

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