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From c1fce65e2b720684ea6ba76ae59921542bd154bb Mon Sep 17 00:00:00 2001
From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Date: Fri, 20 Nov 2015 12:22:14 -0500
Subject: [PATCH 3/3] libvchan: Read prod/cons only once.
We must ensure that the prod/cons are only read once and that
the compiler won't try to optimize the reads. That is split
the read of these in multiple instructions influencing later
branch code. As such insert barriers when fetching the cons
and prod index.
This is part of XSA155.
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
---
tools/libvchan/io.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/tools/libvchan/io.c b/tools/libvchan/io.c
index 8a9629b..381cc05 100644
--- a/tools/libvchan/io.c
+++ b/tools/libvchan/io.c
@@ -117,6 +117,7 @@ static inline int send_notify(struct libxenvchan *ctrl, uint8_t bit)
static inline int raw_get_data_ready(struct libxenvchan *ctrl)
{
uint32_t ready = rd_prod(ctrl) - rd_cons(ctrl);
+ xen_mb(); /* Ensure 'ready' is read only once. */
if (ready > rd_ring_size(ctrl))
/* We have no way to return errors. Locking up the ring is
* better than the alternatives. */
@@ -158,6 +159,7 @@ int libxenvchan_data_ready(struct libxenvchan *ctrl)
static inline int raw_get_buffer_space(struct libxenvchan *ctrl)
{
uint32_t ready = wr_ring_size(ctrl) - (wr_prod(ctrl) - wr_cons(ctrl));
+ xen_mb(); /* Ensure 'ready' is read only once. */
if (ready > wr_ring_size(ctrl))
/* We have no way to return errors. Locking up the ring is
* better than the alternatives. */
--
2.1.0
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