57 lines
2.0 KiB
Diff
57 lines
2.0 KiB
Diff
From 12b11658a9d6a654a1e7acbf2f2d56ce9a396c86 Mon Sep 17 00:00:00 2001
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From: David Vrabel <david.vrabel@citrix.com>
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Date: Fri, 20 Nov 2015 11:59:05 -0500
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Subject: [PATCH 1/3] xen: Add RING_COPY_REQUEST()
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Using RING_GET_REQUEST() on a shared ring is easy to use incorrectly
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(i.e., by not considering that the other end may alter the data in the
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shared ring while it is being inspected). Safe usage of a request
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generally requires taking a local copy.
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Provide a RING_COPY_REQUEST() macro to use instead of
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RING_GET_REQUEST() and an open-coded memcpy(). This takes care of
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ensuring that the copy is done correctly regardless of any possible
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compiler optimizations.
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Use a volatile source to prevent the compiler from reordering or
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omitting the copy.
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This is part of XSA155.
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Signed-off-by: David Vrabel <david.vrabel@citrix.com>
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Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
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---
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v2: Add comment about GCC bug.
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---
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xen/include/public/io/ring.h | 14 ++++++++++++++
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1 file changed, 14 insertions(+)
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diff --git a/xen/include/public/io/ring.h b/xen/include/public/io/ring.h
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index ba9401b..801c0da 100644
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--- a/xen/include/public/io/ring.h
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+++ b/xen/include/public/io/ring.h
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@@ -212,6 +212,20 @@ typedef struct __name##_back_ring __name##_back_ring_t
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#define RING_GET_REQUEST(_r, _idx) \
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(&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))
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+/*
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+ * Get a local copy of a request.
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+ *
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+ * Use this in preference to RING_GET_REQUEST() so all processing is
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+ * done on a local copy that cannot be modified by the other end.
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+ *
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+ * Note that https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 may cause this
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+ * to be ineffective where _req is a struct which consists of only bitfields.
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+ */
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+#define RING_COPY_REQUEST(_r, _idx, _req) do { \
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+ /* Use volatile to force the copy into _req. */ \
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+ *(_req) = *(volatile typeof(_req))RING_GET_REQUEST(_r, _idx); \
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+} while (0)
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+
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#define RING_GET_RESPONSE(_r, _idx) \
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(&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].rsp))
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--
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2.1.0
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