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59 lines
2.2 KiB
59 lines
2.2 KiB
From 76c089d06f5ff8dc7a54c3e5ef7d2f1447ca8ec4 Mon Sep 17 00:00:00 2001
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From: =?UTF-8?q?Marek=20Marczykowski-G=C3=B3recki?=
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<marmarek@invisiblethingslab.com>
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Date: Tue, 15 Dec 2015 21:35:14 +0100
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Subject: [PATCH] xen: Add RING_COPY_RESPONSE()
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MIME-Version: 1.0
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Content-Type: text/plain; charset=UTF-8
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Content-Transfer-Encoding: 8bit
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Using RING_GET_RESPONSE() 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 response
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generally requires taking a local copy.
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Provide a RING_COPY_RESPONSE() macro to use instead of
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RING_GET_RESPONSE() 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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CC: stable@vger.kernel.org
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Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
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---
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include/xen/interface/io/ring.h | 14 ++++++++++++++
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1 file changed, 14 insertions(+)
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diff --git a/include/xen/interface/io/ring.h b/include/xen/interface/io/ring.h
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index 3f40501fc60b..03702f6874df 100644
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--- a/include/xen/interface/io/ring.h
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+++ b/include/xen/interface/io/ring.h
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@@ -201,6 +201,20 @@ struct __name##_back_ring { \
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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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+ * Get a local copy of a response.
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+ *
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+ * Use this in preference to RING_GET_RESPONSE() 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 _rsp is a struct which consists of only bitfields.
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+ */
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+#define RING_COPY_RESPONSE(_r, _idx, _rsp) do { \
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+ /* Use volatile to force the copy into _rsp. */ \
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+ *(_rsp) = *(volatile typeof(_rsp))RING_GET_RESPONSE(_r, _idx); \
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+} while (0)
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+
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/* Loop termination condition: Would the specified index overflow the ring? */
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#define RING_REQUEST_CONS_OVERFLOW(_r, _cons) \
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(((_cons) - (_r)->rsp_prod_pvt) >= RING_SIZE(_r))
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--
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2.21.0
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