linux-kernel - [PATCH v2] kcsan: Fix encoding masks and regain address bit

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Message-ID: <20201106093456.GB2851373@elver.google.com>
Date:   Fri, 6 Nov 2020 10:34:56 +0100
From:   Marco Elver <elver@...gle.com>
To:     paulmck@...nel.org, boqun.feng@...il.com
Cc:     linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        kernel-team@...com, mingo@...nel.org, andreyknvl@...gle.com,
        glider@...gle.com, dvyukov@...gle.com, cai@....pw
Subject: [PATCH v2] kcsan: Fix encoding masks and regain address bit

The watchpoint encoding masks for size and address were off-by-one bit
each, with the size mask using 1 unnecessary bit and the address mask
missing 1 bit. However, due to the way the size is shifted into the
encoded watchpoint, we were effectively wasting and never using the
extra bit.

For example, on x86 with PAGE_SIZE==4K, we have 1 bit for the is-write
bit, 14 bits for the size bits, and then 49 bits left for the address.
Prior to this fix we would end up with this usage:

	[ write<1> | size<14> | wasted<1> | address<48> ]

Fix it by subtracting 1 bit from the GENMASK() end and start ranges of
size and address respectively. The added static_assert()s verify that
the masks are as expected. With the fixed version, we get the expected
usage:

	[ write<1> | size<14> |             address<49> ]

Functionally no change is expected, since that extra address bit is
insignificant for enabled architectures.

Signed-off-by: Marco Elver <elver@...gle.com>
---
v2:
* Use WATCHPOINT_ADDR_BITS to avoid duplicating "BITS_PER_LONG-1 -
  WATCHPOINT_SIZE_BITS" per Boqun's suggestion.
---
 kernel/kcsan/encoding.h | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/kernel/kcsan/encoding.h b/kernel/kcsan/encoding.h
index 4f73db6d1407..7ee405524904 100644
--- a/kernel/kcsan/encoding.h
+++ b/kernel/kcsan/encoding.h
@@ -37,14 +37,12 @@
  */
 #define WATCHPOINT_ADDR_BITS (BITS_PER_LONG-1 - WATCHPOINT_SIZE_BITS)
 
-/*
- * Masks to set/retrieve the encoded data.
- */
-#define WATCHPOINT_WRITE_MASK BIT(BITS_PER_LONG-1)
-#define WATCHPOINT_SIZE_MASK                                                   \
-	GENMASK(BITS_PER_LONG-2, BITS_PER_LONG-2 - WATCHPOINT_SIZE_BITS)
-#define WATCHPOINT_ADDR_MASK                                                   \
-	GENMASK(BITS_PER_LONG-3 - WATCHPOINT_SIZE_BITS, 0)
+/* Bitmasks for the encoded watchpoint access information. */
+#define WATCHPOINT_WRITE_MASK	BIT(BITS_PER_LONG-1)
+#define WATCHPOINT_SIZE_MASK	GENMASK(BITS_PER_LONG-2, WATCHPOINT_ADDR_BITS)
+#define WATCHPOINT_ADDR_MASK	GENMASK(WATCHPOINT_ADDR_BITS-1, 0)
+static_assert(WATCHPOINT_ADDR_MASK == (1UL << WATCHPOINT_ADDR_BITS) - 1);
+static_assert((WATCHPOINT_WRITE_MASK ^ WATCHPOINT_SIZE_MASK ^ WATCHPOINT_ADDR_MASK) == ~0UL);
 
 static inline bool check_encodable(unsigned long addr, size_t size)
 {
-- 
2.29.2.222.g5d2a92d10f8-goog