From: Marco Elver <elver@google.com>
To: paulmck@kernel.org, boqun.feng@gmail.com
Cc: linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com,
kernel-team@fb.com, mingo@kernel.org, andreyknvl@google.com,
glider@google.com, dvyukov@google.com, cai@lca.pw
Subject: [PATCH v2] kcsan: Fix encoding masks and regain address bit
Date: Fri, 6 Nov 2020 10:34:56 +0100 [thread overview]
Message-ID: <20201106093456.GB2851373@elver.google.com> (raw)
In-Reply-To: <20201105220324.15808-3-paulmck@kernel.org>
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@google.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
next prev parent reply other threads:[~2020-11-06 9:35 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-05 22:03 [PATCH kcsan 0/3] KCSAN updates for 5.11 Paul E. McKenney
2020-11-05 22:03 ` [PATCH kcsan 1/3] kcsan: selftest: Ensure that address is at least PAGE_SIZE paulmck
2020-11-05 22:03 ` [PATCH kcsan 2/3] kcsan: Never set up watchpoints on NULL pointers paulmck
2020-11-05 22:03 ` [PATCH kcsan 3/3] kcsan: Fix encoding masks and regain address bit paulmck
2020-11-06 1:23 ` Boqun Feng
2020-11-06 9:03 ` Marco Elver
2020-11-06 10:18 ` Boqun Feng
2020-11-06 10:47 ` Marco Elver
2020-11-06 9:34 ` Marco Elver [this message]
2020-11-06 10:19 ` [PATCH v2] " Boqun Feng
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