From: Marco Elver <elver@google.com>
To: elver@google.com, paulmck@kernel.org
Cc: mark.rutland@arm.com, will@kernel.org, dvyukov@google.com,
glider@google.com, boqun.feng@gmail.com,
kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org
Subject: [PATCH 1/9] kcsan: Simplify value change detection
Date: Wed, 14 Apr 2021 13:28:17 +0200 [thread overview]
Message-ID: <20210414112825.3008667-2-elver@google.com> (raw)
In-Reply-To: <20210414112825.3008667-1-elver@google.com>
From: Mark Rutland <mark.rutland@arm.com>
In kcsan_setup_watchpoint() we store snapshots of a watched value into a
union of u8/u16/u32/u64 sized fields, modify this in place using a
consistent field, then later check for any changes via the u64 field.
We can achieve the safe effect more simply by always treating the field
as a u64, as smaller values will be zero-extended. As the values are
zero-extended, we don't need to truncate the access_mask when we apply
it, and can always apply the full 64-bit access_mask to the 64-bit
value.
Finally, we can store the two snapshots and calculated difference
separately, which makes the code a little easier to read, and will
permit reporting the old/new values in subsequent patches.
There should be no functional change as a result of this patch.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marco Elver <elver@google.com>
---
kernel/kcsan/core.c | 40 ++++++++++++++++------------------------
1 file changed, 16 insertions(+), 24 deletions(-)
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 45c821d4e8bd..d360183002d6 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -407,12 +407,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0;
const bool is_assert = (type & KCSAN_ACCESS_ASSERT) != 0;
atomic_long_t *watchpoint;
- union {
- u8 _1;
- u16 _2;
- u32 _4;
- u64 _8;
- } expect_value;
+ u64 old, new, diff;
unsigned long access_mask;
enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
unsigned long ua_flags = user_access_save();
@@ -468,19 +463,19 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
* Read the current value, to later check and infer a race if the data
* was modified via a non-instrumented access, e.g. from a device.
*/
- expect_value._8 = 0;
+ old = 0;
switch (size) {
case 1:
- expect_value._1 = READ_ONCE(*(const u8 *)ptr);
+ old = READ_ONCE(*(const u8 *)ptr);
break;
case 2:
- expect_value._2 = READ_ONCE(*(const u16 *)ptr);
+ old = READ_ONCE(*(const u16 *)ptr);
break;
case 4:
- expect_value._4 = READ_ONCE(*(const u32 *)ptr);
+ old = READ_ONCE(*(const u32 *)ptr);
break;
case 8:
- expect_value._8 = READ_ONCE(*(const u64 *)ptr);
+ old = READ_ONCE(*(const u64 *)ptr);
break;
default:
break; /* ignore; we do not diff the values */
@@ -506,33 +501,30 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
* racy access.
*/
access_mask = get_ctx()->access_mask;
+ new = 0;
switch (size) {
case 1:
- expect_value._1 ^= READ_ONCE(*(const u8 *)ptr);
- if (access_mask)
- expect_value._1 &= (u8)access_mask;
+ new = READ_ONCE(*(const u8 *)ptr);
break;
case 2:
- expect_value._2 ^= READ_ONCE(*(const u16 *)ptr);
- if (access_mask)
- expect_value._2 &= (u16)access_mask;
+ new = READ_ONCE(*(const u16 *)ptr);
break;
case 4:
- expect_value._4 ^= READ_ONCE(*(const u32 *)ptr);
- if (access_mask)
- expect_value._4 &= (u32)access_mask;
+ new = READ_ONCE(*(const u32 *)ptr);
break;
case 8:
- expect_value._8 ^= READ_ONCE(*(const u64 *)ptr);
- if (access_mask)
- expect_value._8 &= (u64)access_mask;
+ new = READ_ONCE(*(const u64 *)ptr);
break;
default:
break; /* ignore; we do not diff the values */
}
+ diff = old ^ new;
+ if (access_mask)
+ diff &= access_mask;
+
/* Were we able to observe a value-change? */
- if (expect_value._8 != 0)
+ if (diff != 0)
value_change = KCSAN_VALUE_CHANGE_TRUE;
/* Check if this access raced with another. */
--
2.31.1.295.g9ea45b61b8-goog
next prev parent reply other threads:[~2021-04-14 11:29 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-14 11:28 [PATCH 0/9] kcsan: Add support for reporting observed value changes Marco Elver
2021-04-14 11:28 ` Marco Elver [this message]
2021-04-14 11:28 ` [PATCH 2/9] kcsan: Distinguish kcsan_report() calls Marco Elver
2021-04-14 11:28 ` [PATCH 3/9] kcsan: Refactor passing watchpoint/other_info Marco Elver
2021-04-14 11:28 ` [PATCH 4/9] kcsan: Fold panic() call into print_report() Marco Elver
2021-04-14 11:28 ` [PATCH 5/9] kcsan: Refactor access_info initialization Marco Elver
2021-04-14 11:28 ` [PATCH 6/9] kcsan: Remove reporting indirection Marco Elver
2021-04-14 11:28 ` [PATCH 7/9] kcsan: Remove kcsan_report_type Marco Elver
2021-04-14 11:28 ` [PATCH 8/9] kcsan: Report observed value changes Marco Elver
2021-04-14 11:28 ` [PATCH 9/9] kcsan: Document "value changed" line Marco Elver
2021-04-15 11:47 ` [PATCH 0/9] kcsan: Add support for reporting observed value changes Mark Rutland
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