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kcsan: Simplify value change detection 

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>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Mark Rutland 2021-04-14 13:28:17 +02:00 committed by Paul E. McKenney
parent ea0484644e
commit 6f2d98192c
1 changed files with 16 additions and 24 deletions
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40
kernel/kcsan/core.c
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@ -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. */