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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>
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1 changed files with 16 additions and 24 deletions
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@ -407,12 +407,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
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const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0;
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const bool is_write = (type & KCSAN_ACCESS_WRITE) != 0;
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const bool is_assert = (type & KCSAN_ACCESS_ASSERT) != 0;
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const bool is_assert = (type & KCSAN_ACCESS_ASSERT) != 0;
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atomic_long_t *watchpoint;
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atomic_long_t *watchpoint;
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union {
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u64 old, new, diff;
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u8 _1;
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u16 _2;
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u32 _4;
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u64 _8;
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} expect_value;
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unsigned long access_mask;
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unsigned long access_mask;
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enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
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enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
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unsigned long ua_flags = user_access_save();
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unsigned long ua_flags = user_access_save();
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@ -468,19 +463,19 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
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* Read the current value, to later check and infer a race if the data
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* Read the current value, to later check and infer a race if the data
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* was modified via a non-instrumented access, e.g. from a device.
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* was modified via a non-instrumented access, e.g. from a device.
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*/
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*/
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expect_value._8 = 0;
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old = 0;
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switch (size) {
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switch (size) {
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case 1:
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case 1:
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expect_value._1 = READ_ONCE(*(const u8 *)ptr);
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old = READ_ONCE(*(const u8 *)ptr);
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break;
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break;
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case 2:
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case 2:
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expect_value._2 = READ_ONCE(*(const u16 *)ptr);
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old = READ_ONCE(*(const u16 *)ptr);
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break;
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break;
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case 4:
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case 4:
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expect_value._4 = READ_ONCE(*(const u32 *)ptr);
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old = READ_ONCE(*(const u32 *)ptr);
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break;
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break;
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case 8:
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case 8:
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expect_value._8 = READ_ONCE(*(const u64 *)ptr);
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old = READ_ONCE(*(const u64 *)ptr);
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break;
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break;
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default:
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default:
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break; /* ignore; we do not diff the values */
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break; /* ignore; we do not diff the values */
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@ -506,33 +501,30 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
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* racy access.
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* racy access.
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*/
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*/
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access_mask = get_ctx()->access_mask;
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access_mask = get_ctx()->access_mask;
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new = 0;
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switch (size) {
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switch (size) {
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case 1:
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case 1:
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expect_value._1 ^= READ_ONCE(*(const u8 *)ptr);
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new = READ_ONCE(*(const u8 *)ptr);
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if (access_mask)
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expect_value._1 &= (u8)access_mask;
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break;
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break;
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case 2:
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case 2:
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expect_value._2 ^= READ_ONCE(*(const u16 *)ptr);
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new = READ_ONCE(*(const u16 *)ptr);
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if (access_mask)
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expect_value._2 &= (u16)access_mask;
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break;
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break;
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case 4:
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case 4:
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expect_value._4 ^= READ_ONCE(*(const u32 *)ptr);
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new = READ_ONCE(*(const u32 *)ptr);
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if (access_mask)
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expect_value._4 &= (u32)access_mask;
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break;
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break;
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case 8:
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case 8:
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expect_value._8 ^= READ_ONCE(*(const u64 *)ptr);
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new = READ_ONCE(*(const u64 *)ptr);
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if (access_mask)
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expect_value._8 &= (u64)access_mask;
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break;
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break;
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default:
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default:
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break; /* ignore; we do not diff the values */
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break; /* ignore; we do not diff the values */
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}
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}
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diff = old ^ new;
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if (access_mask)
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diff &= access_mask;
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/* Were we able to observe a value-change? */
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/* Were we able to observe a value-change? */
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if (expect_value._8 != 0)
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if (diff != 0)
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value_change = KCSAN_VALUE_CHANGE_TRUE;
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value_change = KCSAN_VALUE_CHANGE_TRUE;
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/* Check if this access raced with another. */
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/* Check if this access raced with another. */
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