mchp: Remove undefined behavior in espi msvw handlers #22
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kiram9
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Jan 5, 2023
DHowett
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The code in espi_msvw[12]_interrupt relies on undefined behavior today.
__builtin_ctz is specified as returning values in the range [0, 31], but
we are checking for 32.
This behavior may be unexpected compared to the CTZ/CLZ instruction on
ARM, which use the value 32 to indicate that there are no ones in the
provided input.
GCC 11+ optimizes the two loops below into infinite loops, as it can see
that the condition will never be met.
After this change, the disassembly of espi_mswv1_interrupt can be
confirmed to contain an exit behind a branch.
... // r4 is loaded with girq24_result and has bits successively cleared
1a: b90c cbnz r4, 20 <espi_mswv1_interrupt+0x20>
1c: e8bd 81f0 ldmia.w sp!, {r4, r5, r6, r7, r8, pc}
20: fa94 f5a4 rbit r5, r4
...
BUG=EmbeddedController#21
BRANCH=hx20-hx30
TEST=Examined the disassembly for espi_msvw[12]_interrupt; see above
Signed-off-by: Dustin L. Howett <dustin@howett.net>
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Accepted upstream at https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/4114450 😄 |
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The code in espi_msvw[12]_interrupt relies on undefined behavior today. __builtin_ctz is specified as returning values in the range [0, 31], but we are checking for 32.
This behavior may be unexpected compared to the CTZ/CLZ instruction on ARM, which use the value 32 to indicate that there are no ones in the provided input.
GCC 11+ optimizes the two loops below into infinite loops, as it can see that the condition will never be met.
After this change, the disassembly of espi_mswv1_interrupt can be confirmed to contain an exit behind a branch.
... // r4 is loaded with girq24_result and has bits successively cleared
1a: b90c cbnz r4, 20 <espi_mswv1_interrupt+0x20>
1c: e8bd 81f0 ldmia.w sp!, {r4, r5, r6, r7, r8, pc}
20: fa94 f5a4 rbit r5, r4
...
BUG=EmbeddedController#21
BRANCH=hx20-hx30
TEST=Examined the disassembly for espi_msvw[12]_interrupt; see above
Signed-off-by: Dustin L. Howett dustin@howett.net