[release/7.0] Fix macOS floating point corruption #75467
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There is a race condition between activation signal handling and returning from a hardware exception handler on macOS. It shows up intermittently in the Regression/coreclr/GitHub_16833 test in the CI and I am able to repro it on my local mac once in several thousands of iterations of the test when running with GC stress C. It turned out the issue is caused by the order in which we set parts of the context in the thread when returning from the hardware exception handler. MacOS can only set the floating point and control / integer portions separately. We were setting the control / integer portion first and the floating point portion after that. In the race condition, the signal handling code in the macOS extracts the context that contains the new control registers, but the old floating point ones (which is the state of those in the PAL_DispatchException). The signal handler for the activation injection then gets executed and when it returns later to our managed code, the floating point registers get restored to the wrong values. The fix is to change the context setting to first set the floating point registers and then the control / integer portion of the context. Close #66568
mangod9
approved these changes
Sep 12, 2022
jeffschwMSFT
approved these changes
Sep 12, 2022
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Backport of #75440 to release/7.0
/cc @janvorli
Customer Impact
Very rare and extremely difficult to diagnose corruption of floating point values.
Testing
CI tests, local coreclr tests and especially running the test Regression/coreclr/GitHub_16833 where that problem was occurring randomly in our CI and that can reproduce the issue once in several thousands iterations.
Risk
Very low, it only changes the order of setting floating point / general purpose + control context parts on a thread that is returning from hardware exception handling.