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Port to 3.1 - Fix JIT_CheckedWriteBarrier on macOS #28051
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In a change to enable Mojave hardened runtime support that was made last year, a bug was introduced into the JIT_CheckedWriteBarrier. A conditional relative jump before an updated piece of code that was jumping after that piece of code was not updated and ended up jumping into the middle of an instruction. Since that condition occurs only with specific memory layout and it is very rare, that problem was not discovered until now.
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| #ifdef FEATURE_WRITEBARRIER_COPY | ||
| // jb NotInHeap | ||
| .byte 0x72, 0x12 | ||
| #else |
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I'm curious, why can this not use a mnemonic?
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The reason is that the clang assembler generates the long version of the relative jump instruction instead of one with byte offset for some reason. We've tried various things to make it generate this short version, but never found a way. So we had to resort to this ugly way.
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Port of dotnet/runtime#38242
In a change to enable Mojave hardened runtime support that was made last
year, a bug was introduced into the JIT_CheckedWriteBarrier. A
conditional relative jump before an updated piece of code that was jumping
after that piece of code was not updated and ended up jumping into the
middle of an instruction. Since that condition occurs only with specific
memory layout and it is very rare, that problem was not discovered until
now.
Customer impact
The bug is causing consistent crashes on some configurations of OSX machines while it works fine on others. Unity was hit by this problem. It was observed on the new Apple silicon devices when running under Rosetta 2 emulator, but it can occur on any x64 macOS device.
Regression?
Yes, introduced in 3.1.0
Testing
The original fix was supplied and tested by Unity
Risk
Low, this fix fixes an obvious bug in assembler helper code where a conditional jump jumps into the middle of an instruction and the app crashes.