Backport bug #20493 to Ruby 3.2 #10797
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There's an exhaustive explanation of this in the linked redmine bug, but the short version is as follows: blocking_region_begin can spill callee-saved registers to the stack for its own use. That means they're not saved to ec->machine by the call to setjmp, since by that point they're already on the stack and new, different values are in the real registers. ec->machine's end-of-stack pointer is also bumped to accomodate this, BUT, after blocking_region_begin returns, that points past the end of the stack! As far as C is concerned, that's fine; the callee-saved registers are restored when blocking_region_begin returns. But, if another thread triggers GC, it is relying on finding references to Ruby objects by walking the stack region pointed to by ec->machine. If the C code in exec; subsequently does things that use that stack memory, then the value will be overwritten and the GC might prematurely collect something it shouldn't. [Bug #20493]
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Backports https://bugs.ruby-lang.org/issues/20493 (#10795) to Ruby 3.2
[Bug #20493]