On 4/4/2016 4:57 PM, 'John Clements' via users-redirect wrote:
FWIW, it appears that the restriction here is much simpler; specifically, pages can’t be writable and executable *simultaneously.* Moreover, a comment by Matthew on the github bug suggests that this might … have a relatively straightforward fix?
There are those who interpret W^X more severely as "once writable - *never* executable". That is fine for static loaded code and even for generated interpreter code [because that is just data]. But this severe interpretation of W^X is incompatible with runtime native code generation.
I agree that the simpler "not simultaneously" interpretation is just a performance issue: a kernel call to change protection costs many thousands of executed instructions. Obviously the overhead can be amortized by changing more pages in a single call, but JIT systems that compile in small units: e.g., function by function as they are 1st called - may suffer significant performance loss. [An issue now that CPU cores aren't getting any faster.]
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