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but I see no benefit from it today. VM_PROT_READ_IS_EXEC was only intended for use on processors that do not distinguish between read and execute permission. On an mmap(2) or mprotect(2), it automatically added execute permission if the caller specified permissions included read permission. The hope was that this would reduce the number of vm map entries needed to implement an address space because there would be fewer neighboring vm map entries that differed only in the presence or absence of VM_PROT_EXECUTE. (See vm/vm_mmap.c revision 1.56.) Today, I don't see any real applications that benefit from VM_PROT_READ_IS_EXEC. In any case, vm map entries are now organized as a self-adjusting binary search tree instead of an ordered list. So, the need for coalescing vm map entries is not as great as it once was. |
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