compiler-rt/lib/enable_execute_stack.c

//===-- enable_execute_stack.c - Implement __enable_execute_stack ---------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#include <stdint.h>
#include <sys/mman.h>


//
// The compiler generates calls to __enable_execute_stack() when creating 
// trampoline functions on the stack for use with nested functions.
// It is expected to mark the page(s) containing the address 
// and the next 48 bytes as executable.  Since the stack is normally rw-
// that means changing the protection on those page(s) to rwx. 
//
void __enable_execute_stack(void* addr)
{
#if __APPLE__
	// On Darwin, pagesize is always 4096 bytes
	const uintptr_t pageSize = 4096;
#else
        // FIXME: We should have a configure check for this.
        const uintptr_t pagesize = getpagesize();
#endif
	const uintptr_t pageAlignMask = ~(pageSize-1);
	uintptr_t p = (uintptr_t)addr;
	unsigned char* startPage = (unsigned char*)(p & pageAlignMask);
	unsigned char* endPage = (unsigned char*)((p+48+pageSize) & pageAlignMask);
	mprotect(startPage, endPage-startPage, PROT_READ | PROT_WRITE | PROT_EXEC);
}