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// Copyright 2014 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "config.h"
#include "wtf/AddressSpaceRandomization.h"
#include "wtf/PageAllocator.h"
#include "wtf/ProcessID.h"
#include "wtf/SpinLock.h"
namespace WTF {
namespace {
// This is the same PRNG as used by tcmalloc for mapping address randomness;
// see http://burtleburtle.net/bob/rand/smallprng.html
struct ranctx {
int lock;
bool initialized;
uint32_t a;
uint32_t b;
uint32_t c;
uint32_t d;
};
#define rot(x, k) (((x) << (k)) | ((x) >> (32 - (k))))
uint32_t ranvalInternal(ranctx* x)
{
uint32_t e = x->a - rot(x->b, 27);
x->a = x->b ^ rot(x->c, 17);
x->b = x->c + x->d;
x->c = x->d + e;
x->d = e + x->a;
return x->d;
}
#undef rot
uint32_t ranval(ranctx* x)
{
spinLockLock(&x->lock);
if (UNLIKELY(!x->initialized)) {
x->initialized = true;
char c;
uint32_t seed = static_cast<uint32_t>(reinterpret_cast<uintptr_t>(&c));
seed ^= static_cast<uint32_t>(getCurrentProcessID());
x->a = 0xf1ea5eed;
x->b = x->c = x->d = seed;
for (int i = 0; i < 20; ++i) {
(void) ranvalInternal(x);
}
}
uint32_t ret = ranvalInternal(x);
spinLockUnlock(&x->lock);
return ret;
}
static struct ranctx s_ranctx;
}
// Calculates a random preferred mapping address. In calculating an
// address, we balance good ASLR against not fragmenting the address
// space too badly.
void* getRandomPageBase()
{
uintptr_t random;
random = static_cast<uintptr_t>(ranval(&s_ranctx));
#if CPU(X86_64)
random <<= 32UL;
random |= static_cast<uintptr_t>(ranval(&s_ranctx));
// This address mask gives a low liklihood of address space collisions.
// We handle the situation gracefully if there is a collision.
#if OS(WIN)
// 64-bit Windows has a bizarrely small 8TB user address space.
// Allocates in the 1-5TB region.
random &= 0x3ffffffffffUL;
random += 0x10000000000UL;
#else
// Linux and OS X support the full 47-bit user space of x64 processors.
random &= 0x3fffffffffffUL;
#endif
#elif CPU(ARM64)
// ARM64 on Linux has 39-bit user space.
random &= 0x3fffffffffUL;
random += 0x1000000000UL;
#else // !CPU(X86_64) && !CPU(ARM64)
// This is a good range on Windows, Linux and Mac.
// Allocates in the 0.5-1.5GB region.
random &= 0x3fffffff;
random += 0x20000000;
#endif // CPU(X86_64)
random &= kPageAllocationGranularityBaseMask;
return reinterpret_cast<void*>(random);
}
}
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