PageAllocator.cpp

/*
 * Copyright (C) 2013 Google Inc. All rights reserved.
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 * modification, are permitted provided that the following conditions are
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#include "config.h"
#include "PageAllocator.h"

#include "AddressSpaceRandomization.h"
#include "Assertions.h"

#include <limits.h>

#if OS(POSIX)

#include <sys/mman.h>

#ifndef MADV_FREE
#define MADV_FREE MADV_DONTNEED
#endif

#ifndef MAP_ANONYMOUS
#define MAP_ANONYMOUS MAP_ANON
#endif

#elif OS(WIN)

#include <windows.h>

#else
#error Unknown OS
#endif // OS(POSIX)

namespace WTF {

// This simple internal function wraps the OS-specific page allocation call so
// that it behaves consistently: the address is a hint and if it cannot be used,
// the allocation will be placed elsewhere.
static void* systemAllocPages(void* addr, size_t len, PageAccessibilityConfiguration pageAccessibility)
{
    ASSERT(!(len & kPageAllocationGranularityOffsetMask));
    ASSERT(!(reinterpret_cast<uintptr_t>(addr) & kPageAllocationGranularityOffsetMask));
    void* ret;
#if OS(WIN)
    int accessFlag = pageAccessibility == PageAccessible ? PAGE_READWRITE : PAGE_NOACCESS;
    ret = VirtualAlloc(addr, len, MEM_RESERVE | MEM_COMMIT, accessFlag);
    if (!ret)
        ret = VirtualAlloc(0, len, MEM_RESERVE | MEM_COMMIT, accessFlag);
#else
    int accessFlag = pageAccessibility == PageAccessible ? (PROT_READ | PROT_WRITE) : PROT_NONE;
    ret = mmap(addr, len, accessFlag, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
    if (ret == MAP_FAILED)
        ret = 0;
#endif
    return ret;
}

static bool trimMapping(void* baseAddr, size_t baseLen, void* trimAddr, size_t trimLen)
{
#if OS(WIN)
    return false;
#else
    char* basePtr = static_cast<char*>(baseAddr);
    char* trimPtr = static_cast<char*>(trimAddr);
    ASSERT(trimPtr >= basePtr);
    ASSERT(trimPtr + trimLen <= basePtr + baseLen);
    size_t preLen = trimPtr - basePtr;
    if (preLen) {
        int ret = munmap(basePtr, preLen);
        RELEASE_ASSERT(!ret);
    }
    size_t postLen = (basePtr + baseLen) - (trimPtr + trimLen);
    if (postLen) {
        int ret = munmap(trimPtr + trimLen, postLen);
        RELEASE_ASSERT(!ret);
    }
    return true;
#endif
}

void* allocPages(void* addr, size_t len, size_t align, PageAccessibilityConfiguration pageAccessibility)
{
    ASSERT(len >= kPageAllocationGranularity);
    ASSERT(!(len & kPageAllocationGranularityOffsetMask));
    ASSERT(align >= kPageAllocationGranularity);
    ASSERT(!(align & kPageAllocationGranularityOffsetMask));
    ASSERT(!(reinterpret_cast<uintptr_t>(addr) & kPageAllocationGranularityOffsetMask));
    size_t alignOffsetMask = align - 1;
    size_t alignBaseMask = ~alignOffsetMask;
    ASSERT(!(reinterpret_cast<uintptr_t>(addr) & alignOffsetMask));
    // If the client passed null as the address, choose a good one.
    if (!addr) {
        addr = getRandomPageBase();
        addr = reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(addr) & alignBaseMask);
    }

    // The common case, which is also the least work we can do, is that the
    // address and length are suitable. Just try it.
    void* ret = systemAllocPages(addr, len, pageAccessibility);
    // If the alignment is to our liking, we're done.
    if (!ret || !(reinterpret_cast<uintptr_t>(ret) & alignOffsetMask))
        return ret;

    // Annoying. Unmap and map a larger range to be sure to succeed on the
    // second, slower attempt.
    freePages(ret, len);

    size_t tryLen = len + (align - kPageAllocationGranularity);
    RELEASE_ASSERT(tryLen > len);

    // We loop to cater for the unlikely case where another thread maps on top
    // of the aligned location we choose.
    int count = 0;
    while (count++ < 100) {
        ret = systemAllocPages(addr, tryLen, pageAccessibility);
        if (!ret)
            return 0;
        // We can now try and trim out a subset of the mapping.
        addr = reinterpret_cast<void*>((reinterpret_cast<uintptr_t>(ret) + alignOffsetMask) & alignBaseMask);

        // On POSIX systems, we can trim the oversized mapping to fit exactly.
        // This will always work on POSIX systems.
        if (trimMapping(ret, tryLen, addr, len))
            return addr;

        // On Windows, you can't trim an existing mapping so we unmap and remap
        // a subset. We used to do for all platforms, but OSX 10.8 has a
        // broken mmap() that ignores address hints for valid, unused addresses.
        freePages(ret, tryLen);
        ret = systemAllocPages(addr, len, pageAccessibility);
        if (ret == addr || !ret)
            return ret;

        // Unlikely race / collision. Do the simple thing and just start again.
        freePages(ret, len);
        addr = getRandomPageBase();
        addr = reinterpret_cast<void*>(reinterpret_cast<uintptr_t>(addr) & alignBaseMask);
    }
    IMMEDIATE_CRASH();
    return 0;
}

void freePages(void* addr, size_t len)
{
    ASSERT(!(reinterpret_cast<uintptr_t>(addr) & kPageAllocationGranularityOffsetMask));
    ASSERT(!(len & kPageAllocationGranularityOffsetMask));
#if OS(POSIX)
    int ret = munmap(addr, len);
    RELEASE_ASSERT(!ret);
#else
    BOOL ret = VirtualFree(addr, 0, MEM_RELEASE);
    RELEASE_ASSERT(ret);
#endif
}

void setSystemPagesInaccessible(void* addr, size_t len)
{
    ASSERT(!(len & kSystemPageOffsetMask));
#if OS(POSIX)
    int ret = mprotect(addr, len, PROT_NONE);
    RELEASE_ASSERT(!ret);
#else
    BOOL ret = VirtualFree(addr, len, MEM_DECOMMIT);
    RELEASE_ASSERT(ret);
#endif
}

bool setSystemPagesAccessible(void* addr, size_t len)
{
    ASSERT(!(len & kSystemPageOffsetMask));
#if OS(POSIX)
    return !mprotect(addr, len, PROT_READ | PROT_WRITE);
#else
    return !!VirtualAlloc(addr, len, MEM_COMMIT, PAGE_READWRITE);
#endif
}

void decommitSystemPages(void* addr, size_t len)
{
    ASSERT(!(len & kSystemPageOffsetMask));
#if OS(POSIX)
    int ret = madvise(addr, len, MADV_FREE);
    RELEASE_ASSERT(!ret);
#else
    setSystemPagesInaccessible(addr, len);
#endif
}

void recommitSystemPages(void* addr, size_t len)
{
    ASSERT(!(len & kSystemPageOffsetMask));
#if OS(POSIX)
    (void) addr;
#else
    RELEASE_ASSERT(setSystemPagesAccessible(addr, len));
#endif
}

void discardSystemPages(void* addr, size_t len)
{
    ASSERT(!(len & kSystemPageOffsetMask));
#if OS(POSIX)
    // On POSIX, the implementation detail is that discard and decommit are the
    // same, and lead to pages that are returned to the system immediately and
    // get replaced with zeroed pages when touched. So we just call
    // decommitSystemPages() here to avoid code duplication.
    decommitSystemPages(addr, len);
#else
    (void) addr;
    (void) len;
    // TODO(cevans): implement this using MEM_RESET for Windows, once we've
    // decided that the semantics are a match.
#endif
}

} // namespace WTF