1 <chapter id="address-space">
2 <title> Address space management </title>
5 Every Win32 process in Wine has its own dedicated native process on the host system, and
6 therefore its own address space. This section explores the layout of the Windows address space
7 and how it is emulated.
11 Firstly, a quick recap of how virtual memory works. Physical memory in RAM chips is split
12 into <emphasis>frames</emphasis>, and the memory that each process sees is split
13 into <emphasis>pages</emphasis>. Each process has its own 4 gigabytes of address space (4gig
14 being the maximum space addressable with a 32 bit pointer). Pages can be mapped or unmapped:
15 attempts to access an unmapped page cause an EXCEPTION_ACCESS_VIOLATION which has the
16 easily recognizable code of 0xC0000005. Any page can be mapped to any frame, therefore you can
17 have multiple addresses which actually "contain" the same memory. Pages can also be mapped to
18 things like files or swap space, in which case accessing that page will cause a disk access to
19 read the contents into a free frame.
23 <title>Initial layout</title>
26 When a Win32 process starts, it does not have a clear address space to use as it pleases. Many pages
27 are already mapped by the operating system. In particular, the EXE file itself and any DLLs it
28 needs are mapped into memory, and space has been reserved for the stack and a couple of heaps
29 (zones used to allocate memory to the app from). Some of these things need to be at a fixed
30 address, and others can be placed anywhere.
34 The EXE file itself is usually mapped at address 0x400000 and up: indeed, most EXEs have
35 their relocation records stripped which means they must be loaded at their base address and
36 cannot be loaded at any other address.
40 DLLs are internally much the same as EXE files but they have relocation records, which means
41 that they can be mapped at any address in the address space. Remember we are not dealing with
42 physical memory here, but rather virtual memory which is different for each
43 process. Therefore OLEAUT32.DLL may be loaded at one address in one process, and a totally
44 different one in another. Ensuring all the functions loaded into memory can find each other
45 is the job of the Windows dynamic linker, which is a part of NTDLL.
49 So, we have the EXE and its DLLs mapped into memory. Two other very important regions also
50 exist: the stack and the process heap. The process heap is simply the equivalent of the libc
51 malloc arena on UNIX: it's a region of memory managed by the OS which malloc/HeapAlloc
52 partitions and hands out to the application. Windows applications can create several heaps but
53 the process heap always exists. It's created as part of process initialization in
54 dlls/ntdll/thread.c:thread_init().
58 There is another heap created as part of process startup, the so-called shared or system
59 heap. This is an undocumented service that exists only on Windows 9x: it is implemented in
60 Wine so native win9x DLLs can be used. The shared heap is unusual in that anything allocated
61 from it will be visible in every other process. This heap is always created at the
62 SYSTEM_HEAP_BASE address or 0x80000000 and defaults to 16 megabytes in size.
66 So far we've assumed the entire 4 gigs of address space is available for the application. In
67 fact that's not so: only the lower 2 gigs are available, the upper 2 gigs are on Windows NT
68 used by the operating system and hold the kernel (from 0x80000000). Why is the kernel mapped
69 into every address space? Mostly for performance: while it's possible to give the kernel its
70 own address space too - this is what Ingo Molnars 4G/4G VM split patch does for Linux - it
71 requires that every system call into the kernel switches address space. As that is a fairly
72 expensive operation (requires flushing the translation lookaside buffers etc) and syscalls are
73 made frequently it's best avoided by keeping the kernel mapped at a constant position in every
74 processes address space.
78 On Windows 9x, in fact only the upper gigabyte (0xC0000000 and up) is used by the kernel, the
79 region from 2 to 3 gigs is a shared area used for loading system DLLs and for file
80 mappings. The bottom 2 gigs on both NT and 9x are available for the programs memory allocation
85 There are a few other magic locations. The bottom 64k of memory is deliberately left unmapped
86 to catch null pointer dereferences. The region from 64k to 1mb+64k are reserved for DOS
87 compatibility and contain various DOS data structures. Finally, the address space also
88 contains mappings for the Wine binary itself, any native libaries Wine is using, the glibc
89 malloc arena and so on.
95 <title> Laying out the address space </title>
98 Up until about the start of 2004, the Linux address space very much resembled the Windows 9x
99 layout: the kernel sat in the top gigabyte, the bottom pages were unmapped to catch null
100 pointer dereferences, and the rest was free. The kernels mmap algorithm was predictable: it
101 would start by mapping files at low addresses and work up from there.
105 The development of a series of new low level patches violated many of these assumptions, and
106 resulted in Wine needing to force the Win32 address space layout upon the system. This
107 section looks at why and how this is done.
111 The exec-shield patch increases security by randomizing the kernels mmap algorithms. Rather
112 than consistently choosing the same addresses given the same sequence of requests, the kernel
113 will now choose randomized addresses. Because the Linux dynamic linker (ld-linux.so.2) loads
114 DSOs into memory by using mmap, this means that DSOs are no longer loaded at predictable
115 addresses, so making it harder to attack software by using buffer overflows. It also attempts
116 to relocate certain binaries into a special low area of memory known as the ASCII armor so
117 making it harder to jump into them when using string based attacks.
121 Prelink is a technology that enhances startup times by precalculating ELF global offset
122 tables then saving the results inside the native binaries themselves. By grid fitting each
123 DSO into the address space, the dynamic linker does not have to perform as many relocations
124 so allowing applications that heavily rely on dynamic linkage to be loaded into memory much
125 quicker. Complex C++ applications such as Mozilla, OpenOffice and KDE can especially benefit
130 The 4G VM split patch was developed by Ingo Molnar. It gives the Linux kernel its own address
131 space, thereby allowing processes to access the maximum addressable amount of memory on a
132 32-bit machine: 4 gigabytes. It allows people with lots of RAM to fully utilise that in any
133 given process at the cost of performance: as mentioned previously the reason behind giving
134 the kernel a part of each processes address space was to avoid the overhead of switching on
139 Each of these changes alter the address space in a way incompatible with Windows. Prelink and
140 exec-shield mean that the libraries Wine uses can be placed at any point in the address
141 space: typically this meant that a library was sitting in the region that the EXE you wanted
142 to run had to be loaded (remember that unlike DLLs, EXE files cannot be moved around in
143 memory). The 4G VM split means that programs could receive pointers to the top gigabyte of
144 address space which some are not prepared for (they may store extra information in the high
145 bits of a pointer, for instance). In particular, in combination with exec-shield this one is
146 especially deadly as it's possible the process heap could be allocated beyond
147 ADDRESS_SPACE_LIMIT which causes Wine initialization to fail.
151 The solution to these problems is for Wine to reserve particular parts of the address space
152 so that areas that we don't want the system to use will be avoided. We later on
153 (re/de)allocate those areas as needed. One problem is that some of these mappings are put in
154 place automatically by the dynamic linker: for instance any libraries that Wine
155 is linked to (like libc, libwine, libpthread etc) will be mapped into memory before Wine even
156 gets control. In order to solve that, Wine overrides the default ELF initialization sequence
157 at a low level and reserves the needed areas by using direct syscalls into the kernel (ie
158 without linking against any other code to do it) before restarting the standard
159 initialization and letting the dynamic linker continue. This is referred to as the
160 preloader and is found in loader/preloader.c.
164 Once the usual ELF boot sequence has been completed, some native libraries may well have been
165 mapped above the 3gig limit: however, this doesn't matter as 3G is a Windows limit, not a
166 Linux limit. We still have to prevent the system from allocating anything else above there
167 (like the heap or other DLLs) though so Wine performs a binary search over the upper gig of
168 address space in order to iteratively fill in the holes with MAP_NORESERVE mappings so the
169 address space is allocated but the memory to actually back it is not. This code can be found
170 in libs/wine/mmap.c:reserve_area.
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