1 <chapter id="address-space">
2 <title> Address space management </title>
5 A good understanding of memory layout in Unix and Windows is
6 required before reading the next section (<xref
7 linkend="arch-mem"> gives some basic insight).
11 <title> Laying out the address space </title>
14 Up until about the start of 2004, the Linux address space very much resembled the Windows 9x
15 layout: the kernel sat in the top gigabyte, the bottom pages were unmapped to catch null
16 pointer dereferences, and the rest was free. The kernels mmap algorithm was predictable: it
17 would start by mapping files at low addresses and work up from there.
21 The development of a series of new low level patches violated many of these assumptions, and
22 resulted in Wine needing to force the Win32 address space layout upon the system. This
23 section looks at why and how this is done.
27 The exec-shield patch increases security by randomizing the kernels mmap algorithms. Rather
28 than consistently choosing the same addresses given the same sequence of requests, the kernel
29 will now choose randomized addresses. Because the Linux dynamic linker (ld-linux.so.2) loads
30 DSOs into memory by using mmap, this means that DSOs are no longer loaded at predictable
31 addresses, so making it harder to attack software by using buffer overflows. It also attempts
32 to relocate certain binaries into a special low area of memory known as the ASCII armor so
33 making it harder to jump into them when using string based attacks.
37 Prelink is a technology that enhances startup times by precalculating ELF global offset
38 tables then saving the results inside the native binaries themselves. By grid fitting each
39 DSO into the address space, the dynamic linker does not have to perform as many relocations
40 so allowing applications that heavily rely on dynamic linkage to be loaded into memory much
41 quicker. Complex C++ applications such as Mozilla, OpenOffice and KDE can especially benefit
46 The 4G VM split patch was developed by Ingo Molnar. It gives the Linux kernel its own address
47 space, thereby allowing processes to access the maximum addressable amount of memory on a
48 32-bit machine: 4 gigabytes. It allows people with lots of RAM to fully utilise that in any
49 given process at the cost of performance: the reason behind
50 giving the kernel a part of each processes address space was to avoid the overhead of switching on
55 Each of these changes alter the address space in a way incompatible with Windows. Prelink and
56 exec-shield mean that the libraries Wine uses can be placed at any point in the address
57 space: typically this meant that a library was sitting in the region that the EXE you wanted
58 to run had to be loaded (remember that unlike DLLs, EXE files cannot be moved around in
59 memory). The 4G VM split means that programs could receive pointers to the top gigabyte of
60 address space which some are not prepared for (they may store extra information in the high
61 bits of a pointer, for instance). In particular, in combination with exec-shield this one is
62 especially deadly as it's possible the process heap could be allocated beyond
63 ADDRESS_SPACE_LIMIT which causes Wine initialization to fail.
67 The solution to these problems is for Wine to reserve particular parts of the address space
68 so that areas that we don't want the system to use will be avoided. We later on
69 (re/de)allocate those areas as needed. One problem is that some of these mappings are put in
70 place automatically by the dynamic linker: for instance any libraries that Wine
71 is linked to (like libc, libwine, libpthread etc) will be mapped into memory before Wine even
72 gets control. In order to solve that, Wine overrides the default ELF initialization sequence
73 at a low level and reserves the needed areas by using direct syscalls into the kernel (ie
74 without linking against any other code to do it) before restarting the standard
75 initialization and letting the dynamic linker continue. This is referred to as the
76 preloader and is found in loader/preloader.c.
80 Once the usual ELF boot sequence has been completed, some native libraries may well have been
81 mapped above the 3gig limit: however, this doesn't matter as 3G is a Windows limit, not a
82 Linux limit. We still have to prevent the system from allocating anything else above there
83 (like the heap or other DLLs) though so Wine performs a binary search over the upper gig of
84 address space in order to iteratively fill in the holes with MAP_NORESERVE mappings so the
85 address space is allocated but the memory to actually back it is not. This code can be found
86 in libs/wine/mmap.c:reserve_area.
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