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Add g++ to package list.
author Rob Landley <rob@landley.net>
date Sun, 17 Dec 2006 17:32:24 -0500
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2907d5193cf6 A real web page, and description of the build process.
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1 <html>
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2 <title>The Firmware Linux build process</title>
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4 <h1>Executive summary</h1>
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5
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6 <p>FWL builds a cross-compiler and then uses it to build a minimal system
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7 containing a native compiler, BusyBox and uClibc. Then it runs this minimal
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8 system under an emulator (QEMU) to natively build the final system. Finally it
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9 packages the resulting system (kernel, initramfs, and root filesystem) into
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10 a single file that can boot and run (on x86 by using a modified version of LILO).</p>
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2907d5193cf6 A real web page, and description of the build process.
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12 <p>Firmware Linux builds in stages:</p>
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14 <h2>Stage 1: Build a cross-compiler.</h2>
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2907d5193cf6 A real web page, and description of the build process.
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16 <p>The first stage builds a cross-compiler,
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17 which runs on the host system and produces binaries that run on the target
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18 system. (See my <a href=/writing/docs/cross-compiling.html>Introduction to
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19 cross compiling</a> if you're unfamiliar with this.)</p>
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21 <p>We have to cross-compile even if the host and target system are both
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22 x86, because the host probably use different C libraries. If the host has
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23 glibc and the target uses uClibc, then the (dynamically linked) target binaries
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24 we produce won't run on the host. This is what distinguishes cross-compiling
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25 from native compiling: different processors are just one reason the binaries
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26 might not run. Of course, as long as we've got the cross-compiling support
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27 anyway, we might as well support building for x86_64, arm, mips, or ppc
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28 targets...</p>
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30 <p>Building a cross-compiler toolchain requires four packages. The bulk of
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31 it is binutils, gcc, and uClibc, but building those requires header files from
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32 the Linux kernel which describe the target system.</p>
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34 <h2>Stage 2: Use the cross-compiler to build a native build environment
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35 for the target.</h2>
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36
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37 <p>Because cross-compiling is persnickety and difficult, we do as little of
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38 it as possible. Instead we use the cross-compiler to generate the smallest
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39 possible native build environment for the target, and then run the rest of the
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40 build in that environment, under an emulator.</p>
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41
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42 <p>The emulator we use is QEMU. The minimal build environment powerful enough
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43 to boot and compile a complete Linux system requires seven packages: the Linux
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44 kernel, binutils, gcc, uClibc, BusyBox, make, and bash.</p>
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45
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46 <h2>Stage 3: Run the target's native build environment under an emulator to
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47 build the final system.</h2>
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48
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49 <p>Running a native build under QEMU is about 1/3 the speed of cross-compiling,
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50 but it's a lot easier and more reliable.</p>
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51
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52 <p>A trick to accelerate the build is to use distcc to call out to the
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53 cross-compiler, feeding the results back into the emulator through the virtual
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54 network. (This is still a TODO item.)</p>
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56 <p>Stage 3 is a fairly straightforward
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57 <a href=http://www.linuxfromscratch.org>Linux From Scratch</a> approach,
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58 except that we use BusyBox and uClibc instead of the gnu packages.</p>
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60 <h2>Stage 4: Package the system into a firmware file.</h2>
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61
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62 <p>The reason for the name Firmware Linux is that the entire operating system
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63 (kernel, initramfs, and read-only squashfs root filesystem) are glued together
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64 into a single file. A modified version of LILO is included which can boot and
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65 run this file on x86.</p>
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66
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67 <hr>
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68
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69 <h1>Evolution of the firmware Linux build process.</h1>
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70
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71 <h2>The basic theory</h2>
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72
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73 <p>The Linux From Scratch approach is to build a minimal intermediate system
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74 with just enough packages to be able to compile stuff, chroot into that, and
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75 build the final system from there. This isolates the host from the target,
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76 which means you should be able to build under a wide variety of distributions.
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77 It also means the final system is built with a known set of tools, so you get
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78 a consistent result.</p>
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79
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80 <p>A minimal build environment consists of a C library, a compiler, and BusyBox.
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81 So in theory you just need three packages:</p>
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82
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83 <ul>
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84 <li>A C library (uClibc)</li>
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85 <li>A toolchain (tcc)</li>
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86 <li>BusyBox</li>
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87 </ul>
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88
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89 <p>Unfortunately, that doesn't work yet.</p>
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90
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91 <h2>Some differences between theory and reality.</h2>
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92
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93 <h3>Environmental dependencies.</h2>
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94
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95 <p>Environmental dependencies are things that need to be installed before you
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96 can build or run a given package. Lots of packages depend on things like zlib,
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97 SDL, texinfo, and all sorts of other strange things. (The GnuCash project
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98 stalled years ago after it released a version with so many environmental
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99 dependencies it was impossible to build or install. Environmental dependencies
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100 have a complexity cost, and are thus something to be minimized.)</p>
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101
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102 <p>A good build system will scan its environment to figure out what it has
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103 available, and disable functionality that depends on stuff that isn't
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104 available. (This is generally done with autoconf, which is disgusting but
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105 suffers from a lack of alternatives.) That way, the complexity cost is
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106 optional: you can build a minimal version of the package if that's all you
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107 need.</p>
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108
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109 <p>A really good build system can be told that the environment
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110 it's building in and the environment the result will run in are different,
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111 so just because it finds zlib on the build system doesn't mean that the
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112 target system will have zlib installed on it. (And even if it does, it may not
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113 be the same version. This is one of the big things that makes cross-compiling
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114 such a pain. One big reason for statically linking programs is to eliminate
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115 this kind of environmental dependency.)</p>
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116
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117 <p>The Firmware Linux build process is structured the way it is to eliminate
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118 as many environmental dependencies as possible. Some are unavoidable (such as
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119 C libraries needing kernel headers or gcc needing binutils), but the
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120 intermediate system is the minimal fully functional Linux development
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121 environment I currently know how to build, and then we switch into that and
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122 work our way back up from there by building more packages in the new
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123 environment.</p>
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124
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125 <h3>Resolving environmental dependencies.</h2>
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126
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127 <p><b>To build uClibc you need kernel headers</b> identifying the syscalls and
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128 such it can make to the OS. Way back when you could use the kernel headers
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129 straight out of the Linux kernel 2.4 tarball and they'd work fine, but sometime
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130 during 2.5 the kernel developers decided that exporting a sane API to userspace
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131 wasn't the kernel's job, and stopped doing it.</p>
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132
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133 <p>The 0.8x series of Firmware Linux used
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134 <a href=http://ep09.pld-linux.org/~mmazur/linux-libc-headers/>kernel
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135 headers manually cleaned up by Mariusz Mazur</a>, but after the 2.6.12 kernel
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136 he had an attack of real life and fell too far behind to catch up again.</p>
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137
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138 <p>The current practice is to use the Linux kernel's "make headers_install"
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139 target, created by David Woodhouse. This runs various scripts against the
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140 kernel headers to sanitize them for use by userspace. This was merged in
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141 2.6.18-rc1, and was more or less debugged by 2.6.19. So can use the Linux
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142 Kernel tarball as a source of headers again.</p>
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143
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144 <p>Another problem is that the busybox shell situation is a mess with four
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145 implementations that share little or no code (depending on how they're
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146 configured). The first question when trying to fix them is "which of the four
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147 do you fix?", and I'm just not going there. So until bbsh goes in we
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148 <b>substitute bash</b>.</p>
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149
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150 <p>Finally, <b>most packages expect gcc</b>. The tcc project isn't a drop-in
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151 gcc replacement yet, and doesn't include a "make" program. Most importantly,
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152 tcc development appears stalled because Fabrice Bellard's other major project
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153 (qemu) is taking up all his time these days. In 2004 Fabrice
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154 <a href=http://fabrice.bellard.free.fr/tcc/tccboot.html>built a modified Linux
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155 kernel with tcc</a>, and
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156 <a href=http://fabrice.bellard.free.fr/tcc/tccboot_readme.html>listed</a>
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157 what needed to be upgraded in TCC to build an unmodified kernel, but
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158 since then he hardly seems to have touched tcc. Hopefully, someday he'll get
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159 back to it and put out a 1.0 release of tcc that's a drop-in gcc replacment.
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160 (And if he does, I'll add a make implementation to toybox so we don't need
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161 to use any of the gnu toolchain). But in the meantime the only open source
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162 compiler that can build a complete Linux system is still the gnu compiler.</p>
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163
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164 <p>The gnu compiler actually consists of three packages <b>(binutils, gcc, and
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165 make)</b>, which is why it's generally called the gnu "toolchain". (The split
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166 between binutils and gcc is for purely historical reasons, and you have
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167 to match the right versions with each other or things break.)</p>
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168
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169 <p>This means that to compile a minimal build environment, you need seven
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170 packages, and to actually run the result we use an eighth package (QEMU).</p>
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171
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172 <p>This can actually be made to work. The next question is how?</p>
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173
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174 <h2>Additional complications</h2>
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175
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176 <h3>Cross-compiling and avoiding root access</h2>
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177
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178 <p>The first problem is that we're cross-compiling. We can't help it.
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179 You're cross-compiling any time you create target binaries that won't run on
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180 the host system. Even when both the host and target are on the same processor,
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181 if they're sufficiently different that one can't run the other's binaries, then
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182 you're cross-compiling. In our case, the host is usually running both a
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183 different C library and an older kernel version than the target, even when
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184 it's the same processor.</p>
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185
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186 <p>The second problem is that we want to avoid requiring root access to build
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187 Firmware Linux. If the build can run as a normal user, it's a lot more
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188 portable and a lot less likely to muck up the host system if something goes
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189 wrong. This means we can't modify the host's / directory (making anything
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190 that requires absolute paths problematic). We also can't mknod, chown, chgrp,
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191 mount (for --bind, loopback, tmpfs)...</p>
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192
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193 <p>In addition, the gnu toolchain (gcc/binutils) is chock-full of hardwired
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194 assumptions, such as what C library it's linking binaries against, where to look
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195 for #included headers, where to look for libraries, the absolute path the
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196 compiler is installed at... Silliest of all, it assumes that if the host and
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197 target use the same processor, you're not cross-compiling (even if they have
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198 a different C library and a different kernel, and even if you ./configure it
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199 for cross-compiling it switches that back off because it knows better than
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200 you do). This makes it very brittle, and it also tends to leak its assumptions
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201 into the programs it builds. New versions may someday fix this, but for now we
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202 have to hit it on the head repeatedly with a metal bar to get anything remotely
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203 useful out of it, and run it in a separate filesystem (chroot environment) so
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204 it can't reach out and grab the wrong headers or wrong libraries despite
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205 everything we've told it.</p>
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206
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207 <p>The absolute paths problem affects target binaries because all dynamically
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208 linked apps expect their shared library loader to live at an absolute path
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209 (in this case /lib/ld-uClibc.so.0). This directory is only writeable by root,
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210 and even if we could install it there polluting the host like that is just
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211 ugly.</p>
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212
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213 <p>The Firmware Linux build has to assume it's cross-compiling because the host
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214 is generally running glibc, and the target is running uClibc, so the libraries
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215 the target binaries need aren't installed on the host. Even if they're
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216 statically linked (which also mitigates the absolute paths problem somewhat),
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217 the target often has a newer kernel than the host, so the set of syscalls
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218 uClibc makes (thinking it's talking to the new kernel, since that's what the
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219 ABI the kernel headers it was built against describe) may not be entirely
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220 understood by the old kernel, leading to segfaults. (One of the reasons glibc
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221 is larger than uClibc is it checks the kernel to see if it supports things
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222 like long filenames or 32-bit device nodes before trying to use them. uClibc
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223 should always work on a newer kernel than the one it was built to expect, but
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224 not necessarily an older one.)</p>
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225
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226 <h2>Ways to make it all work</h2>
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227
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228 <h3>Cross compiling vs native compiling under emulation</h3>
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229
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230 <p>Cross compiling is a pain. There are a lot of ways to get it to sort of
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231 kinda work for certain versions of certain packages built on certain versions
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232 of certain distributions. But making it reliable or generally applicable is
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233 hard to do.</p>
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234
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235 <p>I wrote an <a href=/writing/docs/cross-compiling.html>introduction
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236 to cross-compiling</a> which explains the terminology, plusses and minuses,
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237 and why you might want to do it. Keep in mind that I wrote that for a company
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238 that specializes in cross-compiling. Personally, I consider cross-compiling
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239 a necessary evil to be minimized, and that's how Firmware Linux is designed.
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240 We cross-compile just enough stuff to get a working native build environment
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241 for the new platform, which we then run under emulation.</p>
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242
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243 <h3>Which emulator?</h3>
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244
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245 <p>The emulator Firmware Linux 0.8x used was User Mode Linux (here's a
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246 <a href=http://www.landley.net/writing/docs/UML.html>UML mini-howto</a> I wrote
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247 while getting this to work). Since we already need the linux-kernel source
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248 tarball anyway, building User Mode Linux from it was convenient and minimized
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249 the number of packages we needed to build the minimal system.</p>
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250
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251 <p>The first stage of the build compiled a UML kernel and ran the rest of the
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252 build under that, using UML's hostfs to mount the parent's root filesystem as
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253 the root filesystem for the new UML kernel. This solved both the kernel
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254 version and the root access problems. The UML kernel was the new version, and
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255 supported all the new syscalls and ioctls and such that the uClibc was built to
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256 expect, translating them to calls to the host system's C library as necessary.
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257 Processes running under User Mode Linux had root access (at least as far as UML
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258 was concerned), and although they couldn't write to the hostfs mounted root
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259 partition, they could create an ext2 image file, loopback mount it, --bind
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260 mount in directories from the hostfs partition to get the apps they needed,
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261 and chroot into it. Which is what the build did.</p>
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262
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263 <p>Current Firmware Linux has switched to a different emulator, QEMU, because
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264 as long as we're we're cross-compiling anyway we might as well have the
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265 ability to cross-compile for non-x86 targets. We still build a new kernel
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266 to run the uClibc binaries with the new kernel ABI, we just build a bootable
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267 kernel and run it under QEMU.</p>
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268
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269 <p>The main difference with QEMU is a sharper dividing line between the host
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270 system and the emulated target. Under UML we could switch to the emulated
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271 system early and still run host binaries (via the hostfs mount). This meant
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272 we could be much more relaxed about cross compiling, because we had one
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273 environment that ran both types of binaries. But this doesn't work if we're
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274 building an ARM, PPC, or x86-64 system on an x86 host.</p>
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275
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276 <p>Instead, we need to sequence more carefully. We build a cross-compiler,
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277 use that to cross-compile a minimal intermediate system from the seven packages
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278 listed earlier, and build a kernel and QEMU. Then we run the kernel under QEMU
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279 with the new intermediate system, and have it build the rest natively.</p>
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280
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281 <p>It's possible to use other emulators instead of QEMU, and I have a todo
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282 item to look at armulator from uClinux. (I looked at another nommu system
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283 simulator at Ottawa Linux Symposium, but after resolving the third unnecessary
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284 environmental dependency and still not being able to get it to finish compiling
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285 yet, I gave up. Armulator may be a patch against an obsolete version of gdb,
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286 but I could at least get it to build.)</p>