From 7a5814c74c875d1d4ce80c8d24b0573a8fd51ab5 Mon Sep 17 00:00:00 2001 From: Mike Frysinger Date: Wed, 13 May 2009 01:48:00 -0400 Subject: split website off into dedicated git repo Signed-off-by: Mike Frysinger --- docs/busybox.net/FAQ.html | 1146 --------------------------------------------- 1 file changed, 1146 deletions(-) delete mode 100644 docs/busybox.net/FAQ.html (limited to 'docs/busybox.net/FAQ.html') diff --git a/docs/busybox.net/FAQ.html b/docs/busybox.net/FAQ.html deleted file mode 100644 index 7ed1394ce..000000000 --- a/docs/busybox.net/FAQ.html +++ /dev/null @@ -1,1146 +0,0 @@ - - -

Frequently Asked Questions

- -This is a collection of some of the more frequently asked questions -about BusyBox. Some of the questions even have answers. If you -have additions to this FAQ document, we would love to add them, - -

General questions

-
    -
  1. How can I get started using BusyBox?
    2. -
  2. How do I configure busybox?
    3. -
  3. How do I build BusyBox with a cross-compiler?
    4. -
  4. How do I build a BusyBox-based system?
    5. -
  5. Which Linux kernel versions are supported?
    6. -
  6. Which architectures does BusyBox run on?
    7. -
  7. Which C libraries are supported?
    8. -
  8. Can I include BusyBox as part of the software on my device?
    9. -
  9. Where can I find other small utilities since busybox does not include the features I want?
    10. -
  10. I demand that you to add <favorite feature> right now! How come you don't answer all my questions on the mailing list instantly? I demand that you help me with all of my problems Right Now!
    11. -
  11. I need help with BusyBox! What should I do?
    12. -
  12. I need you to add <favorite feature>! Are the BusyBox developers willing to be paid in order to fix bugs or add in <favorite feature>? Are you willing to provide support contracts?
    13. -
- -

Troubleshooting

-
    -
  1. I think I found a bug in BusyBox! What should I do?!
    2. -
  2. I'm using an ancient version from the dawn of time and something's broken. Can you backport fixes for free?
    3. -
  3. Busybox init isn't working!
    4. -
  4. I can't configure busybox on my system.
    5. -
  5. Why do I keep getting "sh: can't access tty; job control turned off" errors? Why doesn't Control-C work within my shell?
    6. -
- -

Misc. questions

-
    -
  1. How do I change the time zone in busybox?
    2. -
- -

Programming questions

-
    -
  1. What are the goals of busybox?
    2. -
  2. What is the design of busybox?
    3. -
  3. How is the source code organized? - -
    4. -
  4. I want to make busybox even smaller, how do I go about it?
    5. -
  5. Adding an applet to busybox
    6. -
  6. What standards does busybox adhere to?
    7. -
  7. Portability.
    8. -
  8. Tips and tricks. - -
    9. -
  9. Who are the BusyBox developers?
    10. -
- - -
-

General questions

- -
-

How can I get started using BusyBox?

- -

If you just want to try out busybox without installing it, download the - tarball, extract it, run "make defconfig", and then run "make". -

-

- This will create a busybox binary with almost all features enabled. To try - out a busybox applet, type "./busybox [appletname] [options]", for - example "./busybox ls -l" or "./busybox cat LICENSE". Type "./busybox" - to see a command list, and "busybox appletname --help" to see a brief - usage message for a given applet. -

-

- BusyBox uses the name it was invoked under to determine which applet is - being invoked. (Try "mv busybox ls" and then "./ls -l".) Installing - busybox consists of creating symlinks (or hardlinks) to the busybox - binary for each applet in busybox, and making sure these links are in - the shell's command $PATH. The special applet name "busybox" (or with - any optional suffix, such as "busybox-static") uses the first argument - to determine which applet to run, as shown above. -

-

- BusyBox also has a feature called the - "standalone shell", where the busybox - shell runs any built-in applets before checking the command path. This - feature is also enabled by "make allyesconfig", and to try it out run - the command line "PATH= ./busybox ash". This will blank your command path - and run busybox as your command shell, so the only commands it can find - (without an explicit path such as /bin/ls) are the built-in busybox ones. - This is another good way to see what's built into busybox. - Note that the standalone shell requires CONFIG_BUSYBOX_EXEC_PATH - to be set appropriately, depending on whether or not /proc/self/exe is - available or not. If you do not have /proc, then point that config option - to the location of your busybox binary, usually /bin/busybox. - (So if you set it to /proc/self/exe, and happen to be able to chroot into - your rootfs, you must mount /proc beforehand.) -

-

- A typical indication that you set CONFIG_BUSYBOX_EXEC_PATH to proc but - forgot to mount proc is: -

-$ /bin/echo $PATH
-/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/bin/X11
-$ echo $PATH
-/bin/sh: echo: not found
-
- -
-

How do I configure busybox?

- -

Busybox is configured similarly to the linux kernel. Create a default - configuration and then run "make menuconfig" to modify it. The end - result is a .config file that tells the busybox build process what features - to include. So instead of "./configure; make; make install" the equivalent - busybox build would be "make defconfig; make; make install". -

- -

Busybox configured with all features enabled is a little under a megabyte - dynamically linked on x86. To create a smaller busybox, configure it with - fewer features. Individual busybox applets cost anywhere from a few - hundred bytes to tens of kilobytes. Disable unneeded applets to save, - space, using menuconfig. -

- -

The most important busybox configurators are:

- - - -

Some other configuration options are:

- - -

Menuconfig modifies your .config file through an interactive menu where you can enable or disable - busybox features, and get help about each feature. - -

- To build a smaller busybox binary, run "make menuconfig" and disable the - features you don't need. (Or run "make allnoconfig" and then use - menuconfig to add just the features you need. Don't forget to recompile - with "make" once you've finished configuring.) -

- -
-

How do I build BusyBox with a cross-compiler?

- -

- To build busybox with a cross-compiler, specify CROSS_COMPILE=<prefix>. -

-

- CROSS_COMPILE specifies the prefix used for all executables used - during compilation. Only gcc and related binutils executables - are prefixed with $(CROSS_COMPILE) in the makefiles. - CROSS_COMPILE can be set on the command line: -

-
-   make CROSS_COMPILE=arm-linux-uclibcgnueabi-
-
-

- Alternatively CROSS_COMPILE can be set in the environment. - Default value for CROSS_COMPILE is not to prefix executables. -

-

- To store the cross-compiler in your .config, set the variable - CONFIG_CROSS_COMPILER_PREFIX accordingly in menuconfig or by - editing the .config file. -

- -
-

How do I build a BusyBox-based system?

- -

- BusyBox is a package that replaces a dozen standard packages, but it is - not by itself a complete bootable system. Building an entire Linux - distribution from source is a bit beyond the scope of this FAQ, but it - understandably keeps cropping up on the mailing list, so here are some - pointers. -

-

- Start by learning how to strip a working system down to the bare essentials - needed to run one or two commands, so you know what it is you actually - need. An excellent practical place to do - this is the Linux - BootDisk Howto, or for a more theoretical approach try - From - PowerUp to Bash Prompt. -

-

- To learn how to build a working Linux system entirely from source code, - the place to go is the Linux - From Scratch project. They have an entire book of step-by-step - instructions you can - read online - or - download. - Be sure to check out the other sections of their main page, including - Beyond Linux From Scratch, Hardened Linux From Scratch, their Hints - directory, and their LiveCD project. (They also have mailing lists which - are better sources of answers to Linux-system building questions than - the busybox list.) -

-

- If you want an automated yet customizable system builder which produces - a BusyBox and uClibc based system, try - buildroot, which is - another project by the maintainer of the uClibc (Erik Andersen). - Download the tarball, extract it, unset CC, make. - For more instructions, see the website. -

- -
-

Which Linux kernel versions are supported?

- -

- Full functionality requires Linux 2.4.x or better. (Earlier versions may - still work, but are no longer regularly tested.) A large fraction of the - code should run on just about anything. While the current code is fairly - Linux specific, it should be fairly easy to port the majority of the code - to support, say, FreeBSD or Solaris, or Mac OS X, or even Windows (if you - are into that sort of thing). -

- -
-

Which architectures does BusyBox run on?

- -

- BusyBox in general will build on any architecture supported by gcc. - Kernel module loading for 2.4 Linux kernels is currently - limited to ARM, CRIS, H8/300, x86, ia64, x86_64, m68k, MIPS, PowerPC, - S390, SH3/4/5, Sparc, v850e, and x86_64 for 2.4.x kernels. -

-

- With 2.6.x kernels, module loading support should work on all architectures. -

- -
-

Which C libraries are supported?

- -

- On Linux, BusyBox releases are tested against uClibc (0.9.27 or later) and - glibc (2.2 or later). Both should provide full functionality with busybox, - and if you find a bug we want to hear about it. -

-

- Linux-libc5 is no longer maintained (and has no known advantages over - uClibc), dietlibc is known to have numerous unfixed bugs, and klibc is - missing too many features to build BusyBox. If you require a small C - library for Linux, the busybox developers recommend uClibc. -

-

- Some BusyBox applets have been built and run under a combination - of newlib and libgloss (see - this thread). - This is still experimental, but may be supported in a future release. -

- -
-

Can I include BusyBox as part of the software on my device?

- -

- Yes. As long as you fully comply - with the generous terms of the GPL BusyBox license you can ship BusyBox - as part of the software on your device. -

- -
-

Where can I find other small utilities since busybox - does not include the features i want?

- -

- we maintain such a list on this site! -

- -
-

I demand that you to add <favorite feature> right now! How come you don't answer all my questions on the mailing list instantly? I demand that you help me with all of my problems Right Now!

- -

- You have not paid us a single cent and yet you still have the product of - many years of our work. We are not your slaves! We work on BusyBox - because we find it useful and interesting. If you go off flaming us, we - will ignore you. - -

-

I need help with BusyBox! What should I do?

- -

- If you find that you need help with BusyBox, you can ask for help on the - BusyBox mailing list at busybox@busybox.net.

- -

In addition to the mailing list, Erik Andersen (andersee), Manuel Nova - (mjn3), Rob Landley (landley), Mike Frysinger (SpanKY), - Bernhard Reutner-Fischer (blindvt), and other long-time BusyBox developers - are known to hang out on the uClibc IRC channel: #uclibc on - irc.freenode.net. There is a - web archive of - daily logs of the #uclibc IRC channel going back to 2002. -

- -

- Please do not send private email to Rob, Erik, Manuel, or the other - BusyBox contributors asking for private help unless you are planning on - paying for consulting services. -

- -

- When we answer questions on the BusyBox mailing list, it helps everyone - since people with similar problems in the future will be able to get help - by searching the mailing list archives. Private help is reserved as a paid - service. If you need to use private communication, or if you are serious - about getting timely assistance with BusyBox, you should seriously consider - paying for consulting services. -

- -
-

I need you to add <favorite feature>! Are the BusyBox developers willing to be paid in order to fix bugs or add in <favorite feature>? Are you willing to provide support contracts?

- -

- Yes we are. The easy way to sponsor a new feature is to post an offer on - the mailing list to see who's interested. You can also email the project's - maintainer and ask them to recommend someone. -

- -
-

Troubleshooting

- -
-

I think I found a bug in BusyBox! What should I do?

- -

- If you simply need help with using or configuring BusyBox, please submit a - detailed description of your problem to the BusyBox mailing list at busybox@busybox.net. - Please do not send email to individual developers asking - for private help unless you are planning on paying for consulting services. - When we answer questions on the BusyBox mailing list, it helps everyone, - while private answers help only you... -

- -

- Bug reports and new feature patches sometimes get lost when posted to the - mailing list, because the developers of BusyBox are busy people and have - only so much they can keep in their brains at a time. You can post a - polite reminder after 2-3 days without offending anybody. If that doesn't - result in a solution, please use the - BusyBox Bug - and Patch Tracking System to submit a detailed explanation and we'll - get to it as soon as we can. -

- -

- Note that bugs entered into the bug system without being mentioned on the - mailing list first may languish there for months before anyone even notices - them. We generally go through the bug system when preparing for new - development releases, to see what fell through the cracks while we were - off writing new features. (It's a fast/unreliable vs slow/reliable thing. - Saves retransits, but the latency sucks.) -

- -
-

I'm using an ancient version from the dawn of time and something's broken. Can you backport fixes for free?

- -

Variants of this one get asked a lot.

- -

The purpose of the BusyBox mailing list is to develop and improve BusyBox, -and we're happy to respond to our users' needs. But if you're coming to the -list for free tech support we're going to ask you to upgrade to a current -version before we try to diagnose your problem.

- -

If you're building BusyBox 0.50 with uClibc 0.9.19 and gcc 1.27 there's a -fairly large chance that whatever problem you're seeing has already been fixed. -To get that fix, all you have to do is upgrade to a newer version. If you -don't at least _try_ that, you're wasting our time.

- -

The volunteers are happy to fix any bugs you point out in the current -versions because doing so helps everybody and makes the project better. We -want to make the current version work for you. But diagnosing, debugging, and -backporting fixes to old versions isn't something we do for free, because it -doesn't help anybody but you. The cost of volunteer tech support is using a -reasonably current version of the project.

- -

If you don't want to upgrade, you have the complete source code and thus -the ability to fix it yourself, or hire a consultant to do it for you. If you -got your version from a vendor who still supports the older version, they can -help you. But there are limits as to what the volunteers will feel obliged to -do for you.

- -

As a rule of thumb, volunteers will generally answer polite questions about -a given version for about three years after its release before it's so old -we don't remember the answer off the top of our head. And if you want us to -put any _effort_ into tracking it down, we want you to put in a little effort -of your own by confirming it's still a problem with the current version. It's -also hard for us to fix a problem of yours if we can't reproduce it because -we don't have any systems running an environment that old.

- -

A consultant will happily set up a special environment just to reproduce -your problem, and you can always ask on the list if any of the developers -have consulting rates.

- -
-

Busybox init isn't working!

- -

- Init is the first program that runs, so it might be that no programs are - working on your new system because of a problem with your cross-compiler, - kernel, console settings, shared libraries, root filesystem... To rule all - that out, first build a statically linked version of the following "hello - world" program with your cross compiler toolchain: -

-
-#include <stdio.h>
-
-int main(int argc, char *argv)
-{
-  printf("Hello world!\n");
-  sleep(999999999);
-}
-
- -

- Now try to boot your device with an "init=" argument pointing to your - hello world program. Did you see the hello world message? Until you - do, don't bother messing with busybox init. -

- -

- Once you've got it working statically linked, try getting it to work - dynamically linked. Then read the FAQ entry How - do I build a BusyBox-based system?, and the - documentation for BusyBox - init. -

- -
-

I can't configure busybox on my system.

- -

- Configuring Busybox depends on a recent version of sed. Older - distributions (Red Hat 7.2, Debian 3.0) may not come with a - usable version. Luckily BusyBox can use its own sed to configure itself, - although this leads to a bit of a chicken and egg problem. - You can work around this by hand-configuring busybox to build with just - sed, then putting that sed in your path to configure the rest of busybox - with, like so: -

- -
-  tar xvjf sources/busybox-x.x.x.tar.bz2
-  cd busybox-x.x.x
-  make allnoconfig
-  make include/bb_config.h
-  echo "CONFIG_SED=y" >> .config
-  echo "#undef ENABLE_SED" >> include/bb_config.h
-  echo "#define ENABLE_SED 1" >> include/bb_config.h
-  make
-  mv busybox sed
-  export PATH=`pwd`:"$PATH"
-
- -

Then you can run "make defconfig" or "make menuconfig" normally.

- -
-

Why do I keep getting "sh: can't access tty; job control turned off" errors? Why doesn't Control-C work within my shell?

- -

- Job control will be turned off since your shell can not obtain a controlling - terminal. This typically happens when you run your shell on /dev/console. - The kernel will not provide a controlling terminal on the /dev/console - device. Your should run your shell on a normal tty such as tty1 or ttyS0 - and everything will work perfectly. If you REALLY want your shell - to run on /dev/console, then you can hack your kernel (if you are into that - sortof thing) by changing drivers/char/tty_io.c to change the lines where - it sets "noctty = 1;" to instead set it to "0". I recommend you instead - run your shell on a real console... -

- -
-

Misc. questions

- -
-

How do I change the time zone in busybox?

- -

Busybox has nothing to do with the timezone. Please consult your libc -documentation. (http://google.com/search?q=uclibc+glibc+timezone).

- -
-

Development

- -
-

What are the goals of busybox?

- -

Busybox aims to be the smallest and simplest correct implementation of the -standard Linux command line tools. First and foremost, this means the -smallest executable size we can manage. We also want to have the simplest -and cleanest implementation we can manage, be standards -compliant, minimize run-time memory usage (heap and stack), run fast, and -take over the world.

- -
-

What is the design of busybox?

- -

Busybox is like a swiss army knife: one thing with many functions. -The busybox executable can act like many different programs depending on -the name used to invoke it. Normal practice is to create a bunch of symlinks -pointing to the busybox binary, each of which triggers a different busybox -function. (See getting started in the -FAQ for more information on usage, and the -busybox documentation for a list of symlink names and what they do.) - -

The "one binary to rule them all" approach is primarily for size reasons: a -single multi-purpose executable is smaller then many small files could be. -This way busybox only has one set of ELF headers, it can easily share code -between different apps even when statically linked, it has better packing -efficiency by avoding gaps between files or compression dictionary resets, -and so on.

- -

Work is underway on new options such as "make standalone" to build separate -binaries for each applet, and a "libbb.so" to make the busybox common code -available as a shared library. Neither is ready yet at the time of this -writing.

- - - -
-

The applet directories

- -

The directory "applets" contains the busybox startup code (applets.c and -busybox.c), and several subdirectories containing the code for the individual -applets.

- -

Busybox execution starts with the main() function in applets/busybox.c, -which sets the global variable applet_name to argv[0] and calls -run_applet_and_exit() in applets/applets.c. That uses the applets[] array -(defined in include/busybox.h and filled out in include/applets.h) to -transfer control to the appropriate APPLET_main() function (such as -cat_main() or sed_main()). The individual applet takes it from there.

- -

This is why calling busybox under a different name triggers different -functionality: main() looks up argv[0] in applets[] to get a function pointer -to APPLET_main().

- -

Busybox applets may also be invoked through the multiplexor applet -"busybox" (see busybox_main() in libbb/appletlib.c), and through the -standalone shell (grep for STANDALONE_SHELL in applets/shell/*.c). -See getting started in the -FAQ for more information on these alternate usage mechanisms, which are -just different ways to reach the relevant APPLET_main() function.

- -

The applet subdirectories (archival, console-tools, coreutils, -debianutils, e2fsprogs, editors, findutils, init, loginutils, miscutils, -modutils, networking, procps, shell, sysklogd, and util-linux) correspond -to the configuration sub-menus in menuconfig. Each subdirectory contains the -code to implement the applets in that sub-menu, as well as a Config.in -file defining that configuration sub-menu (with dependencies and help text -for each applet), and the makefile segment (Makefile.in) for that -subdirectory.

- -

The run-time --help is stored in usage_messages[], which is initialized at -the start of applets/applets.c and gets its help text from usage.h. During the -build this help text is also used to generate the BusyBox documentation (in -html, txt, and man page formats) in the docs directory. See -adding an applet to busybox for more -information.

- -
-

libbb

- -

Most non-setup code shared between busybox applets lives in the libbb -directory. It's a mess that evolved over the years without much auditing -or cleanup. For anybody looking for a great project to break into busybox -development with, documenting libbb would be both incredibly useful and good -experience.

- -

Common themes in libbb include allocation functions that test -for failure and abort the program with an error message so the caller doesn't -have to test the return value (xmalloc(), xstrdup(), etc), wrapped versions -of open(), close(), read(), and write() that test for their own failures -and/or retry automatically, linked list management functions (llist.c), -command line argument parsing (getopt32.c), and a whole lot more.

- -
-

I want to make busybox even smaller, how do I go about it?

- -

- To conserve bytes it's good to know where they're being used, and the - size of the final executable isn't always a reliable indicator of - the size of the components (since various structures are rounded up, - so a small change may not even be visible by itself, but many small - savings add up). -

- -

The busybox Makefile builds two versions of busybox, one of which - (busybox_unstripped) has extra information that various analysis tools - can use. (This has nothing to do with CONFIG_DEBUG, leave that off - when trying to optimize for size.) -

- -

The "make bloatcheck" option uses Matt Mackall's bloat-o-meter - script to compare two versions of busybox (busybox_unstripped vs - busybox_old), and report which symbols changed size and by how much. - To use it, first build a base version with "make baseline". - (This creates busybox_old, which should have the original sizes for - comparison purposes.) Then build the new version with your changes - and run "make bloatcheck" to see the size differences from the old - version. -

-

- The first line of output has totals: how many symbols were added or - removed, how many symbols grew or shrank, the number of bytes added - and number of bytes removed by these changes, and finally the total - number of bytes difference between the two files. The remaining - lines show each individual symbol, the old and new sizes, and the - increase or decrease in size (which results are sorted by). -

-

- The "make sizes" option produces raw symbol size information for - busybox_unstripped. This is the output from the "nm --size-sort" - command (see "man nm" for more information), and is the information - bloat-o-meter parses to produce the comparison report above. For - defconfig, this is a good way to find the largest symbols in the tree - (which is a good place to start when trying to shrink the code). To - take a closer look at individual applets, configure busybox with just - one applet (run "make allnoconfig" and then switch on a single applet - with menuconfig), and then use "make sizes" to see the size of that - applet's components. -

-

- The "showasm" command (in the scripts directory) produces an assembly - dump of a function, providing a closer look at what changed. Try - "scripts/showasm busybox_unstripped" to list available symbols, and - "scripts/showasm busybox_unstripped symbolname" to see the assembly - for a sepecific symbol. -

- -
-

Adding an applet to busybox

- -

To add a new applet to busybox, first pick a name for the applet and -a corresponding CONFIG_NAME. Then do this:

- - - -
-

What standards does busybox adhere to?

- -

The standard we're paying attention to is the "Shell and Utilities" -portion of the Open -Group Base Standards (also known as the Single Unix Specification version -3 or SUSv3). Note that paying attention isn't necessarily the same thing as -following it.

- -

SUSv3 doesn't even mention things like init, mount, tar, or losetup, nor -commonly used options like echo's '-e' and '-n', or sed's '-i'. Busybox is -driven by what real users actually need, not the fact the standard believes -we should implement ed or sccs. For size reasons, we're unlikely to include -much internationalization support beyond UTF-8, and on top of all that, our -configuration menu lets developers chop out features to produce smaller but -very non-standard utilities.

- -

Also, Busybox is aimed primarily at Linux. Unix standards are interesting -because Linux tries to adhere to them, but portability to dozens of platforms -is only interesting in terms of offering a restricted feature set that works -everywhere, not growing dozens of platform-specific extensions. Busybox -should be portable to all hardware platforms Linux supports, and any other -similar operating systems that are easy to do and won't require much -maintenance.

- -

In practice, standards compliance tends to be a clean-up step once an -applet is otherwise finished. When polishing and testing a busybox applet, -we ensure we have at least the option of full standards compliance, or else -document where we (intentionally) fall short.

- -
-

Portability.

- -

Busybox is a Linux project, but that doesn't mean we don't have to worry -about portability. First of all, there are different hardware platforms, -different C library implementations, different versions of the kernel and -build toolchain... The file "include/platform.h" exists to centralize and -encapsulate various platform-specific things in one place, so most busybox -code doesn't have to care where it's running.

- -

To start with, Linux runs on dozens of hardware platforms. We try to test -each release on x86, x86-64, arm, power pc, and mips. (Since qemu can handle -all of these, this isn't that hard.) This means we have to care about a number -of portability issues like endianness, word size, and alignment, all of which -belong in platform.h. That header handles conditional #includes and gives -us macros we can use in the rest of our code. At some point in the future -we might grow a platform.c, possibly even a platform subdirectory. As long -as the applets themselves don't have to care.

- -

On a related note, we made the "default signedness of char varies" problem -go away by feeding the compiler -funsigned-char. This gives us consistent -behavior on all platforms, and defaults to 8-bit clean text processing (which -gets us halfway to UTF-8 support). NOMMU support is less easily separated -(see the tips section later in this document), but we're working on it.

- -

Another type of portability is build environments: we unapologetically use -a number of gcc and glibc extensions (as does the Linux kernel), but these have -been picked up by packages like uClibc, TCC, and Intel's C Compiler. As for -gcc, we take advantage of newer compiler optimizations to get the smallest -possible size, but we also regression test against an older build environment -using the Red Hat 9 image at "http://busybox.net/downloads/qemu". This has a -2.4 kernel, gcc 3.2, make 3.79.1, and glibc 2.3, and is the oldest -build/deployment environment we still put any effort into maintaining. (If -anyone takes an interest in older kernels you're welcome to submit patches, -but the effort would probably be better spent -trimming -down the 2.6 kernel.) Older gcc versions than that are uninteresting since -we now use c99 features, although -tcc might be worth a -look.

- -

We also test busybox against the current release of uClibc. Older versions -of uClibc aren't very interesting (they were buggy, and uClibc wasn't really -usable as a general-purpose C library before version 0.9.26 anyway).

- -

Other unix implementations are mostly uninteresting, since Linux binaries -have become the new standard for portable Unix programs. Specifically, -the ubiquity of Linux was cited as the main reason the Intel Binary -Compatability Standard 2 died, by the standards group organized to name a -successor to ibcs2: the 86open -project. That project disbanded in 1999 with the endorsement of an -existing standard: Linux ELF binaries. Since then, the major players at the -time (such as AIX, Solaris, and -FreeBSD) -have all either grown Linux support or folded.

- -

The major exceptions are newcomer MacOS X, some embedded environments -(such as newlib+libgloss) which provide a posix environment but not a full -Linux environment, and environments like Cygwin that provide only partial Linux -emulation. Also, some embedded Linux systems run a Linux kernel but amputate -things like the /proc directory to save space.

- -

Supporting these systems is largely a question of providing a clean subset -of BusyBox's functionality -- whichever applets can easily be made to -work in that environment. Annotating the configuration system to -indicate which applets require which prerequisites (such as procfs) is -also welcome. Other efforts to support these systems (swapping #include -files to build in different environments, adding adapter code to platform.h, -adding more extensive special-case supporting infrastructure such as mount's -legacy mtab support) are handled on a case-by-case basis. Support that can be -cleanly hidden in platform.h is reasonably attractive, and failing that -support that can be cleanly separated into a separate conditionally compiled -file is at least worth a look. Special-case code in the body of an applet is -something we're trying to avoid.

- -
-

Programming tips and tricks.

- -

Various things busybox uses that aren't particularly well documented -elsewhere.

- -
-

Encrypted Passwords

- -

Password fields in /etc/passwd and /etc/shadow are in a special format. -If the first character isn't '$', then it's an old DES style password. If -the first character is '$' then the password is actually three fields -separated by '$' characters:

-
-  $type$salt$encrypted_password
-
- -

The "type" indicates which encryption algorithm to use: 1 for MD5 and 2 for SHA1.

- -

The "salt" is a bunch of ramdom characters (generally 8) the encryption -algorithm uses to perturb the password in a known and reproducible way (such -as by appending the random data to the unencrypted password, or combining -them with exclusive or). Salt is randomly generated when setting a password, -and then the same salt value is re-used when checking the password. (Salt is -thus stored unencrypted.)

- -

The advantage of using salt is that the same cleartext password encrypted -with a different salt value produces a different encrypted value. -If each encrypted password uses a different salt value, an attacker is forced -to do the cryptographic math all over again for each password they want to -check. Without salt, they could simply produce a big dictionary of commonly -used passwords ahead of time, and look up each password in a stolen password -file to see if it's a known value. (Even if there are billions of possible -passwords in the dictionary, checking each one is just a binary search against -a file only a few gigabytes long.) With salt they can't even tell if two -different users share the same password without guessing what that password -is and decrypting it. They also can't precompute the attack dictionary for -a specific password until they know what the salt value is.

- -

The third field is the encrypted password (plus the salt). For md5 this -is 22 bytes.

- -

The busybox function to handle all this is pw_encrypt(clear, salt) in -"libbb/pw_encrypt.c". The first argument is the clear text password to be -encrypted, and the second is a string in "$type$salt$password" format, from -which the "type" and "salt" fields will be extracted to produce an encrypted -value. (Only the first two fields are needed, the third $ is equivalent to -the end of the string.) The return value is an encrypted password in -/etc/passwd format, with all three $ separated fields. It's stored in -a static buffer, 128 bytes long.

- -

So when checking an existing password, if pw_encrypt(text, -old_encrypted_password) returns a string that compares identical to -old_encrypted_password, you've got the right password. When setting a new -password, generate a random 8 character salt string, put it in the right -format with sprintf(buffer, "$%c$%s", type, salt), and feed buffer as the -second argument to pw_encrypt(text,buffer).

- -
-

Fork and vfork

- -

On systems that haven't got a Memory Management Unit, fork() is unreasonably -expensive to implement (and sometimes even impossible), so a less capable -function called vfork() is used instead. (Using vfork() on a system with an -MMU is like pounding a nail with a wrench. Not the best tool for the job, but -it works.)

- -

Busybox hides the difference between fork() and vfork() in -libbb/bb_fork_exec.c. If you ever want to fork and exec, use bb_fork_exec() -(which returns a pid and takes the same arguments as execve(), although in -this case envp can be NULL) and don't worry about it. This description is -here in case you want to know why that does what it does.

- -

Implementing fork() depends on having a Memory Management Unit. With an -MMU then you can simply set up a second set of page tables and share the -physical memory via copy-on-write. So a fork() followed quickly by exec() -only copies a few pages of the parent's memory, just the ones it changes -before freeing them.

- -

With a very primitive MMU (using a base pointer plus length instead of page -tables, which can provide virtual addresses and protect processes from each -other, but no copy on write) you can still implement fork. But it's -unreasonably expensive, because you have to copy all the parent process' -memory into the new process (which could easily be several megabytes per fork). -And you have to do this even though that memory gets freed again as soon as the -exec happens. (This is not just slow and a waste of space but causes memory -usage spikes that can easily cause the system to run out of memory.)

- -

Without even a primitive MMU, you have no virtual addresses. Every process -can reach out and touch any other process' memory, because all pointers are to -physical addresses with no protection. Even if you copy a process' memory to -new physical addresses, all of its pointers point to the old objects in the -old process. (Searching through the new copy's memory for pointers and -redirect them to the new locations is not an easy problem.)

- -

So with a primitive or missing MMU, fork() is just not a good idea.

- -

In theory, vfork() is just a fork() that writeably shares the heap and stack -rather than copying it (so what one process writes the other one sees). In -practice, vfork() has to suspend the parent process until the child does exec, -at which point the parent wakes up and resumes by returning from the call to -vfork(). All modern kernel/libc combinations implement vfork() to put the -parent to sleep until the child does its exec. There's just no other way to -make it work: the parent has to know the child has done its exec() or exit() -before it's safe to return from the function it's in, so it has to block -until that happens. In fact without suspending the parent there's no way to -even store separate copies of the return value (the pid) from the vfork() call -itself: both assignments write into the same memory location.

- -

One way to understand (and in fact implement) vfork() is this: imagine -the parent does a setjmp and then continues on (pretending to be the child) -until the exec() comes around, then the _exec_ does the actual fork, and the -parent does a longjmp back to the original vfork call and continues on from -there. (It thus becomes obvious why the child can't return, or modify -local variables it doesn't want the parent to see changed when it resumes.) - -

Note a common mistake: the need for vfork doesn't mean you can't have two -processes running at the same time. It means you can't have two processes -sharing the same memory without stomping all over each other. As soon as -the child calls exec(), the parent resumes.

- -

If the child's attempt to call exec() fails, the child should call _exit() -rather than a normal exit(). This avoids any atexit() code that might confuse -the parent. (The parent should never call _exit(), only a vforked child that -failed to exec.)

- -

(Now in theory, a nommu system could just copy the _stack_ when it forks -(which presumably is much shorter than the heap), and leave the heap shared. -Even with no MMU at all -In practice, you've just wound up in a multi-threaded situation and you can't -do a malloc() or free() on your heap without freeing the other process' memory -(and if you don't have the proper locking for being threaded, corrupting the -heap if both of you try to do it at the same time and wind up stomping on -each other while traversing the free memory lists). The thing about vfork is -that it's a big red flag warning "there be dragons here" rather than -something subtle and thus even more dangerous.)

- -
-

Short reads and writes

- -

Busybox has special functions, bb_full_read() and bb_full_write(), to -check that all the data we asked for got read or written. Is this a real -world consideration? Try the following:

- -
while true; do echo hello; sleep 1; done | tee out.txt
- -

If tee is implemented with bb_full_read(), tee doesn't display output -in real time but blocks until its entire input buffer (generally a couple -kilobytes) is read, then displays it all at once. In that case, we _want_ -the short read, for user interface reasons. (Note that read() should never -return 0 unless it has hit the end of input, and an attempt to write 0 -bytes should be ignored by the OS.)

- -

As for short writes, play around with two processes piping data to each -other on the command line (cat bigfile | gzip > out.gz) and suspend and -resume a few times (ctrl-z to suspend, "fg" to resume). The writer can -experience short writes, which are especially dangerous because if you don't -notice them you'll discard data. They can also happen when a system is under -load and a fast process is piping to a slower one. (Such as an xterm waiting -on x11 when the scheduler decides X is being a CPU hog with all that -text console scrolling...)

- -

So will data always be read from the far end of a pipe at the -same chunk sizes it was written in? Nope. Don't rely on that. For one -counterexample, see rfc 896 -for Nagle's algorithm, which waits a fraction of a second or so before -sending out small amounts of data through a TCP/IP connection in case more -data comes in that can be merged into the same packet. (In case you were -wondering why action games that use TCP/IP set TCP_NODELAY to lower the latency -on their their sockets, now you know.)

- -
-

Memory used by relocatable code, PIC, and static linking.

- -

The downside of standard dynamic linking is that it results in self-modifying -code. Although each executable's pages are mmaped() into a process' address -space from the executable file and are thus naturally shared between processes -out of the page cache, the library loader (ld-linux.so.2 or ld-uClibc.so.0) -writes to these pages to supply addresses for relocatable symbols. This -dirties the pages, triggering copy-on-write allocation of new memory for each -processes' dirtied pages.

- -

One solution to this is Position Independent Code (PIC), a way of linking -a file so all the relocations are grouped together. This dirties fewer -pages (often just a single page) for each process' relocations. The down -side is this results in larger executables, which take up more space on disk -(and a correspondingly larger space in memory). But when many copies of the -same program are running, PIC dynamic linking trades a larger disk footprint -for a smaller memory footprint, by sharing more pages.

- -

A third solution is static linking. A statically linked program has no -relocations, and thus the entire executable is shared between all running -instances. This tends to have a significantly larger disk footprint, but -on a system with only one or two executables, shared libraries aren't much -of a win anyway.

- -

You can tell the glibc linker to display debugging information about its -relocations with the environment variable "LD_DEBUG". Try -"LD_DEBUG=help /bin/true" for a list of commands. Learning to interpret -"LD_DEBUG=statistics cat /proc/self/statm" could be interesting.

- -

For more on this topic, here's Rich Felker:

-
-

Dynamic linking (without fixed load addresses) fundamentally requires -at least one dirty page per dso that uses symbols. Making calls (but -never taking the address explicitly) to functions within the same dso -does not require a dirty page by itself, but will with ELF unless you -use -Bsymbolic or hidden symbols when linking.

- -

ELF uses significant additional stack space for the kernel to pass all -the ELF data structures to the newly created process image. These are -located above the argument list and environment. This normally adds 1 -dirty page to the process size.

- -

The ELF dynamic linker has its own data segment, adding one or more -dirty pages. I believe it also performs relocations on itself.

- -

The ELF dynamic linker makes significant dynamic allocations to manage -the global symbol table and the loaded dso's. This data is never -freed. It will be needed again if libdl is used, so unconditionally -freeing it is not possible, but normal programs do not use libdl. Of -course with glibc all programs use libdl (due to nsswitch) so the -issue was never addressed.

- -

ELF also has the issue that segments are not page-aligned on disk. -This saves up to 4k on disk, but at the expense of using an additional -dirty page in most cases, due to a large portion of the first data -page being filled with a duplicate copy of the last text page.

- -

The above is just a partial list of the tiny memory penalties of ELF -dynamic linking, which eventually add up to quite a bit. The smallest -I've been able to get a process down to is 8 dirty pages, and the -above factors seem to mostly account for it (but some were difficult -to measure).

-
- -
-

Including kernel headers

- -

The "linux" or "asm" directories of /usr/include -contain Linux kernel -headers, so that the C library can talk directly to the Linux kernel. In -a perfect world, applications shouldn't include these headers directly, but -we don't live in a perfect world.

- -

For example, Busybox's losetup code wants linux/loop.c because nothing else -#defines the structures to call the kernel's loopback device setup ioctls. -Attempts to cut and paste the information into a local busybox header file -proved incredibly painful, because portions of the loop_info structure vary by -architecture, namely the type __kernel_dev_t has different sizes on alpha, -arm, x86, and so on. Meaning we either #include <linux/posix_types.h> or -we hardwire #ifdefs to check what platform we're building on and define this -type appropriately for every single hardware architecture supported by -Linux, which is simply unworkable.

- -

This is aside from the fact that the relevant type defined in -posix_types.h was renamed to __kernel_old_dev_t during the 2.5 series, so -to cut and paste the structure into our header we have to #include -<linux/version.h> to figure out which name to use. (What we actually -do is -check if we're building on 2.6, and if so just use the new 64 bit structure -instead to avoid the rename entirely.) But we still need the version -check, since 2.4 didn't have the 64 bit structure.

- -

The BusyBox developers spent two years trying to figure -out a clean way to do all this. There isn't one. The losetup in the -util-linux package from kernel.org isn't doing it cleanly either, they just -hide the ugliness by nesting #include files. Their mount/loop.h -#includes "my_dev_t.h", which #includes <linux/posix_types.h> -and <linux/version.h> just like we do. There simply is no alternative. -

- -

Just because directly #including kernel headers is sometimes -unavoidable doesn't me we should include them when there's a better -way to do it. However, block copying information out of the kernel headers -is not a better way.

- -
-

Who are the BusyBox developers?

- -

The following login accounts currently exist on busybox.net. (I.E. these -people can commit patches -into subversion for the BusyBox, uClibc, and buildroot projects.)

- -
-aldot     :Bernhard Reutner-Fischer
-andersen  :Erik Andersen      - uClibc and BuildRoot maintainer.
-bug1      :Glenn McGrath
-davidm    :David McCullough
-gkajmowi  :Garrett Kajmowicz  - uClibc++ maintainer
-jbglaw    :Jan-Benedict Glaw
-jocke     :Joakim Tjernlund
-landley   :Rob Landley
-lethal    :Paul Mundt
-mjn3      :Manuel Novoa III
-osuadmin  :osuadmin
-pgf       :Paul Fox
-pkj       :Peter Kjellerstedt
-prpplague :David Anders
-psm       :Peter S. Mazinger
-russ      :Russ Dill
-sandman   :Robert Griebl
-sjhill    :Steven J. Hill
-solar     :Ned Ludd
-timr      :Tim Riker
-tobiasa   :Tobias Anderberg
-vapier    :Mike Frysinger
-vda       :Denys Vlasenko     - BusyBox maintainer
-
- -

The following accounts used to exist on busybox.net, but don't anymore so -I can't ask /etc/passwd for their names. Rob Wentworth -<robwen at gmail.com> asked Google and recovered the names:

- -
-aaronl   :Aaron Lehmann
-beppu    :John Beppu
-dwhedon  :David Whedon
-erik     :Erik Andersen
-gfeldman :Gennady Feldman
-jimg     :Jim Gleason
-kraai    :Matt Kraai
-markw    :Mark Whitley
-miles    :Miles Bader
-proski   :Pavel Roskin
-rjune    :Richard June
-tausq    :Randolph Chung
-vodz     :Vladimir N. Oleynik
-
- - -
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