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diff --git a/www/faq.html b/www/faq.html index 23f5d818..5e054f02 100755 --- a/www/faq.html +++ b/www/faq.html @@ -16,6 +16,8 @@ <h2>Using toybox</h2> <ul> +<!-- get binaries --> +<li><h2><a href="#cross">How do I cross compile toybox?</h2></li> <li><h2><a href="#mkroot">How do you build a working Linux system with toybox?</a></h2></li> </ul> @@ -231,6 +233,103 @@ instead of the toybox version (in theory they should work the same), and VERBOSE=1 to see diffs of the expected and actual output when a test fails (VERBOSE=fail to stop at the first such failure)</p> +<a name="cross" /> +<h2>Q: How do I cross compile toybox?</h2> + +<p>A: toybox is tested against three C libraries (bionic, musl, glibc) +with 2 compilers (llvm, gcc). The easy way to get coverage (if not every +combination) is:</p> + +<p><b>1) gcc+glibc = host toolchain</b></p> + +<p>Most Linux distros come with that as a host compiler, just build normally +(<b>make distclean defconfig toybox</b>, or <b>make menuconfig</b> followed +by <b>make</b>).</p> + +<p>You can use LDFLAGS=--static if you want static binaries, but static +glibc is hugely inefficient ("hello world" is 810k on x86-64) and throws a +zillion linker warnings because one of its previous maintainers +<a href=https://www.akkadia.org/drepper/no_static_linking.html>was insane</a> +(which meant at the time he refused to fix +<a href=https://elinux.org/images/2/2d/ELC2010-gc-sections_Denys_Vlasenko.pdf>obvious bugs</a>), plus it uses dlopen() at runtime to implement basic things like +<a href=https://stackoverflow.com/questions/15165306/compile-a-static-binary-which-code-there-a-function-gethostbyname>DNS lookup</a> (which is impossible +to support properly from a static binary because you wind up with two +instances of malloc() managing two heaps which corrupt as soon as a malloc() +from one is free()d into the other), although glibc added +<a href=https://stackoverflow.com/questions/14289488/use-dlsym-on-a-static-binary>improper support</a> which still requires the shared libraries to be +installed on the system alongside the static binary: +<a href=https://www.youtube.com/watch?v=Ih-3vK2qLls>in brief, avoid</a>.) +These days glibc is maintained by a committee instead of a single +maintainer, if you consider that an improvement.</p> + +<p><b>2) gcc+musl = musl-cross-make</b> + +<p>The cross compilers I test this with are built from the +<a href=http://musl.libc.org/>musl-libc</a> maintainer's +<a href=https://github.com/richfelker/musl-cross-make>musl-cross-make</a> +project, built by running toybox's scripts/mcm-buildall.sh in that directory, +and then symlink the resulting "ccc" subdirectory into toybox where +"make root CROSS=" can find them, ala:</p> + +<blockquote><pre> +cd ~ +git clone https://github.com/landley/toybox +git clone https://github.com/richfelker/musl-cross-make +cd musl-cross-make +../toybox/scripts/mcm-buildall.sh # this takes a while +ln -s $(realpath ccc) ../toybox/ccc +</pre></blockquote> + +<p>Instead of symlinking ccc, you can specify a CROSS_COMPILE= prefix +in the same format the Linux kernel build uses. You can either provide a +full path in the CROSS_COMPILE string, or add the appropriate bin directory +to your $PATH. I.E:</p> + +<blockquote> +<p>make LDFLAGS=--static CROSS_COMPILE=~/musl-cross-make/ccc/m68k-linux-musl-cross/bin/m68k-linux-musl- distclean defconfig toybox</p> +</blockquote> + +<p>Is equivalent to:</p> + +<blockquote><p> +export "PATH=~/musl-cross-make/ccc/m68k-linux-musl-cross/bin:$PATH"<br /> +LDFLAGS=--static make distclean defconfig toybox CROSS=m68k-linux-musl- +</p></blockquote> + +<p>Note: a non-static build won't run unless you install musl on your host. +In theory you could "make root" a dynamic root filesystem with musl by copying +the shared libraries out of the toolchain, but I haven't bothered implementing +that yet because a static linked musl hello world is 10k on x86 +(5k if stripped).</p> + +<p><b>3) llvm+bionic = Android NDK</b></p> + +<p>The <a href=https://developer.android.com/ndk/downloads>Android +Native Development Kit</a> provides an llvm toolchain with the bionic +libc used by Android. To turn it into something toybox can use, you +just have to add an appropriately prefixed "cc" symlink to the other +prefixed tools, ala:</p> + +<blockquote><pre> +unzip android-ndk-r21b-linux-x86_64.zip +cd android-ndk-21b/toolchains/llvm/prebuilt/linux-x86_64/bin +ln -s x86_64-linux-android29-clang x86_64-linux-android-cc +PATH="$PWD:$PATH" +cd ~/toybox +make distclean +make LDFLAGS=--static CROSS_COMPILE=x86_64-linux-android- defconfig toybox +</pre></blockquote> + +<p>Again, you need a static link unless you want to install bionic on your +host. Binaries statically linked against bionic are almost as big as with +glibc, but at least it doesn't have the dlopen() issues.</p> + +<p>Unfortunately, although the resulting toybox will run a bionic-based +chroot will not, because even "hello world" statically linked +against bionic will segfault before calling main() if /dev/null isn't +present, and the init script written by mkroot.sh has to run a shell linked +against bionic in order to mount devtmpfs on /dev to provide /dev/null.</p> + <a name="mkroot" /> <h2>Q: How do you build a working Linux system with toybox?</h2> |