investigate cross-compilation with LLVM tools and without GNU binutils

[tpimh]

I'm not sure that cross compilation is possible with llvm tools at this point. I will research the topic and post my findings in this thread.

[nickdesaulniers]

I don't follow. All of Android is cross compiled via LLVM. Which parts are you referring to specifically? LLVM uses a combination of -triple <target triple> and --sysroot <path to cross crt0.o and friends> .

[tpimh]

Right now we are installing binutils-aarch64-linux-gnu, binutils-arm-linux-gnueabi, binutils-powerpc64le-linux-gnu. These packages provide tools specific for each platform with respective cross-prefixes. My idea is to replace these tools with the ones from llvm-8 package with "llvm-" prefix.
For example, instead of using objcopy, aarch64-linux-gnu-objcopy, arm-linux-gnueabi-objcopy, powerpc64le-linux-gnu-objcopy use llvm-objcopy for all platforms.

[nickdesaulniers]

@rupprecht is looking in to such incompatibilties as well. https://github.com/ClangBuiltLinux/linux/issues?q=is%3Aopen+is%3Aissue+label%3Allvm-tools is our list of known issues (separate from Clang, LLD, or integrated-as).

[tpimh]

In order to make sure there is no binutils installed I do dpkg -r --force-depends binutils binutils-common binutils-x86-64-linux-gnu libbinutils for x86_64, then apt remove binutils-* for all other platforms. At the moment it's not possible to build the kernel with clang's integrated as, so this is not very useful.
However, it's possible to use llvm-ar, llvm-nm, llvm-objdump, llvm-objcopy and llvm-strip.

[tpimh]

After very little modification (replacing about 10 lines of unsupported assembly with nops), I finally have built an x86_64 defconfig kernel without using GNU binutils. It obviously should not be bootable, but it was linked successfully.
Currently the issues are the following:

• expected a symbol reference in '.long' directive in include/trace/events/initcall.h linux#413
• error: expected a symbol reference in cpufeature.h (x86_64 with integrated-as) linux#575
• error: expected assembly-time absolute expression in cpufeature.h (x86_64 with integrated-as) linux#576

There may be other issues like invalid code generation, but they are very unlikely to be detected until a kernel is compiled without stubs.

[nickdesaulniers]

cc @gwelymernans

[tpimh]

Hooray! I have successfully build an arm32_v5 kernel without using GNU binutils: travis build log.

[nathanchance]

Awesome! Something like this should reveal if everything is hermetic:

nathanchance@0e2a26e

I suspect we are still implicitly using ld as the host linker when HOSTCC=clang for certain things, as that is what I ran into:

nathanchance@730028d

ClangBuiltLinux/linux#479

Otherwise, I'd expect only LLVM tools to be used.

[tpimh]

Thank you very much for your suggestions. I was sure that GNU binutils were not used during the build, but still wanted to remove them. Turned out the build was not actually hermetic!

[nickdesaulniers]

great find, filed ClangBuiltLinux/linux#766