Cross-compiling for Android doesn't seem to propagate architecture correctly

[korDen]

I have a basic method that I want to invoke through JNI. Think hello world. I have a similar one implemented in C. It works. Rust one fails to link. Let's start with a basic repro.

rust_static_library(
    name = "android-rs",
    srcs = ["src/lib.rs"], // #[no_mangle] pub fn hello_from_jni(...) { ... }
)

// Exact same think but in C++, just as a sanity check.
cc_library(
    name = "android-cc",
    srcs = ["sanity_check.cc"], // extern "C" void hello_from_jni(...) { ... }
)

android_binary(
    name = "android",
    custom_package = "rust.on.android",
    manifest = "AndroidManifest.xml",
    deps = [
        ":android-rs", // fails
        ":android-cc", // succeeds
    ],
)

bazel build :android

Command that fails:
external/androidndk/ndk/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang -shared -o bazel-out/android-armeabi-v7a-fastbuild/bin/_nativedeps/1_3_0_0_3_1603a52a129c5902439281827f3522ba084d3ca86608ba9c80599aee4f3a27a0.so bazel-out/android-armeabi-v7a-fastbuild/bin/platforms/android/libandroid_rs-3993832558.a external/androidndk/ndk/sources/cxx-stl/llvm-libc++/libs/armeabi-v7a/libandroid_support.a external/androidndk/ndk/sources/cxx-stl/llvm-libc++/libs/armeabi-v7a/libc++_static.a external/androidndk/ndk/sources/cxx-stl/llvm-libc++/libs/armeabi-v7a/libc++abi.a external/androidndk/ndk/sources/cxx-stl/llvm-libc++/libs/armeabi-v7a/libunwind.a '-Wl,-soname=libandroid' -lSystem -lresolv -static-libgcc -Wl,-S -target armv7-none-linux-androideabi -gcc-toolchain external/androidndk/ndk/toolchains/arm-linux-androideabi-4.9/prebuilt/darwin-x86_64 -Lexternal/androidndk/ndk/sources/cxx-stl/llvm-libc++/libs/armeabi-v7a -no-canonical-prefixes -Wl,-z,relro -Wl,--gc-sections '--sysroot=external/androidndk/ndk/platforms/android-30/arch-arm'

Errors:

external/androidndk/ndk/toolchains/arm-linux-androideabi-4.9/prebuilt/darwin-x86_64/lib/gcc/arm-linux-androideabi/4.9.x/../../../../arm-linux-androideabi/bin/ld: error: bazel-out/android-armeabi-v7a-fastbuild/bin/platforms/android/libandroid_rs-3993832558.a: no archive symbol table (run ranlib)
external/androidndk/ndk/toolchains/arm-linux-androideabi-4.9/prebuilt/darwin-x86_64/lib/gcc/arm-linux-androideabi/4.9.x/../../../../arm-linux-androideabi/bin/ld: error: cannot find -lSystem
external/androidndk/ndk/toolchains/arm-linux-androideabi-4.9/prebuilt/darwin-x86_64/lib/gcc/arm-linux-androideabi/4.9.x/../../../../arm-linux-androideabi/bin/ld: error: cannot find -lresolv

the library that .rs file compiled into is this:
bazel-out/android-armeabi-v7a-fastbuild/bin/platforms/android/libandroid_rs-3993832558.a

which at first glance appears to detect architecture correct, but running lipo -info on it gives the following information:
Non-fat file: bazel-out/android-armeabi-v7a-fastbuild/bin/platforms/android/libandroid_rs-3993832558.a is architecture: x86_64

Which is totally wrong. Also otool says it's an AR file and each object file in it has a Mach header.

By comparison, if I use android_cc and not android_rs, everything compiles and links successfully, and the produced the following:

1. libandroid.so is ELF 32-bit LSB shared object, ARM, EABI5 version 1 (SYSV), dynamically linked, not stripped
2. main.pic.o ELF 32-bit LSB relocatable, ARM, EABI5 version 1 (SYSV), not stripped

The conclusion I'm getting here is that:

• lib.rs was compiled for the host architecture (MacOS) and not for the target architecture (android armebi-v7a)
• incorrect libraries are being linked in (-lSystem and -lresolv)

[korDen]

Digged a little bit deeper and the android-rs target is being explicitly compiled with --target=x86_64-apple-darwin flag.

I tried to override target by passing my own target via rustc_flags and it failed with error:

Option 'target' given more than once

Here is complete build configuration when compiling android_rs:

bazel-out/darwin-opt-exec-2B5CBBC6/bin/external/rules_rust/util/process_wrapper/process_wrapper --subst 'pwd=${pwd}' -- bazel-out/android-armeabi-v7a-fastbuild/bin/external/rust_darwin_x86_64/toolchain_for_x86_64-apple-darwin_impl/bin/rustc platforms/android/src/lib.rs '--crate-name=android_rs' '--crate-type=staticlib' '--error-format=human' '--codegen=metadata=-3993832558' '--out-dir=bazel-out/android-armeabi-v7a-fastbuild/bin/platforms/android' '--codegen=extra-filename=-3993832558' '--codegen=opt-level=0' '--codegen=debuginfo=0' '--remap-path-prefix=${pwd}=.' '--emit=dep-info,link' '--color=always' '--target=x86_64-apple-darwin' -L bazel-out/android-armeabi-v7a-fastbuild/bin/external/rust_darwin_x86_64/toolchain_for_x86_64-apple-darwin_impl/lib/rustlib/x86_64-apple-darwin/lib '--edition=2018' '--codegen=linker=external/androidndk/ndk/toolchains/llvm/prebuilt/darwin-x86_64/bin/clang' --codegen 'link-args=-static-libgcc -target armv7-none-linux-androideabi -gcc-toolchain external/androidndk/ndk/toolchains/arm-linux-androideabi-4.9/prebuilt/darwin-x86_64 -Lexternal/androidndk/ndk/sources/cxx-stl/llvm-libc++/libs/armeabi-v7a -no-canonical-prefixes -Wl,-z,relro -Wl,--gc-sections --sysroot=external/androidndk/ndk/platforms/android-30/arch-arm' '-Lnative=external/androidndk/ndk/sources/cxx-stl/llvm-libc++/libs/armeabi-v7a' '-Lnative=external/androidndk/ndk/sources/cxx-stl/llvm-libc++/libs/armeabi-v7a' '-Lnative=external/androidndk/ndk/sources/cxx-stl/llvm-libc++/libs/armeabi-v7a' '-Lnative=external/androidndk/ndk/sources/cxx-stl/llvm-libc++/libs/armeabi-v7a' '-lstatic=android_support' '-lstatic=c++_static' '-lstatic=c++abi' '-lstatic=unwind')

[korDen]

I noticed that --codegen does have the right target (well, almost, rust wants a target triple but android is using quadruple), and used it as the target with a small patch like this:

-    rustc_flags.add("--target=" + toolchain.target_flag_value)
+    target_flag_value = toolchain.target_flag_value

(obtain link_args and then)
+            for i in range(0, len(link_args) - 1):
+                if link_args[i] == "-target":
+                    target_flag_value = link_args[i + 1].replace("-none-", "-") # armv7-none-linux-androideabi  -> armv7-linux-androideabi 
+                    break

(at the end)
+    rustc_flags.add("--target=" + target_flag_value)

This fixed the rustc invocation, replacing host target triple with correct target triple, but now I'm facing another issue:

error[E0461]: couldn't find crate std with expected target triple armv7-linux-androideabi

This is something I could fix with rustup (e.g rustup target add armv7-linux-androideabi) but no idea how to fix it with bazel :(

[korDen]

#1181 is aiming to fix this issue.