get basic no_std example to build

.gitignore

@@ -1,3 +1,10 @@
+# ignore built APE and debug executables
+*.com
+*.com.dbg
+# download cosmo amalgamation
+/libcosmo/*
+!/libcosmo/README.md
+
 # Generated by Cargo
 # will have compiled files and executables
 /target/

Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "hello_world"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

README.md

@@ -0,0 +1,111 @@
+# Actually Portable Executables with Cosmopolitan Libc and Rust
+
+This repository contains a simple `Hello world!` example in the [Rust][rust]
+programming language, that builds with [Cosmopolitan Libc][cosmo]. 
+
+I created a [custom compilation target][custom-target] for Rust, called
+`x86_64-unknown-linux-cosmo`, to provide a build process the Cosmopolitan Libc
+amalgamation and `cargo`. I followed the documentation in the [Rust
+Embedonomicon][custom-embed] to create the target.
+
+An alternative method to build APEs with Rust would be to avoid `cargo`, just
+use `rustc` or equivalent compiler to generate `.o` files, and then write a
+shell script that does the linking with the expected flags. I have not tried
+this method.
+
+## Build steps
+
+1. Download the Cosmopolitan Libc [amalgamation][amalg-download] into the `libcosmo` folder:
+
+```bash
+cd libcosmo
+wget https://justine.lol/cosmopolitan/cosmopolitan.zip
+unzip cosmopolitan.zip
+cd ../
+```
+
+2. Download the necessary toolchain(s) and source code for Rust:
+
+```bash
+rustup toolchain install nightly-x86_64-unknown-linux-gnu
+rustup component add rust-src --toolchain nightly-x86_64-unknown-linux-gnu
+```
+
+3. run `cargo build` to get the debug executable. This uses a bash script that
+   removes unnecessary linker arguments. A recent version of `gcc` and `ld.bfd`
+   is required.
+
+```bash
+cargo +nightly build -Zbuild-std=core,libc --target=./x86_64-unknown-linux-cosmo.json
+```
+
+4. run `objcopy` on the debug binary to obtain the APE:
+
+```bash
+objcopy -SO binary ./target/x86_64-unknown-linux-cosmo/debug/hello_world.com.dbg ./hello_world.com
+```
+
+## What about the `std` crate?
+
+The `std` crate compiles successfully, but fails at the linker stage. Here's how
+that can be tested:
+
+1. Change the source code in `src/main.rs` to use the commented out `main`
+   function and `#![restricted_std]`.
+
+2. In the source code for Rust's `std` crate, change a `cfg_if` in
+   `$HOME/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/backtrace/src/backtrace/mod.rs`
+   to not use the `noop` trace instead of depending on `libunwind`. 
+
+```diff
+--- mod.rs	2022-06-21 12:52:21.724053459 +0530
++++ mod.rs	2022-06-21 13:05:50.948777093 +0530
+@@ -132,7 +132,7 @@
+         pub(crate) mod miri;
+         use self::miri::trace as trace_imp;
+         pub(crate) use self::miri::Frame as FrameImp;
+-    } else if #[cfg(
++    } /* else if #[cfg(
+         any(
+             all(
+                 unix,
+@@ -154,7 +154,7 @@
+         pub(crate) use self::dbghelp::Frame as FrameImp;
+         #[cfg(target_env = "msvc")] // only used in dbghelp symbolize
+         pub(crate) use self::dbghelp::StackFrame;
+-    } else {
++    } */ else {
+         mod noop;
+         use self::noop::trace as trace_imp;
+         pub(crate) use self::noop::Frame as FrameImp;
+```
+
+I haven't figured out what config I should give to `cargo` so I don't need to do
+this. I find it surprising that `std` cannot be built without relying on
+`libunwind`.
+
+3. The build command now changes to
+
+```bash
+cargo +nightly build -Zbuild-std=core,alloc,panic_abort,libc,std -Zbuild-std-features=  --target=./x86_64-unknown-linux-cosmo.json
+```
+
+4. At the linker stage you might find some of the following symbols are missing:
+  
+  - If `Unwind_Backtrace` or similar is missing, you need to check if step 2 is
+    done properly.
+
+  - `open64`, `stat64`, `fstat64`, `__xpg_strerror_r`: these can be added to
+    Cosmopolitan Libc via aliases of the existing functions.
+
+  - `pthread_key_create`,`pthread_setspecific`,`pthread_key_delete`,`pthread_getspecific`
+    -- these functions should not be needed in a single-threaded program, but
+    somehow they are still linked. Support for threads is currently being added
+    to Cosmopolitan Libc, but possibly the related code in the `std` crate can
+    be changed to avoid this issue.
+
+[rust]: https://rust-lang.org
+[cosmo]: https://github.com/jart/cosmopolitan
+[amalg-download]: https://justine.lol/cosmopolitan/download.html
+[custom-target]: https://doc.rust-lang.org/rustc/targets/custom.html
+[custom-embed]: https://docs.rust-embedded.org/embedonomicon/custom-target.html

gcc-linker-wrapper.bash

@@ -0,0 +1,21 @@
+#!/bin/bash
+set -eu
+
+declare -a args
+args=()
+for o in "$@" ; do
+    case $o in
+        "-lunwind") continue;;
+        "-lutil") continue;;
+        "-lrt") continue;;
+        "-lc") continue;;
+        "-lm") continue;;
+        "-lpthread") continue;;
+        "-lgcc") continue;;
+        "-Wl,-Bdynamic") continue;;
+        "-Wl,-Bstatic") continue;;
+    esac
+    args+=("$o")
+done
+
+gcc "${args[@]}"

src/main.rs

@@ -0,0 +1,31 @@
+#![no_main]
+#![no_std]
+#![feature(rustc_private)]
+
+extern crate libc;
+
+#[no_mangle]
+pub extern "C" fn main(_argc: isize, _argv: *const *const u8) -> isize {
+    // Since we are passing a C string the final null character is mandatory
+    const HELLO: &'static str = "Hello, world! %d + %d = %d\n\0";
+    let x: i32 = 1;
+    let y: i32 = 2;
+    unsafe {
+        libc::printf(HELLO.as_ptr() as *const _, x, y, x+y);
+    }
+    0
+}
+
+#[panic_handler]
+fn my_panic(_info: &core::panic::PanicInfo) -> ! {
+    loop {}
+}
+
+/* std crate requires some external changes
+
+#![feature(restricted_std)]
+
+pub fn main() {
+    println!("hello pls cosmo rust APE");
+}
+*/

x86_64-unknown-linux-cosmo.json

@@ -0,0 +1,62 @@
+{
+  "llvm-target": "x86_64-unknown-linux-musl",
+  "target-pointer-width": "64",
+  "arch": "x86_64",
+  "data-layout": "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128",
+  "cpu": "x86-64",
+  "os":"linux",
+  "env": "musl",
+  "panic-strategy":"abort",
+  "requires-uwtable":false,
+  "dynamic-linking": false,
+  "executables": true,
+  "exe-suffix": ".com.dbg",
+  "emit-debug-gdb-scripts":false,
+  "crt-static-default": true,
+  "crt-static-respected": false,
+  "linker-is-gnu":true,
+  "allows-weak-linkage":true,
+  "has-rpath": false,
+  "has-thread-local": false,
+  "is-builtin": false,
+  "trap-unreachable":true,
+  "position-independent-executables": false,
+  "static-position-independent-executables": false,
+  "relocation-model": "static",
+  "disable-redzone":true,
+  "frame-pointer":"always",
+  "requires-lto":false,
+  "eh-frame-header":false,
+  "no-default-libraries":true,
+  "max-atomic-width":64,
+  "linker-flavor":"gcc",
+  "linker": "./gcc-linker-wrapper.bash",
+  "late-link-args": {
+      "gcc": []
+  },
+  "pre-link-args": {
+      "gcc": [
+          "-static",
+          "-nostdinc",
+          "-nostdlib",
+          "-pg",
+          "-mnop-mcount"
+      ]
+  },
+  "post-link-args": {
+      "gcc": [
+          "-Wl,--gc-sections",
+          "-fuse-ld=bfd",
+          "-Wl,-T,./libcosmo/ape.lds",
+          "./libcosmo/crt.o",
+          "./libcosmo/ape-no-modify-self.o",
+          "./libcosmo/cosmopolitan.a"
+      ]
+  },
+  "stack-probes": {
+    "kind": "none"
+  },
+  "target-family": [
+    "unix"
+  ]
+}