Auto merge of #56066 - jethrogb:jb/sgx-target, r=alexcrichton

Add SGX target to std and dependencies

This PR adds tier 3 `std` support for the `x86_64-fortanix-unknown-sgx` target.

### Background

Intel Software Guard Extensions (SGX) is an instruction set extension for x86 that allows executing code in fully-isolated *secure enclaves*. These enclaves reside in the address space of a regular user process, but access to the enclave's address space from outside (by e.g. the OS or a hypervisor) is blocked.

From within such enclaves, there is no access to the operating system or hardware peripherals. In order to communicate with the outside world, enclaves require an untrusted “helper” program that runs as a normal user process.

SGX is **not** a sandboxing technology: code inside SGX has full access to all memory belonging to the process it is running in.

### Overview

The Fortanix SGX ABI (compiler target `x86_64-fortanix-unknown-sgx`) is an interface for Intel SGX enclaves. It is a small yet functional interface suitable for writing larger enclaves. In contrast to other enclave interfaces, this interface is primarly designed for running entire applications in an enclave. The interface has been under development since early 2016 and builds on Fortanix's significant experience running enclaves in production.

Also unlike other enclave interfaces, this is the only implementation of an enclave interface that is nearly pure-Rust (except for the entry point code).

A description of the ABI may be found at https://docs.rs/fortanix-sgx-abi/ and https://github.com/fortanix/rust-sgx/blob/master/doc/FORTANIX-SGX-ABI.md.

The following parts of `std` are not supported and most operations will error when used:

* `std::fs`
* `std::process`
* `std::net::UdpSocket`

### Future plans

A separate PR (#56067) will add the SGX target to the rust compiler. In the very near future, I expect to upgrade this target to tier 2.

This PR is just the initial support to make things mostly work. There will be more work coming in the future, for example to add interfaces to the native SGX primitives, implement unwinding, optimize usercalls.

UDP and some form of filesystem support may be added in the future, but process support seems unlikely given the platform's constraints.

### Testing build

1. Install [Xargo](https://github.com/japaric/xargo): `cargo install xargo`
2. Create a new Cargo project, for example: `cargo new --bin sgxtest`.
3. Put the following in a file `Xargo.toml` next to your `Cargo.toml`:

```toml
[target.x86_64-fortanix-unknown-sgx.dependencies.std]
git = "https://github.com/jethrogb/rust"
branch = "jb/sgx-target"
```

NB. This can be quite slow. Instead, you can have a local checkout of that branch and use `path = "/path/to/rust/src/libstd"` instead. Don't forget to checkout the submodules too!

4. Build:

```sh
xargo build --target x86_64-fortanix-unknown-sgx
```

### Testing execution

Execution is currently only supported on x86-64 Linux, but support for Windows is planned.

1. Install pre-requisites. In order to test execution, you'll need to have a CPU with Intel SGX support. SGX support needs to be enabled in the BIOS. You'll also need to install the SGX driver and Platform Software (PSW) from [Intel](https://01.org/intel-software-guard-extensions).

2. Install toolchain, executor:
```sh
cargo install sgxs-tools --version 0.6.0-rc1
cargo install fortanix-sgx-tools --version 0.1.0-rc1
```

3. Start the enclave:

```sh
ftxsgx-elf2sgxs target/x86_64-fortanix-unknown-sgx/debug/sgxtest --heap-size 0x20000 --ssaframesize 1 --stack-size 0x20000 --threads 1 --debug
sgxs-append -i target/x86_64-fortanix-unknown-sgx/debug/sgxtest.sgxs
ftxsgx-runner target/x86_64-fortanix-unknown-sgx/debug/sgxtest.sgxs
```

Author: bors

.gitmodules

@@ -61,10 +61,12 @@
 	path = src/tools/clang
 	url = https://github.com/rust-lang-nursery/clang.git
 	branch = rust-release-80-v2
-
 [submodule "src/doc/rustc-guide"]
 	path = src/doc/rustc-guide
 	url = https://github.com/rust-lang/rustc-guide.git
 [submodule "src/doc/edition-guide"]
 	path = src/doc/edition-guide
 	url = https://github.com/rust-lang-nursery/edition-guide
+[submodule "src/rust-sgx"]
+	path = src/rust-sgx
+	url = https://github.com/fortanix/rust-sgx

Cargo.lock

@@ -797,6 +797,14 @@ name = "foreign-types-shared"
 version = "0.1.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 
+[[package]]
+name = "fortanix-sgx-abi"
+version = "0.0.0"
+dependencies = [
+ "compiler_builtins 0.0.0",
+ "core 0.0.0",
+]
+
 [[package]]
 name = "fs2"
 version = "0.4.3"
@@ -2773,6 +2781,7 @@ dependencies = [
  "compiler_builtins 0.0.0",
  "core 0.0.0",
  "dlmalloc 0.0.0",
+ "fortanix-sgx-abi 0.0.0",
  "libc 0.0.0",
  "panic_abort 0.0.0",
  "panic_unwind 0.0.0",

src/bootstrap/dist.rs

@@ -874,6 +874,7 @@ impl Step for Src {
             "src/rustc/compiler_builtins_shim",
             "src/rustc/libc_shim",
             "src/rustc/dlmalloc_shim",
+            "src/rustc/fortanix-sgx-abi_shim",
             "src/libtest",
             "src/libterm",
             "src/libprofiler_builtins",

src/dlmalloc

@@ -66,6 +66,12 @@ pub unsafe extern fn __rust_start_panic(_payload: usize) -> u32 {
     unsafe fn abort() -> ! {
         core::intrinsics::abort();
     }
+
+    #[cfg(target_env="sgx")]
+    unsafe fn abort() -> ! {
+        extern "C" { pub fn panic_exit() -> !; }
+        panic_exit();
+    }
 }
 
 // This... is a bit of an oddity. The tl;dr; is that this is required to link

src/libcompiler_builtins

@@ -62,7 +62,7 @@ cfg_if! {
     if #[cfg(target_os = "emscripten")] {
         #[path = "emcc.rs"]
         mod imp;
-    } else if #[cfg(target_arch = "wasm32")] {
+    } else if #[cfg(any(target_arch = "wasm32", target_env = "sgx"))] {
         #[path = "dummy.rs"]
         mod imp;
     } else if #[cfg(all(target_env = "msvc", target_arch = "aarch64"))] {

src/liblibc

@@ -35,9 +35,12 @@ rustc_lsan = { path = "../librustc_lsan" }
 rustc_msan = { path = "../librustc_msan" }
 rustc_tsan = { path = "../librustc_tsan" }
 
-[target.'cfg(all(target_arch = "wasm32", not(target_os = "emscripten")))'.dependencies]
+[target.'cfg(any(all(target_arch = "wasm32", not(target_os = "emscripten")), target_env = "sgx"))'.dependencies]
 dlmalloc = { path = '../rustc/dlmalloc_shim' }
 
+[target.x86_64-fortanix-unknown-sgx.dependencies]
+fortanix-sgx-abi = { path = "../rustc/fortanix-sgx-abi_shim" }
+
 [build-dependencies]
 cc = "1.0"
 build_helper = { path = "../build_helper" }

src/libpanic_abort/lib.rs

@@ -184,7 +184,7 @@ pub enum ErrorKind {
 }
 
 impl ErrorKind {
-    fn as_str(&self) -> &'static str {
+    pub(crate) fn as_str(&self) -> &'static str {
         match *self {
             ErrorKind::NotFound => "entity not found",
             ErrorKind::PermissionDenied => "permission denied",