Use Restricted std in UEFI

8 min read June 16, 2022 #rust #tianocore #gsoc22 #uefi

Hello Everyone; in my last post, I set up the development environment for working on adding Rust support for UEFI. In this post, I will get a restricted version of std (basically a glorified core + alloc) to work for the x86_64 UEFI target. We will be starting with the no_std hello_world program from the last post.

Add env module for UEFI in std

Add uefi Module

First, I will add uefi module under library/std/src/sys. To do this, we will create a new file library/std/src/sys/uefi/mod.rs. We will point everything other than the env module to the unsupported module. The contents of library/std/src/sys/uefi/mod.rs are given below:

//! Platform-specific extensions to `std` for UEFI platforms.
//!
//! Provides access to platform-level information on UEFI platforms, and
//! exposes Unix-specific functions that would otherwise be inappropriate as
//! part of the core `std` library.
//!
//! It exposes more ways to deal with platform-specific strings ([`OsStr`],
//! [`OsString`]), allows to set permissions more granularly, extract low-level
//! file descriptors from files and sockets, and has platform-specific helpers
//! for spawning processes.
//!
//! [`OsStr`]: crate::ffi::OsStr
//! [`OsString`]: crate::ffi::OsString

#![deny(unsafe_op_in_unsafe_fn)]

#[path = "../unsupported/alloc.rs"]
pub mod alloc;
#[path = "../unsupported/args.rs"]
pub mod args;
#[path = "../unix/cmath.rs"]
pub mod cmath;
pub mod env;
#[path = "../unsupported/fs.rs"]
pub mod fs;
#[path = "../unsupported/io.rs"]
pub mod io;
#[path = "../unsupported/locks/mod.rs"]
pub mod locks;
#[path = "../unsupported/net.rs"]
pub mod net;
#[path = "../unsupported/os.rs"]
pub mod os;
#[path = "../windows/os_str.rs"]
pub mod os_str;
#[path = "../unix/path.rs"]
pub mod path;
#[path = "../unsupported/pipe.rs"]
pub mod pipe;
#[path = "../unsupported/process.rs"]
pub mod process;
#[path = "../unsupported/stdio.rs"]
pub mod stdio;
#[path = "../unsupported/thread.rs"]
pub mod thread;
#[path = "../unsupported/thread_local_key.rs"]
pub mod thread_local_key;
#[path = "../unsupported/time.rs"]
pub mod time;

#[path = "../unsupported/common.rs"]
#[deny(unsafe_op_in_unsafe_fn)]
mod common;
pub use common::*;

As you can see, cmath, os_str, and path point to the unix or windows module instead of unsupported. This is because unsupported does not provide any definition for those. The cmath unix module API can be provided by compiler-builtins, so that should work. However, os_str and path are simply placeholders for now.

Implement env module

The env module is very basic and we just need to define a few constants for it. Here are the contents of library/std/src/sys/uefi/env.rs:

pub mod os {
    pub const FAMILY: &str = "";
    pub const OS: &str = "";
    pub const DLL_PREFIX: &str = "";
    pub const DLL_SUFFIX: &str = "";
    pub const DLL_EXTENSION: &str = "";
    pub const EXE_SUFFIX: &str = ".efi";
    pub const EXE_EXTENSION: &str = "efi";
}

Export UEFI module contents

This can be done by adding the following lines to library/std/sys/mod.rs:

     } else if #[cfg(target_family = "wasm")] {
         mod wasm;
         pub use self::wasm::*;
+    } else if #[cfg(target_os = "uefi")] {
+        mod uefi;
+        pub use self::uefi::*;
     } else if #[cfg(all(target_vendor = "fortanix", target_env = "sgx"))] {
         mod sgx;
         pub use self::sgx::*;

And we now have the env module implemented. These constants are exposed under std::env::consts.

Build the new toolchain

We will now need to build the toolchain again:

./x.py build --stage 1

stage1 should now be pointing to this new toolchain.

Now, we will try to print these constants to UEFI in a very primitive manner (since io, strings, etc have not been implemented yet).


Compile hello_world with std

Firstly, we will remove the no_std attribute and panic_handler from src/main.rs. Then we will update .cargo/config.toml with the following contents:

[unstable]
build-std = ["std", "compiler_builtins"]
build-std-features = ["compiler-builtins-mem"]

On trying to compile the project now, we get the following error:

error[E0463]: can't find crate for `panic_abort`

error[E0658]: use of unstable library feature 'restricted_std'
  |
  = help: add `#![feature(restricted_std)]` to the crate attributes to enable

Some errors have detailed explanations: E0463, E0658.
For more information about an error, try `rustc --explain E0463`.
error: could not compile `hello_world` due to 2 previous errors

The first error is a bug in build-std. It doesn’t know how to handle panic_abort vs panic_unwind, so it doesn’t use either crate, resulting in the above error. It can be fixed by adding panic_abort to .cargo/config.toml.

The second error can be fixed by adding the restricted_std feature to the src/main.rs.

Trying to build now and ………. It fails. We get the following error:

error: linking with `rust-lld` failed: exit status: 1
  |
  = note: "rust-lld" "-flavor" "link" "/NOLOGO" "/entry:efi_main" "/subsystem:efi_application" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/hello_world-bf96c91d419b97ff.3aumljakx58twtt1.rcgu.o" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/hello_world-bf96c91d419b97ff.11wx9iybsb4s2x54.rcgu.o" "/LIBPATH:/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps" "/LIBPATH:/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/debug/deps" "/LIBPATH:/var/home/ayush/Documents/Programming/Rust/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/target/lib" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/libr_efi-44f0e2c98ca397ed.rlib" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/libstd-0f82fcd1446bb823.rlib" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/libpanic_abort-6267bb336da2fa77.rlib" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/librustc_demangle-12ba4dfb1ce0974e.rlib" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/libstd_detect-af3466d26b0b584c.rlib" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/libhashbrown-282ed18a03cc159e.rlib" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/librustc_std_workspace_alloc-d47af0fe9b191470.rlib" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/libunwind-8ad628841136827a.rlib" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/libcfg_if-f793ff480fd551b6.rlib" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/liblibc-249d18e9ef84acfd.rlib" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/liballoc-4b1b3794d0343e8a.rlib" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/librustc_std_workspace_core-3a7d1ce70363f171.rlib" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/libcore-73e7a0474be04fb7.rlib" "/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/libcompiler_builtins-4f95060227077e02.rlib" "/NXCOMPAT" "/LIBPATH:/var/home/ayush/Documents/Programming/Rust/rust/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/target/lib" "/OUT:/var/home/ayush/Documents/Programming/Rust/uefi/hello_world/target/target/debug/deps/hello_world-bf96c91d419b97ff.efi" "/OPT:REF,NOICF" "/DEBUG" "/NODEFAULTLIB"
  = note: rust-lld: error: undefined symbol: __CxxFrameHandler3
          >>> referenced by libstd-0f82fcd1446bb823.rlib(std-0f82fcd1446bb823.std.ea9c30f6-cgu.2.rcgu.o):(.xdata)
          >>> referenced by libstd-0f82fcd1446bb823.rlib(std-0f82fcd1446bb823.std.ea9c30f6-cgu.2.rcgu.o):(.xdata)
          

error: could not compile `hello_world` due to previous error

This error is, well, a bit weird. We can fix the build for now by providing a blank implementation of __CxxFrameHandler3, but this needs more research. The following lines need to be added to src/main.rs:

#[no_mangle]
pub extern "C" fn __CxxFrameHandler3() {}

The application builds and runs fine now.

Now we will print EXE_EXTENSION to the console. Since we do not have io and string implemented yet, we will have to do it in a primitive way using u16 arrays. The final src/main.rs is given below:

#![no_main]
#![feature(restricted_std)]

use r_efi::efi;
use std::env::consts;

#[no_mangle]
pub extern "C" fn __CxxFrameHandler3() {}

fn print_efi(s: &[u16], st: *mut efi::SystemTable) -> Result<(), r_efi::base::Status> {
    let r = unsafe { ((*(*st).con_out).output_string)((*st).con_out, s.as_ptr() as *mut efi::Char16) };

    if r.is_error() {
        Err(r)
    } else {
        Ok(())
    }
}

fn print_newline(st: *mut efi::SystemTable) -> Result<(), r_efi::base::Status> {
    let mut s = [0;2];
    create_const_uefi_str("\n", &mut s);
    print_efi(&s, st)
}

fn create_const_uefi_str(const_str: &str, uefi_array: &mut [u16]) {
    let mut i = 0;
    for v in const_str.bytes() {
        uefi_array[i] = v as u16;
        i += 1;
    }

    uefi_array[i] = 0x0000u16;
}

fn print_env_constants(st: *mut efi::SystemTable) -> Result<(), r_efi::base::Status> {
    let mut exe_extension_heading = [0; 16];
    create_const_uefi_str("exe_extension: ", &mut exe_extension_heading);
    print_efi(&exe_extension_heading, st)?;
    let mut exe_extension = [0; 5];
    create_const_uefi_str(consts::EXE_EXTENSION, &mut exe_extension);
    print_efi(&exe_extension, st)?;
    print_newline(st)
}

#[export_name = "efi_main"]
pub extern "C" fn main(_h: efi::Handle, st: *mut efi::SystemTable) -> efi::Status {
    let r = print_env_constants(st);

    if let Err(x) = r {
        return x;
    }

   efi::Status::SUCCESS
}

Here is the program running under qemu:

With this, we are using our new std for UEFI.


Edit

The __CxxFrameHandler3 blank implementation is no longer required in the master branch. I was previously basing my changes on the v1.61.0 tag. However, from now, I am going to work on master directly.


Conclusion

Technically, we are now using std (even though none of it has yet been implemented). Now I will slowly start implementing parts of std starting with allocation. I also wanted to find a way to use the normal Rust main function instead of the current efi_main. However, this still does not seem possible (see #29633). So, let’s get allocation working and replace all the arrays with vectors in this code.

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