Module twizzler_abi::rt1
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Runtime functions for Twizzler userspace. This part of the code is a bit arcane and somewhat tricky, so buckle up.
We need to start executing in a reasonable environment before we call into the Rust runtime init. Rust actually expects a fair bit of us right off the bat — thread-local storage (TLS), env vars, args, etc. So our goal will be to set up an environment where we can serve the right underlying runtime to Rust.
Execution will start at the _start symbol, provided in arch::_start. This will almost immediately call twz_runtime_start. From there, we: 0. Initialize global context.
- Process the aux array.
- Find the TLS template region and store that info.
- Create a TLS region for ourselves, the main thread.
- Set the TLS region via the kernel.
- Run the pre-init array, _init(), and the init_array.
- Call std_runtime_start, which jumps into the Rust standard lib.
- Exit our thread should we return here.
And all of that has to happen without a panic runtime, so any errors we encounter we need to abort(). This is what we get for not linking to libc.
This does not encompass all of the runtime pieces. We also have:
- crti and crtn, which we provide, see toolchain/src/crti.rs etc.
- crtbegin and friends. These are provided by LLVM’s crtstuff and distributed with the toolchain. They have interesting linking requirements, see below.
- libunwind. Not strictly required as part of the runtime, but we build with panic_unwind for userspace by default, so I’m including it. This also comes from llvm.
For information about linking order and how the linking actually happens, take a look at toolchain/src/rust/compiler/rustc_target/spec/{twizzler_base.rs, x86_64-unknown-twizzler.rs, x86_64-unknown-twizzler-linker-script.ld}.