Use global_asm! instead of external assembly files (#4306)

* Use `global_asm!` instead of external assembly files

This commit moves the external assembly files of the `wasmtime-fiber`
crate into `global_asm!` blocks defined in Rust. The motivation for
doing this is not very strong at this time, but the points in favor of
this are:

* One less tool needed to cross-compile Wasmtime. A linker is still
  needed but perhaps one day that will improve as well.
* A "modern" assembler, built-in to LLVM, is used instead of whatever
  appears on the system.

The first point hasn't really cropped up that much and typically getting
an assembler is just as hard as getting a linker nowadays. The second
point though has us using `hint #xx` in aarch64 assembly instead of the
actual instructions for assembler compatibility, and I believe that's no
longer necessary because the LLVM assembler supports the modern
instruction names.

The translation of the x86/x86_64 assembly has been done to Intel
syntax as well as opposed to the old AT&T syntax since that's Rust's
default. Additionally s390x still remains in an external assembler file
because `global_asm!` is still unstable in Rust on that platform.

* Simplify alignment specification

* Temporarily disable fail-fast

* Add `.cfi_def_cfa_offset 0` to fix CI

* Turn off fail-fast

* Review comments

crates/fiber/src/unix/aarch64.rs

@@ -0,0 +1,175 @@
+// A WORD OF CAUTION
+//
+// This entire file basically needs to be kept in sync with itself. It's not
+// really possible to modify just one bit of this file without understanding
+// all the other bits. Documentation tries to reference various bits here and
+// there but try to make sure to read over everything before tweaking things!
+//
+// Also at this time this file is heavily based off the x86_64 file, so you'll
+// probably want to read that one as well.
+//
+// Finally, control flow integrity hardening has been applied to the code using
+// the Pointer Authentication (PAuth) and Branch Target Identification (BTI)
+// technologies from the Arm instruction set architecture:
+// * All callable functions start with either the `BTI c` or `PACIASP`/`PACIBSP`
+//   instructions
+// * Return addresses are signed and authenticated using the stack pointer
+//   value as a modifier (similarly to the salt in a HMAC operation); the
+//   `DW_CFA_AARCH64_negate_ra_state` DWARF operation (aliased with the
+//   `.cfi_window_save` assembler directive) informs an unwinder about this
+
+cfg_if::cfg_if! {
+    if #[cfg(target_os = "macos")] {
+        macro_rules! cfi_window_save { () => (); }
+        macro_rules! pacia1716 { () => (); }
+        macro_rules! paciasp { () => (); }
+        macro_rules! autiasp { () => (); }
+    } else {
+        macro_rules! cfi_window_save { () => (".cfi_window_save\n"); }
+        macro_rules! pacia1716 { () => ("pacia1716\n"); }
+        macro_rules! paciasp { () => ("paciasp\n"); }
+        macro_rules! autiasp { () => ("autiasp\n"); }
+    }
+}
+
+// fn(top_of_stack(%x0): *mut u8)
+asm_func!(
+    "wasmtime_fiber_switch",
+    "
+        .cfi_startproc
+    ",
+    paciasp!(),
+    cfi_window_save!(),
+    "
+        // Save all callee-saved registers on the stack since we're
+        // assuming they're clobbered as a result of the stack switch.
+        stp x29, x30, [sp, -16]!
+        stp x20, x19, [sp, -16]!
+        stp x22, x21, [sp, -16]!
+        stp x24, x23, [sp, -16]!
+        stp x26, x25, [sp, -16]!
+        stp x28, x27, [sp, -16]!
+        stp d9, d8, [sp, -16]!
+        stp d11, d10, [sp, -16]!
+        stp d13, d12, [sp, -16]!
+        stp d15, d14, [sp, -16]!
+
+        // Load our previously saved stack pointer to resume to, and save
+        // off our current stack pointer on where to come back to
+        // eventually.
+        ldr x8, [x0, -0x10]
+        mov x9, sp
+        str x9, [x0, -0x10]
+
+        // Switch to the new stack and restore all our callee-saved
+        // registers after the switch and return to our new stack.
+        mov sp, x8
+        ldp d15, d14, [sp], 16
+        ldp d13, d12, [sp], 16
+        ldp d11, d10, [sp], 16
+        ldp d9, d8, [sp], 16
+        ldp x28, x27, [sp], 16
+        ldp x26, x25, [sp], 16
+        ldp x24, x23, [sp], 16
+        ldp x22, x21, [sp], 16
+        ldp x20, x19, [sp], 16
+        ldp x29, x30, [sp], 16
+    ",
+    autiasp!(),
+    cfi_window_save!(),
+    "
+        ret
+        .cfi_endproc
+    ",
+);
+
+// fn(
+//    top_of_stack(%x0): *mut u8,
+//    entry_point(%x1): extern fn(*mut u8, *mut u8),
+//    entry_arg0(%x2): *mut u8,
+// )
+// We set up the newly initialized fiber, so that it resumes execution
+// from wasmtime_fiber_start(). As a result, we need a signed address
+// of this function, so there are 2 requirements:
+// * The fiber stack pointer value that is used by the signing operation
+//   must match the value when the pointer is authenticated inside
+//   wasmtime_fiber_switch(), otherwise the latter would fault
+// * We would like to use an instruction that is executed as a no-op by
+//   processors that do not support PAuth, so that the code is
+//   backward-compatible and there is no duplication; `PACIA1716` is a
+//   suitable one, which has the following operand register
+//   conventions:
+//   * X17 contains the pointer value to sign
+//   * X16 contains the modifier value
+//
+// TODO: Use the PACGA instruction to authenticate the saved register
+// state, which avoids creating signed pointers to
+// wasmtime_fiber_start(), and provides wider coverage.
+#[rustfmt::skip]
+asm_func!(
+    "wasmtime_fiber_init",
+    "
+        .cfi_startproc
+        hint #34 // bti c
+        sub x16, x0, #16
+        adr x17, ", asm_sym!("wasmtime_fiber_start"), "
+    ",
+    pacia1716!(),
+    "
+        str x17, [x16, -0x8] // x17 => lr
+        str x0, [x16, -0x18] // x0 => x19
+        stp x2, x1, [x0, -0x38] // x1 => x20, x2 => x21
+
+        // `wasmtime_fiber_switch` has an 0xa0 byte stack, and we add 0x10 more for
+        // the original reserved 16 bytes.
+        add x8, x0, -0xb0
+        str x8, [x0, -0x10]
+        ret
+        .cfi_endproc
+    ",
+);
+
+// See the x86_64 file for more commentary on what these CFI directives are
+// doing. Like over there note that the relative offsets to registers here
+// match the frame layout in `wasmtime_fiber_switch`.
+asm_func!(
+    "wasmtime_fiber_start",
+    "
+        .cfi_startproc simple
+        .cfi_def_cfa_offset 0
+        .cfi_escape 0x0f,    /* DW_CFA_def_cfa_expression */ \
+            5,               /* the byte length of this expression */ \
+            0x6f,            /* DW_OP_reg31(%sp) */ \
+            0x06,            /* DW_OP_deref */ \
+            0x23, 0xa0, 0x1  /* DW_OP_plus_uconst 0xa0 */
+        .cfi_rel_offset x29, -0x10
+        .cfi_rel_offset x30, -0x08
+    ",
+    cfi_window_save!(),
+    "
+        .cfi_rel_offset x19, -0x18
+        .cfi_rel_offset x20, -0x20
+        .cfi_rel_offset x21, -0x28
+        .cfi_rel_offset x22, -0x30
+        .cfi_rel_offset x23, -0x38
+        .cfi_rel_offset x24, -0x40
+        .cfi_rel_offset x25, -0x48
+        .cfi_rel_offset x26, -0x50
+        .cfi_rel_offset x27, -0x58
+
+        // Load our two arguments from the stack, where x1 is our start
+        // procedure and x0 is its first argument. This also blows away the
+        // stack space used by those two arguments.
+        mov x0, x21
+        mov x1, x19
+
+        // ... and then we call the function! Note that this is a function call
+        // so our frame stays on the stack to backtrace through.
+        blr x20
+        // Unreachable, here for safety. This should help catch unexpected
+        // behaviors.  Use a noticeable payload so one can grep for it in the
+        // codebase.
+        brk 0xf1b3
+        .cfi_endproc
+    ",
+);