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Add SEH for aarch64 UEFI target #75

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69bea16
Add SEH for aarch64 UEFI target
joschock Mar 18, 2026
98db1a3
load value from parent link field.
joschock Apr 3, 2026
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89 changes: 75 additions & 14 deletions src/arch/aarch64.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -80,17 +80,40 @@ use crate::unwind::{
InitialFunc, StackCallFunc, TrapHandler,
};
use crate::util::EncodedValue;
// The cfi!() and seh!() macros ensure that DWARF CFI and SEH directives are
// mutually exclusive: UEFI uses SEH (.pdata/.xdata), all other platforms use
// DWARF CFI (.eh_frame).
cfg_if::cfg_if! {
if #[cfg(target_os = "uefi")] {
macro_rules! seh {
($asm:expr) => { $asm }
}
macro_rules! cfi {
($asm:expr) => { "" }
}
} else {
macro_rules! seh {
($asm:expr) => { "" }
}
macro_rules! cfi {
($asm:expr) => { $asm }
}
}
}

pub const STACK_ALIGNMENT: usize = 16;
pub const PARENT_STACK_OFFSET: usize = 0;
pub const PARENT_LINK_OFFSET: usize = 16;
pub type StackWord = u64;

// On non-UEFI platforms we use DWARF CFI directives. On UEFI we use SEH
// directives instead (see the seh!() and cfi!() macros above).
global_asm!(
".balign 4",
asm_function_begin!("stack_init_trampoline"),
".cfi_startproc",
cfi_signal_frame!(),
cfi!(".cfi_startproc"),
seh!(".seh_proc stack_init_trampoline"),
cfi!(cfi_signal_frame!()),
// At this point our register state contains the following:
// - SP points to the top of the parent stack.
// - LR contains the return address in the parent context.
Expand All @@ -110,21 +133,52 @@ global_asm!(
// Switch to the coroutine stack and pop the padding and initial PC.
"add sp, x2, #32",
// Set up the frame pointer to point at the parent link. This is needed for
// the unwinding code below.
// the unwinding code below: the parent link is dynamically updated on each
// stack switch so that the unwinder always finds the current parent frame.
"mov x29, x1",
//
// SEH unwind directives (UEFI only)
//
// The SEH unwinder processes the prologue codes in reverse order to
// perform a double indirection through the parent link:
// 1. .seh_set_fp: SP = FP (= parent link pointer).
// 2. .seh_save_fplr 0: X29 = [SP], LR = [SP + 8]. Dereferences the
// parent link so X29 = parent stack pointer.
// (LR gets a temporary incorrect value here.)
// 3. .seh_set_fp: SP = X29 (= parent stack frame).
// 4. .seh_nop: No effect.
// 5. .seh_save_reg x19, 16: X19 = [SP + 16].
// 6. .seh_save_fplr_x 32: X29 = [SP], LR = [SP + 8], SP += 32.
//
// After processing, SP = original parent SP, X29/LR/X19 are all correctly
// restored (LR's temporary wrong value from step 2 is overwritten in step
// 6), and the unwinder sets PC = LR to continue on the parent stack.
//
// Note: precise unwinding from within the prologue is not possible with
// the current trampoline structure, but full-prologue unwind is correct.
seh!(".seh_save_fplr_x 32"),
seh!(".seh_save_reg x19, 16"),
seh!(".seh_nop"),
seh!(".seh_set_fp"),
seh!(".seh_save_fplr 0"),
seh!(".seh_set_fp"),
seh!(".seh_endprologue"),
//
// DWARF CFI unwind directives (non-UEFI platforms)
//
// The actual meanings of the magic bytes are:
// 0x0f: DW_CFA_def_cfa_expression
// 5: byte length of the following DWARF expression
// 0x8d 0x00: DW_OP_breg29 (x29 + 0)
// 0x06: DW_OP_deref
// 0x23, 0x20: DW_OP_plus_uconst 32
".cfi_escape 0x0f, 5, 0x8d, 0x00, 0x06, 0x23, 0x20",
cfi!(".cfi_escape 0x0f, 5, 0x8d, 0x00, 0x06, 0x23, 0x20"),
// Now we can tell the unwinder how to restore the 3 registers that were
// pushed on the parent stack. These are described as offsets from the CFA
// that we just calculated.
".cfi_offset x19, -16",
".cfi_offset lr, -24",
".cfi_offset x29, -32",
cfi!(".cfi_offset x19, -16"),
cfi!(".cfi_offset lr, -24"),
cfi!(".cfi_offset x29, -32"),
// Set up the 3rd argument to the initial function to point to the object
// that init_stack() set up on the stack.
"mov x2, sp",
Expand All @@ -139,7 +193,8 @@ global_asm!(
asm_function_alt_entry!("stack_init_trampoline_return"),
// This BRK is necessary because of our use of .cfi_signal_frame earlier.
"brk #0",
".cfi_endproc",
seh!(".seh_endproc"),
cfi!(".cfi_endproc"),
asm_function_end!("stack_init_trampoline"),
);

Expand All @@ -148,8 +203,9 @@ global_asm!(
// used here.
".balign 4",
asm_function_begin!("stack_call_trampoline"),
".cfi_startproc",
cfi_signal_frame!(),
cfi!(".cfi_startproc"),
seh!(".seh_proc stack_call_trampoline"),
cfi!(cfi_signal_frame!()),
// At this point our register state contains the following:
// - SP points to the top of the parent stack.
// - X29 holds its value from the parent context.
Expand All @@ -160,9 +216,13 @@ global_asm!(
// Create a stack frame and point the frame pointer at it.
"stp x29, x30, [sp, #-16]!",
"mov x29, sp",
".cfi_def_cfa x29, 16",
".cfi_offset x30, -8",
".cfi_offset x29, -16",
seh!(".seh_save_fplr_x 16"),
seh!(".seh_set_fp"),
seh!(".seh_endprologue"),
// DWARF CFI (non-UEFI)
cfi!(".cfi_def_cfa x29, 16"),
cfi!(".cfi_offset x30, -8"),
cfi!(".cfi_offset x29, -16"),
// Switch to the new stack.
"mov sp, x1",
// Call the function pointer. The argument is already in the correct
Expand All @@ -173,7 +233,8 @@ global_asm!(
"mov sp, x29",
"ldp x29, x30, [sp], #16",
"ret",
".cfi_endproc",
seh!(".seh_endproc"),
cfi!(".cfi_endproc"),
asm_function_end!("stack_call_trampoline"),
);

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