239 lines
9.7 KiB
Rust
239 lines
9.7 KiB
Rust
// This file is part of libfringe, a low-level green threading library.
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// Copyright (c) edef <edef@edef.eu>,
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// whitequark <whitequark@whitequark.org>
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// Amanieu d'Antras <amanieu@gmail.com>
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// Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or
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// http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or
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// http://opensource.org/licenses/MIT>, at your option. This file may not be
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// copied, modified, or distributed except according to those terms.
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// To understand the machine code in this file, keep in mind these facts:
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// * i686 SysV C ABI requires the stack to be aligned at function entry,
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// so that `%esp+4` is a multiple of 16. Aligned operands are a requirement
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// of SIMD instructions, and making this the responsibility of the caller
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// avoids having to maintain a frame pointer, which is necessary when
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// a function has to realign the stack from an unknown state.
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// * i686 SysV C ABI passes the first argument on the stack. This is
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// unfortunate, because unlike every other architecture we can't reuse
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// `swap` for the initial call, and so we use a trampoline.
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//
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// To understand the DWARF CFI code in this file, keep in mind these facts:
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// * CFI is "call frame information"; a set of instructions to a debugger or
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// an unwinder that allow it to simulate returning from functions. This implies
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// restoring every register to its pre-call state, as well as the stack pointer.
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// * CFA is "call frame address"; the value of stack pointer right before the call
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// instruction in the caller. Everything strictly below CFA (and inclusive until
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// the next CFA) is the call frame of the callee. This implies that the return
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// address is the part of callee's call frame.
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// * Logically, DWARF CFI is a table where rows are instruction pointer values and
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// columns describe where registers are spilled (mostly using expressions that
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// compute a memory location as CFA+n). A .cfi_offset pseudoinstruction changes
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// the state of a column for all IP numerically larger than the one it's placed
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// after. A .cfi_def_* pseudoinstruction changes the CFA value similarly.
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// * Simulating return is as easy as restoring register values from the CFI table
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// and then setting stack pointer to CFA.
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//
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// A high-level overview of the function of the trampolines when unwinding is:
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// * The 2nd init trampoline puts a controlled value (written in swap to `new_cfa`)
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// into %ebp. This is then used as the CFA for the 1st trampoline.
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// * This controlled value points to the bottom of the stack of the parent context,
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// which holds the saved %ebp and return address from the call to swap().
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// * The 1st init trampoline tells the unwinder to restore %ebp and its return
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// address from the stack frame at %ebp (in the parent stack), thus continuing
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// unwinding at the swap call site instead of falling off the end of context stack.
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use core::mem;
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use stack::Stack;
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pub const STACK_ALIGNMENT: usize = 16;
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#[derive(Debug, Clone, Copy)]
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#[repr(transparent)]
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pub struct StackPointer(*mut usize);
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pub unsafe fn init(stack: &Stack, f: unsafe extern "C" fn(usize, StackPointer) -> !) -> StackPointer {
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#[cfg(not(target_vendor = "apple"))]
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#[naked]
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unsafe extern "C" fn trampoline_1() {
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asm!(
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r#"
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# gdb has a hardcoded check that rejects backtraces where frame addresses
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# do not monotonically decrease. It is turned off if the function is called
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# "__morestack" and that is hardcoded. So, to make gdb backtraces match
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# the actual unwinder behavior, we call ourselves "__morestack" and mark
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# the symbol as local; it shouldn't interfere with anything.
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__morestack:
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.local __morestack
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# Set up the first part of our DWARF CFI linking stacks together. When
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# we reach this function from unwinding, %ebp will be pointing at the bottom
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# of the parent linked stack. This link is set each time swap() is called.
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# When unwinding the frame corresponding to this function, a DWARF unwinder
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# will use %ebp+8 as the next call frame address, restore return address
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# from CFA-4 and restore %ebp from CFA-8. This mirrors what the second half
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# of `swap_trampoline` does.
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.cfi_def_cfa %ebp, 8
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.cfi_offset %ebp, -8
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# This nop is here so that the initial swap doesn't return to the start
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# of the trampoline, which confuses the unwinder since it will look for
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# frame information in the previous symbol rather than this one. It is
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# never actually executed.
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nop
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# Stack unwinding in some versions of libunwind doesn't seem to like
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# 1-byte symbols, so we add a second nop here. This instruction isn't
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# executed either, it is only here to pad the symbol size.
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nop
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.Lend:
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.size __morestack, .Lend-__morestack
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"#
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: : : : "volatile")
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}
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#[cfg(target_vendor = "apple")]
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#[naked]
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unsafe extern "C" fn trampoline_1() {
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asm!(
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r#"
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# Identical to the above, except avoids .local/.size that aren't available on Mach-O.
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__morestack:
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.private_extern __morestack
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.cfi_def_cfa %ebp, 8
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.cfi_offset %ebp, -8
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nop
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nop
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"#
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: : : : "volatile")
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}
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#[naked]
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unsafe extern "C" fn trampoline_2() {
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asm!(
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r#"
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# Set up the second part of our DWARF CFI.
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# When unwinding the frame corresponding to this function, a DWARF unwinder
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# will restore %ebp (and thus CFA of the first trampoline) from the stack slot.
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# This stack slot is updated every time swap() is called to point to the bottom
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# of the stack of the context switch just switched from.
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.cfi_def_cfa %ebp, 8
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.cfi_offset %ebp, -8
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# This nop is here so that the return address of the swap trampoline
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# doesn't point to the start of the symbol. This confuses gdb's backtraces,
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# causing them to think the parent function is trampoline_1 instead of
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# trampoline_2.
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nop
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# Push arguments.
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pushl %esi
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pushl %edi
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# Call the provided function.
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calll *16(%esp)
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"#
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: : : : "volatile")
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}
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unsafe fn push(sp: &mut StackPointer, val: usize) {
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sp.0 = sp.0.offset(-1);
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*sp.0 = val
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}
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// We set up the stack in a somewhat special way so that to the unwinder it
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// looks like trampoline_1 has called trampoline_2, which has in turn called
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// swap::trampoline.
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//
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// There are 2 call frames in this setup, each containing the return address
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// followed by the %ebp value for that frame. This setup supports unwinding
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// using DWARF CFI as well as the frame pointer-based unwinding used by tools
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// such as perf or dtrace.
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let mut sp = StackPointer(stack.base() as *mut usize);
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push(&mut sp, 0 as usize); // Padding to ensure the stack is properly aligned
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push(&mut sp, 0 as usize); // Padding to ensure the stack is properly aligned
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push(&mut sp, 0 as usize); // Padding to ensure the stack is properly aligned
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push(&mut sp, f as usize); // Function that trampoline_2 should call
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// Call frame for trampoline_2. The CFA slot is updated by swap::trampoline
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// each time a context switch is performed.
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push(&mut sp, trampoline_1 as usize + 2); // Return after the 2 nops
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push(&mut sp, 0xdead0cfa); // CFA slot
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// Call frame for swap::trampoline. We set up the %ebp value to point to the
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// parent call frame.
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let frame = sp;
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push(&mut sp, trampoline_2 as usize + 1); // Entry point, skip initial nop
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push(&mut sp, frame.0 as usize); // Pointer to parent call frame
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sp
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}
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#[inline(always)]
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pub unsafe fn swap(arg: usize, new_sp: StackPointer,
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new_stack: Option<&Stack>) -> (usize, StackPointer) {
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// Address of the topmost CFA stack slot.
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let mut dummy: usize = mem::uninitialized();
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let new_cfa = if let Some(new_stack) = new_stack {
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(new_stack.base() as *mut usize).offset(-6)
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} else {
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// Just pass a dummy pointer if we aren't linking the stack
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&mut dummy
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};
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#[naked]
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unsafe extern "C" fn trampoline() {
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asm!(
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r#"
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# Save frame pointer explicitly; the unwinder uses it to find CFA of
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# the caller, and so it has to have the correct value immediately after
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# the call instruction that invoked the trampoline.
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pushl %ebp
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.cfi_adjust_cfa_offset 4
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.cfi_rel_offset %ebp, 0
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# Link the call stacks together by writing the current stack bottom
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# address to the CFA slot in the new stack.
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movl %esp, (%ecx)
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# Pass the stack pointer of the old context to the new one.
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movl %esp, %esi
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# Load stack pointer of the new context.
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movl %edx, %esp
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# Restore frame pointer of the new context.
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popl %ebp
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.cfi_adjust_cfa_offset -4
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.cfi_restore %ebp
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# Return into the new context. Use `pop` and `jmp` instead of a `ret`
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# to avoid return address mispredictions (~8ns per `ret` on Ivy Bridge).
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popl %eax
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.cfi_adjust_cfa_offset -4
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.cfi_register %eip, %eax
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jmpl *%eax
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"#
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: : : : "volatile")
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}
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let ret: usize;
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let ret_sp: *mut usize;
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asm!(
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r#"
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# Push instruction pointer of the old context and switch to
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# the new context.
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call ${2:c}
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"#
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: "={edi}" (ret)
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"={esi}" (ret_sp)
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: "s" (trampoline as usize)
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"{edi}" (arg)
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"{edx}" (new_sp.0)
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"{ecx}" (new_cfa)
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: "eax", "ebx", "ecx", "edx", /*"esi", "edi", "ebp", "esp",*/
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"mm0", "mm1", "mm2", "mm3", "mm4", "mm5", "mm6", "mm7",
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"xmm0", "xmm1", "xmm2", "xmm3", "xmm4", "xmm5", "xmm6", "xmm7",
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"cc", "dirflag", "fpsr", "flags", "memory"
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: "volatile");
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(ret, StackPointer(ret_sp))
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}
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