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| // This file is part of libfringe, a low-level green threading library. | ||
| // Copyright (c) Nathan Zadoks <nathan@nathan7.eu>, | ||
| // whitequark <whitequark@whitequark.org> | ||
| // Amanieu d'Antras <amanieu@gmail.com> | ||
| // Licensed under the Apache License, Version 2.0, <LICENSE-APACHE or | ||
| // http://apache.org/licenses/LICENSE-2.0> or the MIT license <LICENSE-MIT or | ||
| // http://opensource.org/licenses/MIT>, at your option. This file may not be | ||
| // copied, modified, or distributed except according to those terms. | ||
|
|
||
| // To understand the machine code in this file, keep in mind these facts: | ||
| // * ARM AAPCS ABI passes the first argument in r0. We also use r0 to pass a value | ||
| // while swapping context; this is an arbitrary choice | ||
| // (we clobber all registers and could use any of them) but this allows us | ||
| // to reuse the swap function to perform the initial call. | ||
| // | ||
| // To understand the ARM EHABI CFI code in this file, keep in mind these facts: | ||
| // * CFI is "call frame information"; a set of instructions to a debugger or | ||
| // an unwinder that allow it to simulate returning from functions. This implies | ||
| // restoring every register to its pre-call state, as well as the stack pointer. | ||
| // * CFA is "call frame address"; the value of stack pointer right before the call | ||
| // instruction in the caller. Everything strictly below CFA (and inclusive until | ||
| // the next CFA) is the call frame of the callee. This implies that the return | ||
| // address is the part of callee's call frame. | ||
| // * Logically, ARM EHABI CFI is a table where rows are instruction pointer values and | ||
| // columns describe where registers are spilled (mostly using expressions that | ||
| // compute a memory location as CFA+n). A .save pseudoinstruction changes | ||
| // the state of a column for all IP numerically larger than the one it's placed | ||
| // after. A .pad or .setfp pseudoinstructions change the CFA value similarly. | ||
| // * Simulating return is as easy as restoring register values from the CFI table | ||
| // and then setting stack pointer to CFA. | ||
| // | ||
| // A high-level overview of the function of the trampolines is: | ||
| // * The 2nd init trampoline puts a controlled value (written in swap to `new_cfa`) | ||
| // into r11. This is then used as the CFA for the 1st trampoline. | ||
| // * This controlled value points to the bottom of the stack of the parent context, | ||
| // which holds the saved r11 and lr from the call to swap(). | ||
| // * The 1st init trampoline tells the unwinder to restore r11 and lr | ||
| // from the stack frame at r11 (in the parent stack), thus continuing | ||
| // unwinding at the swap call site instead of falling off the end of context stack. | ||
| use core::mem; | ||
| use arch::StackPointer; | ||
| use unwind; | ||
|
|
||
| pub const STACK_ALIGNMENT: usize = 8; | ||
|
|
||
| pub unsafe fn init(stack_base: *mut u8, f: unsafe fn(usize, StackPointer)) -> StackPointer { | ||
| #[cfg(not(target_vendor = "apple"))] | ||
| #[naked] | ||
| unsafe extern "C" fn trampoline_1() { | ||
| asm!( | ||
| r#" | ||
| # gdb has a hardcoded check that rejects backtraces where frame addresses | ||
| # do not monotonically decrease. It is turned off if the function is called | ||
| # "__morestack" and that is hardcoded. So, to make gdb backtraces match | ||
| # the actual unwinder behavior, we call ourselves "__morestack" and mark | ||
| # the symbol as local; it shouldn't interfere with anything. | ||
| __morestack: | ||
| .local __morestack | ||
| # Set up the first part of our ARM EHABI CFI linking stacks together. When | ||
| # we reach this function from unwinding, r11 will be pointing at the bottom | ||
| # of the parent linked stack. This link is set each time swap() is called. | ||
| # When unwinding the frame corresponding to this function, a ARM EHABI unwinder | ||
| # will use r11+16 as the next call frame address, restore return address (lr) | ||
| # from CFA-8 and restore r11 from CFA-16. This mirrors what the second half | ||
| # of `swap_trampoline` does. | ||
| # .setfp fp, sp | ||
| # .save {fp, lr} | ||
| # This nop is here so that the initial swap doesn't return to the start | ||
| # of the trampoline, which confuses the unwinder since it will look for | ||
| # frame information in the previous symbol rather than this one. It is | ||
| # never actually executed. | ||
| nop | ||
| .Lend: | ||
| .size __morestack, .Lend-__morestack | ||
| "# | ||
| : : : : "volatile") | ||
| } | ||
|
|
||
| #[cfg(target_vendor = "apple")] | ||
| #[naked] | ||
| unsafe extern "C" fn trampoline_1() { | ||
| asm!( | ||
| r#" | ||
| # Identical to the above, except avoids .local/.size that aren't available on Mach-O. | ||
| __morestack: | ||
| .private_extern __morestack | ||
| # .setfp fp, sp | ||
| # .save {fp, lr} | ||
| nop | ||
| "# | ||
| : : : : "volatile") | ||
| } | ||
|
|
||
| #[naked] | ||
| unsafe extern "C" fn trampoline_2() { | ||
| asm!( | ||
| r#" | ||
| # Set up the second part of our ARM EHABI CFI. | ||
| # When unwinding the frame corresponding to this function, a DWARF unwinder | ||
| # will restore r11 (and thus CFA of the first trampoline) from the stack slot. | ||
| # This stack slot is updated every time swap() is called to point to the bottom | ||
| # of the stack of the context switch just switched from. | ||
| # .setfp fp, sp | ||
| # .save {fp, lr} | ||
| # This nop is here so that the return address of the swap trampoline | ||
| # doesn't point to the start of the symbol. This confuses gdb's backtraces, | ||
| # causing them to think the parent function is trampoline_1 instead of | ||
| # trampoline_2. | ||
| nop | ||
| # Call unwind_wrapper with the provided function and the stack base address. | ||
| add r2, sp, #16 | ||
| ldr r3, [sp, #8] | ||
| bl ${0} | ||
| # Restore the stack pointer of the parent context. No CFI adjustments | ||
| # are needed since we have the same stack frame as trampoline_1. | ||
| ldr sp, [sp] | ||
| # Load frame and instruction pointers of the parent context. | ||
| pop {fp, lr} | ||
| # If the returned value is nonzero, trigger an unwind in the parent | ||
| # context with the given exception object. | ||
| cmp r0, #0 | ||
| bne ${1} | ||
| # Clear the stack pointer. We can't call into this context any more once | ||
| # the function has returned. | ||
| mov r1, #0 | ||
| # Return into the new context. Use `r12` instead of `lr` to avoid | ||
| # return address mispredictions. | ||
| mov r12, lr | ||
| bx r12 | ||
| "# | ||
| : | ||
| : "s" (unwind::unwind_wrapper as usize) | ||
| "s" (unwind::start_unwind as usize) | ||
| : : "volatile") | ||
| } | ||
|
|
||
| // We set up the stack in a somewhat special way so that to the unwinder it | ||
| // looks like trampoline_1 has called trampoline_2, which has in turn called | ||
| // swap::trampoline. | ||
| // | ||
| // There are 2 call frames in this setup, each containing the return address | ||
| // followed by the r11 value for that frame. This setup supports unwinding | ||
| // using DWARF CFI as well as the frame pointer-based unwinding used by tools | ||
| // such as perf or dtrace. | ||
| let mut sp = StackPointer::new(stack_base); | ||
|
|
||
| sp.push(0 as usize); // Padding to ensure the stack is properly aligned | ||
| sp.push(f as usize); // Function that trampoline_2 should call | ||
|
|
||
| // Call frame for trampoline_2. The CFA slot is updated by swap::trampoline | ||
| // each time a context switch is performed. | ||
| sp.push(trampoline_1 as usize + 4); // Return after the nop | ||
| sp.push(0xdead0cfa); // CFA slot | ||
|
|
||
| // Call frame for swap::trampoline. We set up the r11 value to point to the | ||
| // parent call frame. | ||
| let frame = sp.offset(0); | ||
| sp.push(trampoline_2 as usize + 4); // Entry point, skip initial nop | ||
| sp.push(frame as usize); // Pointer to parent call frame | ||
|
|
||
| sp | ||
| } | ||
|
|
||
| #[inline(always)] | ||
| pub unsafe fn swap_link(arg: usize, new_sp: StackPointer, | ||
| new_stack_base: *mut u8) -> (usize, Option<StackPointer>) { | ||
| let ret: usize; | ||
| let ret_sp: usize; | ||
| asm!( | ||
| r#" | ||
| # Set up the link register | ||
| adr lr, 0f | ||
| # Save the frame pointer and link register; the unwinder uses them to find | ||
| # the CFA of the caller, and so they have to have the correct value immediately | ||
| # after the call instruction that invoked the trampoline. | ||
| push {fp, lr} | ||
| # Pass the stack pointer of the old context to the new one. | ||
| mov r1, sp | ||
| # Link the call stacks together by writing the current stack bottom | ||
| # address to the CFA slot in the new stack. | ||
| str sp, [r3, #-16] | ||
| # Load stack pointer of the new context. | ||
| mov sp, r2 | ||
| # Load frame and instruction pointers of the new context. | ||
| pop {fp, r12} | ||
| # Return into the new context. Use `r12` instead of `lr` to avoid | ||
| # return address mispredictions. | ||
| bx r12 | ||
| 0: | ||
| "# | ||
| : "={r0}" (ret) | ||
| "={r1}" (ret_sp) | ||
| : "{r0}" (arg) | ||
| "{r2}" (new_sp.0) | ||
| "{r3}" (new_stack_base) | ||
| :/*r0, r1,*/ "r2", "r3", "r4", "r5", "r6", "r7", | ||
| "r8", "r9", "r10",/*r11,*/"r12",/*sp,*/ "lr", /*pc,*/ | ||
| "d0", "d1", "d2", "d3", "d4", "d5", "d6", "d7", | ||
| "d8", "d9", "d10", "d11", "d12", "d13", "d14", "d15", | ||
| "d16", "d17", "d18", "d19", "d20", "d21", "d22", "d23", | ||
| "d24", "d25", "d26", "d27", "d28", "d29", "d30", "d31", | ||
| "cc", "memory" | ||
| : "volatile"); | ||
| (ret, mem::transmute(ret_sp)) | ||
| } | ||
|
|
||
| #[inline(always)] | ||
| pub unsafe fn swap(arg: usize, new_sp: StackPointer) -> (usize, StackPointer) { | ||
| // This is identical to swap_link, but without the write to the CFA slot. | ||
| let ret: usize; | ||
| let ret_sp: usize; | ||
| asm!( | ||
| r#" | ||
| adr lr, 0f | ||
| push {fp, lr} | ||
| mov r1, sp | ||
| mov sp, r2 | ||
| pop {fp, r12} | ||
| bx r12 | ||
| 0: | ||
| "# | ||
| : "={r0}" (ret) | ||
| "={r1}" (ret_sp) | ||
| : "{r0}" (arg) | ||
| "{r2}" (new_sp.0) | ||
| :/*r0, r1,*/ "r2", "r3", "r4", "r5", "r6", "r7", | ||
| "r8", "r9", "r10",/*r11,*/"r12",/*sp,*/ "lr", /*pc,*/ | ||
| "d0", "d1", "d2", "d3", "d4", "d5", "d6", "d7", | ||
| "d8", "d9", "d10", "d11", "d12", "d13", "d14", "d15", | ||
| "d16", "d17", "d18", "d19", "d20", "d21", "d22", "d23", | ||
| "d24", "d25", "d26", "d27", "d28", "d29", "d30", "d31", | ||
| "cc", "memory" | ||
| // We need the "alignstack" attribute here to ensure that the stack is | ||
| // properly aligned if a call to start_unwind needs to be injected into | ||
| // our stack context. | ||
| : "volatile", "alignstack"); | ||
| (ret, mem::transmute(ret_sp)) | ||
| } | ||
|
|
||
| #[inline(always)] | ||
| pub unsafe fn unwind(new_sp: StackPointer, new_stack_base: *mut u8) { | ||
| // Argument to pass to start_unwind, based on the stack base address. | ||
| let arg = unwind::unwind_arg(new_stack_base); | ||
|
|
||
| // This is identical to swap_link, except that it performs a tail call to | ||
| // start_unwind instead of returning into the target context. | ||
| asm!( | ||
| r#" | ||
| adr lr, 0f | ||
| push {fp, lr} | ||
| str sp, [r3, #-16] | ||
| mov sp, r2 | ||
| pop {fp, r12} | ||
| # Jump to the start_unwind function, which will force a stack unwind in | ||
| # the target context. This will eventually return to us through the | ||
| # stack link. | ||
| b ${0} | ||
| 0: | ||
| "# | ||
| : | ||
| : "s" (unwind::start_unwind as usize) | ||
| "{r0}" (arg) | ||
| "{r2}" (new_sp.0) | ||
| "{r3}" (new_stack_base) | ||
| : "r0", "r1", "r2", "r3", "r4", "r5", "r6", "r7", | ||
| "r8", "r9", "r10",/*r11,*/"r12",/*sp,*/ "lr", /*pc,*/ | ||
| "d0", "d1", "d2", "d3", "d4", "d5", "d6", "d7", | ||
| "d8", "d9", "d10", "d11", "d12", "d13", "d14", "d15", | ||
| "d16", "d17", "d18", "d19", "d20", "d21", "d22", "d23", | ||
| "d24", "d25", "d26", "d27", "d28", "d29", "d30", "d31", | ||
| "cc", "memory" | ||
| : "volatile"); | ||
| } |
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