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+; MIT license:
+;
+; Copyright (c) 2023 Marcus R.A. Newman (prefetch@prefetch.eu)
+;
+; Permission is hereby granted, free of charge, to any person obtaining a copy
+; of this software and associated documentation files (the "Software"), to deal
+; in the Software without restriction, including without limitation the rights
+; to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+; copies of the Software, and to permit persons to whom the Software is
+; furnished to do so, subject to the following conditions:
+;
+; The above copyright notice and this permission notice shall be included in
+; all copies or substantial portions of the Software.
+;
+; THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+; AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+; LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+; OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+; SOFTWARE.
+
+
+; Cheat sheet for Linux' x86_64 calling convention:
+;
+; - free to overwrite; the caller should save them:
+;       rax, rcx, rdx, rsi, rdi, r8-r11, xmm0-xmm15
+; - caller expects no change; callee should save them:
+;       rbx, rbp, r12-r15
+;
+; - for passing paramters to functions:
+;       rdi, rsi, rdx, rcx, r8, r9, xmm0-xmm7
+; - for getting return values from functions:
+;       rax, rdx, xmm0
+;
+; - for passing parameters to syscalls:
+;       rax, rdi, rsi, rdx, r10, r8, r9
+; - for getting return values from syscalls:
+;       rax, rdx
+; - overwritten by syscalls (all others preserved):
+;       rcx, r11
+
+
+section .text
+
+
+; Relevant system call IDs
+%define SYS_MUNMAP  11
+%define SYS_FUTEX  202
+
+; Relevant flags for futex
+%define FUTEX_WAIT         0x00
+%define FUTEX_PRIVATE_FLAG 0x80
+
+
+%define STACK_SIZE 2097152 ; 2 MiB stack
+%define GUARD_PAGE    4096 ; 4 KiB guard page
+
+
+; Wait for thread to exit, save its return value, and clean up. Arguments:
+;	rdi: u32*   = handle of the thread to wait for
+;	rsi: void** = where to put the void* returned by the thread
+; Returns zero on success, or a standard error code.
+global linen_thread_finish
+linen_thread_finish:
+		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+		;;;; Check validity of arguments ;;;;
+		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+		; Return EINVAL if rdi is NULL or otherwise invalid
+		mov eax, -22 ; (EINVAL = -22)
+
+		test rdi, rdi
+		jz join_end ; rdi is NULL
+
+		; rdi is nonzero, so let's just assume it's a valid pointer.
+		; If that assumption is wrong we'll get a segmentation fault.
+		; But we don't yet trust that [rdi] is a valid thread handle!
+		; To verify this, we check the canary value stored at [rdi + 4].
+		mov ecx, [rdi + 4]
+		cmp ecx, 0xDEADBEEF
+		jnz join_end
+
+		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+		;;;; Wait until thread is finished ;;;;
+		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+		; We'll clobber rsi if we need to set up a futex call
+		mov r8, rsi
+
+		; When spawning, we set CLONE_CHILD_SETTID and CLONE_CHILD_CLEARTID:
+		; [rdi] contains the child thread's TID, and will get automatically
+		; cleared (to 0) when the child exits; this is what we'll watch for.
+
+		; Atomically check whether the target thread is still running.
+		; if ([rdi] == 0) { goto join_done; } else { eax = [rdi]; }
+		xor eax, eax
+		lock cmpxchg [rdi], eax
+		jz join_done
+
+		; The thread is still busy, so block until it's done.
+		; The futex system call waits until the dword at an
+		; address (rdi) deviates from an expected value (eax).
+		; See: man 2 futex
+
+		; futex: rdi = uaddr: address of the dword to watch
+		; futex: rsi = futex_op: which futex operation we want
+		; - FUTEX_WAIT:         block until the value at [rdi] changes
+		; - FUTEX_PRIVATE_FLAG: FIXME waits forever, I don't understand why
+		mov esi, FUTEX_WAIT
+		; futex: rdx = val: the expected value at [rdi] before it changes
+		mov edx, eax
+		; futex: r10 = timeout: in case we had a deadline (we don't)
+		xor r10, r10
+		; futex: r8 = uaddr2: ignored when FUTEX_WAIT is used
+		; futex: r9 = val3:   ignored when FUTEX WAIT is used
+		; futex: rax = system call ID
+		mov eax, SYS_FUTEX
+		; futex: rax = futex(rdi, rsi, rdx, r10, (r8), (r9))
+		syscall
+
+		; Check result of futex: negative means failure
+		test rax, rax
+		jnz join_end
+
+		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+		;;;; Clean up after thread's exit ;;;;
+		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+	join_done:
+		; The thread left its function return value on the stack, read it
+		mov rdx, [rdi - 8]
+
+		; The munmap system call destroys mappings created by mmap.
+		; In this case that means deallocating the stack buffer.
+		; See: man 2 munmap
+
+		; munmap: rdi = addr: lowest address of region to unmap
+		; Our rdi is near the buffer's top, so we must subtract
+		sub rdi, (STACK_SIZE + GUARD_PAGE - 8)
+		; munmap: rsi = length: size of region starting from rdi
+		mov esi, (STACK_SIZE + GUARD_PAGE)
+		; munmap: rax = system call ID
+		mov eax, SYS_MUNMAP
+		; munmap: rax = munmap(rdi, rsi)
+		syscall
+
+		; Check result of munmap: nonzero means failure
+		test rax, rax
+		jnz join_end
+
+		; Check if caller gave a location (r8) to save the return value (rdx)
+		test r8, r8
+		jz join_end ; caller doesn't care: gave NULL pointer
+		mov [r8], rdx
+		; Note: if munmap failed, the buffer is still there, so we
+		; can safely return an error without losing the return value.
+
+	join_end:
+		ret
+