From 94e3e3652f9f04810126ee754fa9a788289e2897 Mon Sep 17 00:00:00 2001
From: Prefetch
Date: Mon, 24 Jul 2023 19:21:30 +0200
Subject: Reduce total code size by 53 bytes (big deal, right?)

---
 src/thread_finish.asm | 30 +++++++++++++++++++-----------
 1 file changed, 19 insertions(+), 11 deletions(-)

(limited to 'src/thread_finish.asm')

diff --git a/src/thread_finish.asm b/src/thread_finish.asm
index 860b0a4..c453d40 100644
--- a/src/thread_finish.asm
+++ b/src/thread_finish.asm
@@ -43,12 +43,15 @@ section .text
 ; Returns zero on success, or a standard error code.
 global linen_thread_finish
 linen_thread_finish:
+		; It's handy to have a register that's 0 for a while
+		xor ecx, ecx
+
 		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 		;;;; Check validity of arguments ;;;;
 		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
-		; Return EINVAL if rdi is NULL or otherwise invalid
-		mov eax, -22 ; (EINVAL = -22)
+		; Return EINVAL (-22) if rdi is NULL or otherwise invalid
+		lea eax, [rcx - 22] ; mov eax, -22
 
 		test rdi, rdi
 		jz finish_return ; rdi is NULL
@@ -57,8 +60,7 @@ linen_thread_finish:
 		; 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
+		cmp dword [rdi + 4], 0xDEADBEEF ; Oh CISC...
 		jnz finish_return
 
 		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
@@ -86,9 +88,9 @@ linen_thread_finish:
 
 		; 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_WAIT = 0:     block until the value at [rdi] changes
 		; - FUTEX_PRIVATE_FLAG: FIXME waits forever, I don't understand why
-		mov esi, FUTEX_WAIT
+		xor esi, esi ; 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)
@@ -96,17 +98,18 @@ linen_thread_finish:
 		; 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
+		xor eax, eax
+		mov al, SYS_FUTEX
 		; futex: rax = futex(rdi, rsi, rdx, r10, (r8), (r9))
 		syscall
 
 		; Sometimes the thread exits after the "lock cmpxchg" instruction
 		; but before the futex call. In that case, futex returns EAGAIN.
-		cmp rax, -11 ; (EAGAIN = -11)
+		cmp eax, -11 ; (EAGAIN = -11)
 		je finish_retry
 
 		; Any other nonzero return value means failure
-		test rax, rax
+		test eax, eax
 		jnz finish_return
 
 		;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
@@ -117,6 +120,11 @@ linen_thread_finish:
 		; The thread left its function return value on the stack, read it
 		mov rdx, [rdi - 8]
 
+		; Remove the canary value so the thread handle becomes invalid. We're about
+		; to deallocate this memory anyway, so it's optional, but maybe the address
+		; becomes valid again later in the program's life, who knows? Play it safe.
+		mov [rdi + 4], eax ; eax = 0 for all paths
+
 		; The munmap system call destroys mappings created by mmap.
 		; In this case that means deallocating the stack buffer.
 		; See: man 2 munmap
@@ -127,12 +135,12 @@ linen_thread_finish:
 		; munmap: rsi = length: size of region starting from rdi
 		mov esi, (STACK_SIZE + GUARD_PAGE)
 		; munmap: rax = system call ID
-		mov eax, SYS_MUNMAP
+		mov al, SYS_MUNMAP
 		; munmap: rax = munmap(rdi, rsi)
 		syscall
 
 		; Check result of munmap: nonzero means failure
-		test rax, rax
+		test eax, eax
 		jnz finish_return
 
 		; Check if caller gave a location (r8) to save the return value (rdx)
-- 
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