Implement __chkstk

core/runtime/procs_windows_amd64.asm

@@ -1,3 +1,5 @@
+bits 64
+
 global __chkstk
 global _tls_index
 global _fltused
@@ -5,9 +7,73 @@ global _fltused
 section .data
 	_tls_index: dd 0
 	_fltused:   dd 0x9875
-	
 
 section .text
-__chkstk: ; proc "c" (rawptr)
-	; TODO implement correctly
-	ret
+; NOTE(flysand): The function call to __chkstk is called
+; by the compiler, when we're allocating arrays larger than
+; a page size. The reason is because the OS doesn't map the
+; whole stack into memory all at once, but does so page-by-page.
+; When the next page is touched, the CPU generates a page fault,
+; which *the OS* is handling by allocating the next page in the
+; stack until we reach the limit of stack size.
+;
+; This page is called the guard page, touching it will extend
+; the size of the stack and overwrite the stack limit in the TEB.
+;
+; If we allocate a large enough array and start writing from the
+; bottom of it, it's possible that we may start touching
+; non-contiguous pages which are unmapped. OS only maps the stack
+; page into the memory if the page above it was also mapped.
+;
+; Therefore the compilers insert this routine, the sole purpose
+; of which is to step through the stack starting from the RSP
+; down to the new RSP after allocation, and touch every page
+; of the new allocation so that the stack is fully mapped for
+; the new allocation
+;
+; I've gotten this code by disassembling the output of MSVC long
+; time ago. I don't remember if I've cleaned it up, but it definately
+; stinks.
+;
+; Additional notes:
+;   RAX (passed as parameter) holds the allocation's size
+;   GS:[0x10] references the current stack limit
+;     (i.e. bottom of the stack (i.e. lowest address accessible))
+;
+; Also this stuff is windows-only kind of thing, because linux people
+; didn't think stack that grows is cool enough for them, but the kernel
+; totally supports this kind of stack.
+__chkstk:
+	;; Allocate 16 bytes to store values of r10 and r11
+    sub   rsp, 0x10
+    mov   [rsp], r10
+    mov   [rsp+0x8], r11
+    ;; Set r10 to point to the stack as of the moment of the function call
+    lea   r10, [rsp+0x18]
+    ;; Subtract r10 til the bottom of the stack allocation, if we overflow
+    ;; reset r10 to 0, we'll crash with segfault anyway
+    xor   r11, r11
+    sub   r10, rax
+    cmovb r10, r11
+    ;; Load r11 with the bottom of the stack (lowest allocated address)
+    mov   r11, gs:[0x10] ; NOTE(flysand): gs:[0x10] is stack limit
+    ;; If the bottom of the allocation is above the bottom of the stack,
+    ;; we don't need to probe
+    cmp   r10, r11
+    jnb   .end
+    ;; Align the bottom of the allocation down to page size
+    and   r10w, 0xf000
+.loop:
+	;; Move the pointer to the next guard page, and touch it by loading 0
+	;; into that page
+    lea   r11, [r11-0x1000]
+    mov   byte [r11], 0x0
+    ;; Did we reach the bottom of the allocation?
+    cmp   r10, r11
+    jnz   .loop
+.end:
+	;; Restore previous r10 and r11 and return
+    mov   r10, [rsp]
+    mov   r11, [rsp+0x8]
+    add   rsp, 0x10
+    ret