Demonstrate stack walker to find current position in frame

With top-of-stack pointer

Author: bdw

asm/Makefile

@@ -27,8 +27,9 @@ librot13.so: rot13.s
 libroman.so: roman.s
 	$(CC) -g -shared -o $@ $<
 
-test_walk_stack: walk_stack.S test_walk_stack.c
-	$(CC) -g -o $@ test_walk_stack.c walk_stack.S
+
+test_stack_walk: stack_walk.s test_stack_walk.c
+	$(CC) -g -fno-omit-frame-pointer -o $@ stack_walk.s test_stack_walk.c
 
 clean:
 	rm -f $(OBJECTS) *.so *.o

asm/stack_walk.s

@@ -0,0 +1,27 @@
+	.intel_syntax noprefix
+	.text
+        .globl stack_find_return_address_in_frame_posix
+	.globl _stack_find_return_address_in_frame_posix
+stack_find_return_address_in_frame_posix:
+_stack_find_return_address_in_frame_posix:
+	mov rcx, rdi /* arg0 = start-of-code region */
+	mov r8,  rdx /* arg2 = stack base-pointer */
+	mov rdx, rsi /* arg1 = size-of-code */
+_stack_find_return_address_in_frame_win64:
+	push rbp
+	mov r9, rsp
+        add rdx, rcx /* compute end-of-code region */
+        /* rcx = start, rdx = end, r8 = stack base */
+loop:
+        cmp r8, r9 /* have we gone past our start-of-stack pointer? */
+	jle done /* stack grows downwards */
+	mov rax, qword ptr [r9+0x8]
+	mov r9, qword ptr [r9]
+	cmp rax, rcx
+	jl  loop
+	cmp rax, rdx
+	jg  loop
+done:
+	/* rax is now within range by definition, or, we're to deep */
+	pop rbp
+	ret

asm/test_stack_walk.c

@@ -0,0 +1,32 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <inttypes.h>
+
+void * stack_find_return_address_in_frame_posix(void *base, int size, void *top);
+void bar(int *);
+int main(int, char**);
+
+void quix(int *a) {
+    void *pos = stack_find_return_address_in_frame_posix(&bar, 200, a);
+    void *mnl = stack_find_return_address_in_frame_posix(&main, 200, a);
+    int ofs = (char*)pos - (char*)bar;
+    fprintf(stderr, "pos=%"PRIx64", ofs=%d\n", pos, ofs);
+    fprintf(stderr, "mnl=%"PRIx64", ofs=%d\n", mnl, (char*)mnl - (char*)main);
+}
+
+
+void bar(int *a) {
+    fprintf(stderr, "Hello, world\n");
+    quix(a);
+    fprintf(stderr, "Goodbye!\n");
+}
+
+void foo(void) {
+    int a = 1;
+    bar(&a);
+}
+
+int main(int argc, char **argv) {
+    foo();
+    return 0;
+}