printf

Author: h908714124

README.md

@@ -10,6 +10,8 @@
 
 [Linux clobbered registers](https://stackoverflow.com/questions/69515893/when-does-linux-x86-64-syscall-clobber-r8-r9-and-r10)
 
+[Calling printf in x86_64 using GNU assembler](https://stackoverflow.com/questions/38335212/calling-printf-in-x86-64-using-gnu-assembler)
+
 ```
 /usr/include/asm-generic/fcntl.h
 ```

draft/draft.c

@@ -7,69 +7,14 @@
 #include <sys/syscall.h>
 #include <sys/types.h>
 
-char* in;
-char* out;
-
-int get_zero() {
-	int result = 128;
-	while(true) {
-		result = result / 2;
-		if (result == 16) {
-			return 0;
-		}
-	}
-	return 1;
-}
-
-int get_one() {
-	int result = 128;
-	while(true) {
-		result = result / 2;
-		if (result == 8) {
-			return 1;
-		}
-	}
-	return 2;
-}
-
-int get_two() {
-	int result = 128;
-	while(true) {
-		result = result / 2;
-		if (result == 4) {
-			return 2;
-		}
-	}
-	return 1;
-}
-
-int get_three() {
-	int result = 128;
-	while(true) {
-		result = result / 2;
-		if (result == 32) {
-			return 3;
-		}
-	}
-	return 2;
-}
-
 int main(int argc, char *argv[])
 {
-	in = malloc(4);
-	out = malloc(4);
-	in[0] = '0';
-	in[1] = 'x';
-	in[2] = 'a';
-	in[3] = '\n';
-	int zero = get_zero();
-	int one = get_one();
-	int two = get_two();
-	int three = get_three();
-	out[zero] = in[zero];
-	out[one] = in[one];
-	out[two] = in[two];
-	out[three] = in[three];
-	syscall(__NR_write, 1, out, 4);
-	return 0;
+	char message[3];
+	message[0] = 0x41;
+	message[1] = 0x55;
+	message[2] = 0x41;
+	printf("%.*s", 3, message);
+	fflush(stdout);
+-	syscall(__NR_exit, 0);
+	//return 0;
 }

group3/.gdbinit

@@ -3,4 +3,4 @@ lay asm
 define hook-quit
 	set confirm off
 end
-break print_lines_loop
+break write_string

group3/Makefile

@@ -1,6 +1,5 @@
 default:
-	$(CC) group3.s -c -g
-	$(LD) -o group3 group3.o -e main
+	$(CC) group3.s -ggdb -o group3
 
 clean:
 	@rm -f group3.o group3

group3/group3.s

@@ -51,6 +51,15 @@ get_memory:
 	pop_all
 	return
 
+sflush:
+	enter
+	push_all
+	xor	%eax, %eax
+	mov	stdout(%rip), %rdi
+	syscall
+	pop_all
+	return
+
 exit:
 	enter
 	push_all
@@ -222,6 +231,7 @@ main:
 	mov	%rax, linebuf(%rip)
 	call	init_file
 	call	print_lines
+	call 	sflush
 
 	call	close
 	call	munmap

print_int/.gdbinit

@@ -3,4 +3,5 @@ lay asm
 define hook-quit
 	set confirm off
 end
-break string_compare
+break write_string
+display $rsi

print_int/print_int.s

@@ -86,6 +86,9 @@
 	pop	%rsi
 	.endm
 
+	.data
+	format: .asciz "%.*s"
+
 	.text
 	.globl main
 
@@ -133,11 +136,11 @@ write_string:
 	enter
 	push_all
 	push	%rax
-	mov	24(%rbp), %rsi			# param: address
-	mov	16(%rbp), %rdx			# param: size
-	mov	$WRITE, %rax
-	mov	$STDOUT, %rdi
-	syscall
+	lea	format(%rip), %rdi
+	mov	24(%rbp), %rdx			# param: address
+	mov	16(%rbp), %rsi			# param: size
+	mov	$2, %al				# number of vector arguments
+	call	printf
 	pop	%rax
 	pop_all
 	return