NASM basic input-output program crashes - linux

Question

Following this thread, How do i read single character input from keyboard using nasm (assembly) under ubuntu? ,I'm trying to compile a program that echoes the input in NASM.
I've made following files:
my_load2.asm:
%include "testio.inc"
global _start
section .text
_start: mov eax, 0
call canonical_off
call canonical_on
testio.inc:
termios: times 36 db 0
stdin: equ 0
ICANON: equ 1<<1
ECHO: equ 1<<3
canonical_off:
call read_stdin_termios
; clear canonical bit in local mode flags
push rax
mov eax, ICANON
not eax
and [termios+12], eax
pop rax
call write_stdin_termios
ret
echo_off:
call read_stdin_termios
; clear echo bit in local mode flags
push rax
mov eax, ECHO
not eax
and [termios+12], eax
pop rax
call write_stdin_termios
ret
canonical_on:
call read_stdin_termios
; set canonical bit in local mode flags
or dword [termios+12], ICANON
call write_stdin_termios
ret
echo_on:
call read_stdin_termios
; set echo bit in local mode flags
or dword [termios+12], ECHO
call write_stdin_termios
ret
read_stdin_termios:
push rax
push rbx
push rcx
push rdx
mov eax, 36h
mov ebx, stdin
mov ecx, 5401h
mov edx, termios
int 80h
pop rdx
pop rcx
pop rbx
pop rax
ret
write_stdin_termios:
push rax
push rbx
push rcx
push rdx
mov eax, 36h
mov ebx, stdin
mov ecx, 5402h
mov edx, termios
int 80h
pop rdx
pop rcx
pop rbx
pop rax
ret
Then I run:
[root#localhost asm]# nasm -f elf64 my_load2.asm
[root#localhost asm]# ld -m elfx86_64 my_load2.o -o my_load2
When I try to run it i get:
[root#localhost asm]# ./my_load2
Segmentation fault
Debugger says:
(gdb) run
Starting program: /root/asm/my_load2
Program received signal SIGSEGV, Segmentation fault.
0x00000000004000b1 in canonical_off ()
Can someone explain why is it crashing without on "import" step?
Also, I am running RHEL in Virtualbox under Win7 64 bit. Can this cause problems with compilation?

Answer 1

Firstly, let's address the issue of not exiting, as mentioned by Daniel. Let's comment out the two call instructions, so the program essentially does nothing:
%include "testio.inc"
global _start
section .text
_start: mov eax, 0
;call canonical_off
;call canonical_on
When we run this:
$ ./my_load2
Segmentation fault (core dumped)
It still dies! Daniel is right - you need to exit:
%include "testio.inc"
global _start
section .text
_start: mov eax, 0
;call canonical_off
;call canonical_on
mov eax, 1
mov ebx, 0
int 0x80
This time:
$ ./my_load2
$
No segfault. So let's uncomment the calls:
%include "testio.inc"
global _start
section .text
_start: mov eax, 0
call canonical_off
call canonical_on
mov eax, 1
mov ebx, 0
int 0x80
And run it again:
$ ./my_load2
Segmentation fault (core dumped)
We get a segfault again. But at least we can be (fairly) sure that's coming from inside one of the called routines.
Running the executable with strace is also quite informative:
$ strace ./my_load2
execve("./my_load2", ["./my_load2"], [/* 57 vars */]) = 0
setsockopt(0, SOL_IP, 0x400080 /* IP_??? */, NULL, 0) = -1 EFAULT (Bad address)
--- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_ACCERR, si_addr=0x40008c} ---
+++ killed by SIGSEGV (core dumped) +++
Segmentation fault (core dumped)
The setsockopt line is due to the ioctl request that happens in read_stdin_termios. strace tells us the return value was EFAULT. The setsockopt(2) man page tells us this happens when:
The address pointed to by optval is not in a valid part of the process address space.
Actually this is telling us that the block of memory into which the termios structure is written is read-only. Frank is correct; everything in the program - including the termios space, and all the code - is in the read-only .text section. You can see this with:
$ objdump -h my_load2.o
my_load2.o: file format elf64-x86-64
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 000000cd 0000000000000000 0000000000000000 000001c0 2**4
CONTENTS, ALLOC, LOAD, RELOC, READONLY, CODE
i.e. there's only one section, .text, and it's READONLY.
The line that actually causes the segfault, however, is this one:
and [termios+12], eax
because it also tries to write to the (read-only) termios memory.
The quickest way to fix this is to put the termios memory in the .data section, and everything else in the .text section:
section .data
termios: times 36 db 0
section .text
stdin: equ 0
ICANON: equ 1<<1
ECHO: equ 1<<3
canonical_off:
call read_stdin_termios
[...]
(stdin, ICANON, and ECHO can be in the read-only .text section, because they're just used as constants - i.e. we don't write to those bits of memory.)
Having made these changes:
$ ./my_load2
$
The program runs and exits normally.