I'm new to GNU Debugger. I've been playing around with it, debugging Assembly Files (x86_64 Linux) for a day or so and just a few hours ago I ''discovered'' the TUI interface.
My first attempt using the TUI interface was to see the register changes as I execute each line at a time of a simple Hello World program (in asm). Here is the code of the program
section .data
text db "Hello, World!", 10
len equ $-text
section .text
global _start
_start:
nop
call _printText
mov rax, 60
mov rdi, 0
syscall
_printText:
nop
mov rax, 1
mov rdi, 1
mov rsi, text
mov rdx, len
syscall
ret
After creating the executable file in the terminal of linux I write
$ gdb -q ./hello -tui
Then I created three breakpoints: one right of the _start, another right after _printText and the last just above the mov rax, 60 for the SYS_EXIT.
After this:
1) I run the program.
2) On gdb mode I write layout asm to see the written code.
3) I write layout regs.
4) Finally I use stepi to see how the register change according the the written hello world program.
The thing is that when the RIP register points to the address of ret, corresponding to SYS_EXIT and I hit Enter I get the following message in console
[Inferior 1 (process 2059) exited normally]
/build/gdb-cXfXJ3/gdb-7.11.1/gdb/thread.c:1100: internal-error: finish_thread_st
ate: Assertion `tp' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n)
If I type n It appears this (as it says, it quits if I type y):
This is a bug, please report it. For instructions, see:
<http://www.gnu.org/software/gdb/bugs/>.
/build/gdb-cXfXJ3/gdb-7.11.1/gdb/thread.c:1100: internal-error: finish_thread_st
ate: Assertion `tp' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Create a core file of GDB? (y or n)
As I don't know what a core file of GDB (and what is useful for), so I type n and the debugging session closes.
Does anyone know why this is happening and how can be fixed?
By the way, I'm new in Assembly also, so if this occurs because of something wrong in the program I'd also appreciate if anyone can point that out.
I use the same GDB version as you and I always use the TUI features; but I've never had this problem. However, when I use your code the internal GDB error occurs. But if I make one change in your write syscall function, the error does not manifest.
Although you are not calling another function from within a function, I generally create a stack frame by including at least the "push rbp", "mov rbp, rsp", and "leave" instructions in my x86-64 function calls. This may be a band-aide or a work around with respect to the "bug".
_printText:
push rbp
mov rbp, rsp
mov rax, 1
mov rdi, 1
mov rsi, text
mov rdx, len
syscall
leave
ret
Does anyone know why this is happening
It's happening because there is a bug in GDB (more precisely, an assertion that GDB internal variable tp is not NULL has been violated).
and how can be fixed?
You should try to reproduce this with current version of GDB (the bug may have already been fixed), and file a bug report (like the message tells you).
I don't know what a core file of GDB (and what is useful for),
It's only useful to GDB developers.
Related
I wrote the following code to check if the 1st number- 'x' is greater than the 2nd number- 'y'. For x>y output should be 1 and for x<=y output should be 0.
section .txt
global _start
global checkGreater
_start:
mov rdi,x
mov rsi,y
call checkGreater
mov rax,60
mov rdi,0
syscall
checkGreater:
mov r8,rdi
mov r9,rsi
cmp r8,r9
jg skip
mov [c],byte '0'
skip:
mov rax,1
mov rdi,1
mov rsi,c
mov rdx,1
syscall
ret
section .data
x db 7
y db 5
c db '1',0
But due to some reasons(of course from my end), the code always gives 0 as the output when executed.
I am using the following commands to run the code on Ubuntu 20.04.1 LTS with nasm 2.14.02-1
nasm -f elf64 fileName.asm
ld -s -o fileName fileName.o
./fileName
Where did I make a mistake?
And how should one debug assembly codes, I looked for printing received arguments in checkGreater, but it turns out that's a disturbing headache itself.
Note: If someone wondering why I didn't directly use x and y in checkGreater, I want to extend the comparison to user inputs, and so wrote code in that way only.
The instructions
mov rdi,x
mov rsi,y
write the address of x into rdi, and of y into rsi. The further code then goes on to compare the addresses, which are always x<y, since x is defined above y.
What you should have written instead is
mov rdi,[x]
mov rsi,[y]
But then you have another problem: x and y variables are 1 byte long, while the destination registers are 8 bytes long. So simply doing the above fix will read extraneous bytes, leading to useless results. The final correction is to either fix the size of the variables (writing dq instead of db), or read them as bytes:
movzx rdi,byte [x]
movzx rsi,byte [y]
As for
And how should one debug assembly codes
The main tool for you is an assembly-level debugger, like EDB on Linux or x64dbg on Windows. But in fact, most debuggers, even the ones intended for languages like C++, are capable of displaying disassembly for the program being debugged. So you can use e.g. GDB, or even a GUI wrapper for it like Qt Creator or Eclipse. Just be sure to switch to machine code mode, or use the appropriate commands like GDB's disassemble, stepi, info registers etc..
Note that you don't have to build EDB or GDB from source (as the links above might suggest): they are likely already packaged in the Linux distribution you use. E.g. on Ubuntu the packages are called edb-debugger and gdb.
I'm trying to make JonesForth run on a recent MacBook out of the box, just using Mac tools.
I started to convert everything 64 bits and attend to the Mac assembler syntax.
I got things to assemble, but I immediately run into a curious segmentation fault:
/* NEXT macro. */
.macro NEXT
lodsq
jmpq *(%rax)
.endm
...
/* Assembler entry point. */
.text
.globl start
.balign 16
start:
cld
mov %rsp,var_SZ(%rip) // Save the initial data stack pointer in FORTH variable S0.
mov return_stack_top(%rip),%rbp // Initialise the return stack.
//call set_up_data_segment
mov cold_start(%rip),%rsi // Initialise interpreter.
NEXT // Run interpreter!
.const
cold_start: // High-level code without a codeword.
.quad QUIT
QUIT is defined like this via macro defword:
.macro defword
.const_data
.balign 8
.globl name_$3
name_$3 :
.quad $4 // Link
.byte $2+$1 // Flags + length byte
.ascii $0 // The name
.balign 8 // Padding to next four-byte boundary
.globl $3
$3 :
.quad DOCOL // Codeword - the interpreter
// list of word pointers follow
.endm
// QUIT must not return (ie. must not call EXIT).
defword "QUIT",4,,QUIT,name_TELL
.quad RZ,RSPSTORE // R0 RSP!, clear the return stack
.quad INTERPRET // Interpret the next word
.quad BRANCH,-16 // And loop (indefinitely)
...more code
When I run this, I get a segmentation fault the first time in the NEXT macro:
(lldb) run
There is a running process, kill it and restart?: [Y/n] y
Process 83000 exited with status = 9 (0x00000009)
Process 83042 launched: '/Users/klapauciusisgreat/jonesforth64/jonesforth' (x86_64)
Process 83042 stopped
* thread #1, stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT)
frame #0: 0x0000000100000698 jonesforth`start + 24
jonesforth`start:
-> 0x100000698 <+24>: jmpq *(%rax)
0x10000069a <+26>: nopw (%rax,%rax)
jonesforth`code_DROP:
0x1000006a0 <+0>: popq %rax
0x1000006a1 <+1>: lodsq (%rsi), %rax
Target 0: (jonesforth) stopped.
rax does point to what I think is the dereferenced address, DOCOL:
(lldb) register read
General Purpose Registers:
rax = 0x0000000100000660 jonesforth`DOCOL
So one mystery is:
Why does RAX point to DOCOL instead of QUIT? My guess is that the instruction was halfway executed and the result of the indirection was stored in rax. What are some good pointers to documentation?
Why the segmentation fault?
I commented out the original segment setup code in the original that called brk to set up a data segment. Another [implementation] also did not call it at all, so I thought I could as well ignore this. Is there any magic on how to set up segment permissions with syscalls in a 64-bit binary on Catalina? The make command is pretty much the standard JonesForth one:
jonesforth: jonesforth.S
gcc -nostdlib -g -static $(BUILD_ID_NONE) -o $# $<
P.S.: Yes, I can get JonesForth to work perfectly in Docker images, but that's besides the point. I really want it to work in 64 bit on Catalina, out of the box.
The original code had something like
mov $cold_start,%rsi
And the Apple assembler complains about not being able to use 32 immediate addressing in 64-bit binaries.
So I tried
mov $cold_start(%rip),%rsi
but that also doesn't work.
So I tried
mov cold_start(%rip),%rsi
which assembles, but of course it dereferences cold start, which is not something I need.
The correct way of doing this is apparently
lea cold_start(%rip),%rsi
This seems to work as intended.
While trying to make a very tiny program with NASM and GCC on my Ubuntu machine, I noticed something weird.
The following code compiles fine under 64-bit NASM and GCC:
global main
extern puts
section .text
main:
push rax
mov rdi, message
call puts
jmp exit
exit:
;return stack memory
pop rax
ret
message:
db "Hello from NASM!", 0
But when trying to compile the same code (only with registers changed) under 32-bit NASM and GCC, it will either result segmentation fault and/or random characters. Why is this happening? Does the x64 architecture have different way in storing memory to the stack than i386? If so, how could this behaviour be prevented?
When in 32-bit mode, most calling conventions (cdecl, stdcall, etc...) expect arguments to be pushed on the stack, not in registers, unlike in 64-bit mode, and also, you would need to adjust the stack pointer after calling puts, so you would need to do something like:
lea edx, #message
push edx
call puts
add esp, 4
For the program to produce the same output in 32-bit mode. I may not have the NASM syntax right as I usually write assembly code in MASM and GAS.
The book Assembly Language Step by Step provides the following code as a sandbox:
section .data
section .text
global _start
_start:
nop
//insert sandbox code here
nop
Any example that I include in the space for sandbox is creating a segmentation fault. For example, adding this code:
mov ax, 067FEh
mov bx, ax
mov cl, bh
mov ch, bl
Then compiling with:
nasm -f macho sandbox.asm
ld -o sandbox -e _start sandbox.o
creates a seg fault when I run it on my OS/X. Is there a way to get more information about what's causing the segmentation fault?
The problem you have is that you have created a program that runs past the end of the code that you have written.
When your program executes, the loader will end up issuing a jmp to your _start. Your code then runs, but you do not have anything to return to the OS at the end, so it will simply continue running, executing whatever bytes happen to be in RAM after your code.
The simplest fix would be to properly exit the code. For example:
mov eax, 0x1 ; system call number for exit
sub esp, 4 ; OS X system calls needs "extra space" on stack
int 0x80
Since you are not generating any actual output, you would need to step through with a debugger to see what's going on. After compiling you could use lldb to step through.
lldb ./sandbox
image dump sections
Make note of the address listed that is of type code for your executable (not dyld). It will likely be 0x0000000000001fe6. Continuing within lldb:
b s -a 0x0000000000001fe6
run
register read
step
register read
step
register read
At this point you should be past the NOPs and see things changing in registers. Have fun!
So i was wondering if there is any? I know afd on windows but not sure anything about mac?
And this his how i am using nasam on the following code: nasm a.asm -o a.com -l a.lst
[org 0x100]
mov ax, 5
mov bx, 10
add ax, bx
mov bx, 15
add ax, bx
mov ax, 0x4c00
int 0x21
On windows i know a debugger name afd which help me to step through each statement but not sure how i can do this using gdb.
And neither i am able to execute this .com file, am i supposed to make some other file here?
Why are you writing 16-bit code that makes DOS syscalls? If you want to know how to write asm that's applicable to your OS, take a look the code generated by "gcc -S" on some C code... (Note that code generated this way will have operands reversed, and is meant to be assembled with as instead of nasm)
Further, are you aware what this code is doing? It reads to me like this:
ax = 5
bx = 10
ax += bx
bx = 15
ax += bx
ax = 0x4c00
int 21h
Seems like this code is equivalent to:
mov bx, 15
mov ax, 4c00
int 21h
Which according to what I see here, is exit(0). You didn't need to change bx either...
But. This doesn't even apply to what you were trying to do, because Mac OS X is not MS-DOS, does not know about DOS APIs, cannot run .COM files, etc. I wasn't even aware that it can run 16 bit code. You will want to look at nasm's -f elf option, and you will want to use registers like eax rather than ax.
I've not done assembly programming on OS X, but you could theoretically do something like this:
extern exit
global main
main:
push dword 0
call exit
; This will never get called, but hey...
add esp, 4
xor eax, eax
ret
Then:
nasm -f elf foo.asm -o foo.o
ld -o foo foo.o -lc
Of course this is relying on the C library, which you might not want to do. I've omitted the "full" version because I don't know what the syscall interface looks like on Mac. On many platforms your entry point is the symbol _start and you do syscalls with int 80h or sysenter.
As for debugging... I would also suggest GDB. You can advance by a single instruction with stepi, and the info registers command will dump register state. The disassemble command is also helpful.
Update: Just remembered, I don't think Mac OS X uses ELF... Well.. Much of what I wrote still applies. :-)
Xcode ships with GDB, the GNU Debugger.
Xcode 4 and newer ships with LLDB instead.
As others have said, use GDB, the gnu debugger. In debugging assembly source, I usually find it useful to load a command file that contains something like the following:
display/5i $pc
display/x $eax
display/x $ebx
...
display/5i will display 5 instructions starting with the next to be executed. You can use the stepi command to step execution one instruction at a time. display/x $eax displays the contents of the eax register in hex. You will also likely want to use the x command to examine the contents of memory: x/x $eax, for example, prints the contents of the memory whose address is stored in eax.
These are a few of many commands. Download the GDB manual and skim through it to find other commands you may be interested in using.
IDA Pro does work on the Mac after a fashion (UI still runs on Windows; see an example).