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glibc scanf Segmentation faults when called from a function that doesn't align RSP
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I have problem with assembly program on my VirtualBox.
On VB I have Linux Mint(64 bit). I tried to compile this assembler code(in file my_file.asm):
extern printf
extern scanf
section .data
format: db "%d",0
napis: db "a/b=%d",10,0
section .bss
a: resd 1
b: resd 1
global main
section .text
main:
mov rdi, format
mov rsi, a
xor rax, rax
call scanf
mov rdi, format
mov rsi, b
xor rax, rax
call scanf
mov eax, [a]
xor edx, edx
div dword[b]
ret
At first I type: nasm -f elf64 myfile.asm
Then I type: gcc myfile.o -o myfile
And after this i have this error message:
"/usr/bin/ld: plik.o: relocation R_X86_64_32S against .bss' can not be used when making a PIE object; recompile with -fPIC
/usr/bin/ld: final link failed: Nonrepresentable section on output
collect2: error: ld returned 1 exit status
"
So i add -no-pie to gcc command: gcc -o test test.o -no-pie
Now, when I use ./my_file I have this error: Segmentation fault (core dumped) and I don't know what to do.
Related
I coding assembly with Nasm, i want debug the program using gdb, but it not works when i put a breakpoint and run the program.
The program compile fine and link too, the problem is gdb.
Here is the commands to compile:
nasm -f elf64 -F dwarf -g types.asm
nasm -f elf64 -F dwarf -g functions.asm
nasm -f elf64 -F dwarf -g Hello.asm
ld -g -o Hello Hello.o functions.o types.o
This is the file i want debug Hello.asm:
%include "functions.asm"
section .bss
res: resb 1
fout: resb 1
section .text
global _start: ;must be declared for linker (ld)
section .data
msg db 'Hello, world!', 0xa ;string to be printed
len equ $ - msg ;length of the string
filename db 'hello.txt'
_start: ;tells linker entry point
mov ecx,5
mov edx,4
call sum
mov [res],eax
mov edx,1 ;message length
mov ecx,res ;message to write
mov ebx,1 ;file descriptor (stdout)
mov eax,4 ;system call number (sys_write)
int 0x80 ;call kernel
write_string msg,len
create_file filename
mov [fout],eax
close_file [fout]
call print_msg
mov eax,1 ;system call number (sys_exit)
int 0x80 ;call kernel
sum:
mov eax,ecx
add eax,edx
add eax,'0'
ret
Next i open gdb:
gdb Hello
(gdb) break _start
Function «_start» not defined
¿Compilación de breakpoint pendiente hasta futura cargada de biblioteca compartida? (y or [n]) y
Punto de interrupción 1 (_start) pendiente.
(gdb) run
Starting program: /asm/Hello
9Hello, world!
Hello, world!from another file
[Inferior 1 (process 5811) exited with code 01]
(gdb)
I solved it, i only change position section .data to section .text and the debugger works.I don't know why, but now the gdb take the .start.
I have the following assembly, which I assemble with NASM and then link with gcc:
section .text
extern printf
global main
main:
sub rsp, 8 ;align stack pointer
mov rax, 0 ;no vector arguments
mov rdi, intro_message ;First argument
call printf
mov rax, 60 ;exit
syscall
section .data
intro_message:
db 'Hello world',0
When I run ./a.out, nothing is printed, and the instruction pointer seems to have moved to an invalid location. What have I done wrong?
Im trying to run this shellcode but it throws me: "Segmentation fault" error
The shellcode is the following:
shellcode.asm:
global _start
_start:
jmp short ca
doit:
pop ebx
xor eax, eax
cdq
mov byte [ebx+7], al
mov long [ebx+8], ebx
mov long [ebx+12], eax
lea ecx, [ebx+8]
mov byte al, 0x0b
int 0x80
ca:
call doit
db '/bin/sh'
i compile it with : 'nasm -f elf shellcode.asm'
and link it with: ' ld -m elf_i386 -s -o shellcode shellcode.o
I think the error is when I use mov [ebx+x], al/eax/ebx
because when I erase it from the code y get no error
Thank you
Your problem is that the .text section is not writable by default. The easiest thing to do is put your code into a new custom section that is marked as writable. Add this line at the top of your asm file:
section .shellcode progbits alloc exec write align=16
You could also pass the -N switch to the linker.
Alternatively, you could rewrite the shellcode so that it uses the stack to create the arguments.
[section .data]
strHello db "Hello World"
STRLEN equ $-strHello
MessageLength equ 9
Message db "hi!!!! "
[section .text]
global main
main:
mov edx,STRLEN;
mov ecx,strHello;
mov ebx,1
mov eax,4
int 0x80
call DispStr
mov ebx,0
mov eax,1
int 0x80
DispStr:
mov ax,MessageLength
mov dh,0
mul dh
add ax,Message
mov bp,ax
mov ax,ds
mov es,ax
mov cx,MessageLength
mov ax,01301h
mov bx,0007h
mov dl,0
int 10h
ret
Compile and run:
$ nasm -f elf64 helloworld.asm -o helloworld.o
$ gcc -s -o helloworld helloworld.o
helloworld.o: In function `DispStr':
helloworld.asm:(.text+0x31): relocation truncated to fit: R_X86_64_16 against `.data'
collect2: ld return 1
This exact error happens because at:
add ax,Message
ax is only 16-bit wide, but Message is a 64-bit wide address, so it won't fit during relocation.
I have explained this error in detail at: https://stackoverflow.com/a/32639540/895245
The solution in this case is to use a linker script as mentioned at: Using .org directive with data in .data section: In connection with ld
This repository contains working examples of boot sectors and BIOS: https://github.com/cirosantilli/x86-bare-metal-examples/tree/d217b180be4220a0b4a453f31275d38e697a99e0
Since you're in 64-bit mode, you won't be able to use BIOS functions (i.e. the int 10h instruction). Even if you could, BIOS uses a different addressing mechanism, so attempting to use the address of Message wouldn't work anyway.
Also, wouldn't the first 3 lines of the DispStr function zero out ax? (since you're multiplying by dh, which was just set to zero)
I'm doing a project that attaches a subroutine that I wrote to a main file included by the teacher. He gave us the instructions for making our subroutine global but apparently I'm an idiot. The two asm files are in the same folder, I'm using nasm -f elf -g prt_dec.asm and ld prt_dec and then doing the same for main.asm. Here's the relevant code in the main.asm:
SECTION .text ; Code section.
global _start ; let loader see entry point
extern prt_dec
_start:
mov ebx, 17
mov edx, 214123
mov edi, 2223187809
mov ebp, 1555544444
mov eax, dword 0x0
call prt_dec
call prt_lf
The line call prt_dec throws "undefined reference to prt_dec" when i use ld main.o
Here's the a code segment from my prt_dec.asm:
Section .text
global prt_dec
global _start
start:
prt_dec:
(pushing some stuff)
L1_top:
(code continues)
You want to call a routine in another asm file or object file?
if you are Assembling prt_dec.asm and are linking multiple asm files to use in your main program, here is a sample, 2 asm files Assembled and linked together... * NOTE * hello.asm *DOES NOT * have a start label!
Main asm file: hellothere.asm
sys_exit equ 1
extern Hello
global _start
section .text
_start:
call Hello
mov eax, sys_exit
xor ebx, ebx
int 80H
Second asm file: hello.asm
sys_write equ 4
stdout equ 1
global Hello
section .data
szHello db "Hello", 10
Hello_Len equ ($ - szHello)
section .text
Hello:
mov edx, Hello_Len
mov ecx, szHello
mov eax, sys_write
mov ebx, stdout
int 80H
ret
makefile:
APP = hellothere
$(APP): $(APP).o hello.o
ld -o $(APP) $(APP).o hello.o
$(APP).o: $(APP).asm
nasm -f elf $(APP).asm
hello.o: hello.asm
nasm -f elf hello.asm
Now, if you just want to separate your code into multiple asm files, you can include them into your main source: with %include "asmfile.asm" at the beginning of your main source file and just assemble and link your main file.