Author Topic: Segmentation fault at 3376 bytes  (Read 3762 times)

Offline razor3x

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Segmentation fault at 3376 bytes
« on: August 07, 2018, 11:11:18 PM »
I'm trying to make a simple HEX editor, but it prints only first 3376 bytes and throws segfault exception. However, in GDB it works correctly and prints the whole file!

Code: [Select]
section .bss
stat resq 18
buf resq 6
raw resb 0

section .data
no_args db 'No arguments', 10
desc dq 0

section .text

global _start
_start:
pop r14 ; argc

cmp r14, 2
jl _no_args

pop rdi ; first arg is the program name

; get the file size

mov rax, 4 ; SYS_NEWSTAT
pop rdi
push rdi
mov rsi, stat
syscall

mov r14, qword [stat + 48]

; allocate the memory in heap

mov rax, 12 ; SYS_BRK
xor rdi, rdi
syscall

lea rdi, [rax + r14]
mov rax, 12
syscall

; get all bytes from file

mov rax, 2 ; SYS_OPEN
pop rdi
mov rsi, 666q
syscall

mov [desc], rax

xor rax, rax
mov rdi, [desc]
mov rsi, raw
mov rdx, r14
syscall

mov rax, 3 ; SYS_CLOSE
mov rdi, [desc]
syscall

; prepare main registers to SYS_WRITE calls
;
; 1 syscall = 1 string = 16 bytes separated by space and line feed at end
; r12 - current line index, r13 - global index, r14 - byte count

mov rdi, 1
mov rsi, buf
mov rdx, 48

; first space position
mov r12, 2
_space:
mov byte [rsi + r12], ' '

add r12, 3

cmp r12, 47
jl _space

; add line break

mov byte [rsi + r12], 10
_print:
call _line

cmp r13, r14
jl _print

jmp _exit
_no_args:
mov rax, 1
mov rdi, 1
mov rsi, no_args
mov rdx, 13
syscall
_exit:
mov rax, 60
xor rdi, rdi
syscall
_line:
xor r12, r12
_byte:
mov bl, byte [raw + r13]
shr bl, 4

call _nibble

inc r12

mov bl, byte [raw + r13]
and bl, 15

call _nibble

inc r13

cmp r13, r14
je _break

add r12, 2

cmp r12, 47
jl _byte

jmp _write
_break:
inc r12

mov byte [rsi + r12], 10

mov rdx, r12
inc rdx
_write:
mov rax, 1 ; SYS_WRITE
syscall

ret
_nibble:
cmp rbx, 10
jl _less

add rbx, 7
_less:
add rbx, 48

mov byte [rsi + r12], bl
ret

Offline Frank Kotler

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Re: Segmentation fault at 3376 bytes
« Reply #1 on: August 08, 2018, 04:58:58 AM »
Hi razor3x,
Welcome to the Forum.

I do not see the problem with your program. I'll look more.

I think I see an issue with the parameters to sys_open. 666q looks like a mode to me. Should be in rdx? Ignored? In rsi should be flags - O_RDONLY - 0 - probably? This is not the problem and doesn't seem to make any difference, but I thought I'd mention it...

Nice work! Good luck finding the problem. I'll look more but don't count on me!

Best,
Frank


Offline Frank Kotler

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Re: Segmentation fault at 3376 bytes
« Reply #2 on: August 08, 2018, 09:06:44 PM »
The 3376 bytes before the segfault might be the remainder of the 4096 the .bss probably allocates before the sys_brk's. I increased raw to resb 100000 - big enough for my test file. No segfault! So... looks like something in your allocation isn't working? Dunno why or why it works in GDB... Thought I'd pass that along. Maybe more later...

Best,
Frank


Offline razor3x

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  • Posts: 2
Re: Segmentation fault at 3376 bytes
« Reply #3 on: August 09, 2018, 12:27:01 AM »
Nevermind, i just forgot to free the memory and access the buffer by value (not by address!). Thank you so much, you saved my time :)

Code: [Select]
; print the whole file

section .bss
stat resq 18
brk resb 0

section .data
no_args db 'No arguments', 10
desc dq 0

section .text

global _start
_start:
pop r14 ; argc

cmp r14, 2
jl _no_args

pop rdi

mov rax, 4 ; SYS_NEWSTAT
pop rdi
push rdi
mov rsi, stat
syscall

mov r14, qword [stat + 48]

mov rax, 12 ; SYS_BRK
xor rdi, rdi
syscall

mov [brk], rax

push rcx ; caller-saved register
mov rcx, r14
lea rdi, [rcx + rax]
mov rax, 12
syscall

pop rcx

mov rax, 2 ; SYS_OPEN
pop rdi
xor rsi, rsi
syscall

mov [desc], rax

xor rax, rax
mov rdi, [desc]
mov rsi, [brk]
mov rdx, r14
syscall

mov rax, 3 ; SYS_CLOSE
mov rdi, [desc]
syscall

mov rax, 1
mov rdi, 1
syscall

mov rax, 12
mov rdi, r14
syscall

jmp _exit
_no_args:
mov rax, 1
mov rdi, 1
mov rsi, no_args
mov rdx, 13
syscall
_exit:
mov rax, 60
xor rdi, rdi
syscall