Seg Fault in ARM Assembly
So, I am trying to learn ARM assembly and basically what I want to do is turn on the LEDs of my BeagleBone Black using pure assembly. I know how to program in C very well, but I am new to ARM assembly if that makes any difference.
Basically I am just trying to modify a character in a string, but it doesn't seem to be working. Maybe it is because I do not fully understand the memory management instructions.
When I run the code it gives me a segmentation fault.
Here is my code:
.syntax unified
.global main
main:
push {ip, lr}
mov r0, beagle_bone_0
mov r1, #0x65
strb r1, [r0]
ldr r0, =beagle_bone_0
bl printf
pop {ip, pc}
beagle_bone_0:
.asciz "/sys/class/leds/beaglebone:green:usr0/brightness"
objdump -x output:
helloworld: file format elf32-littlearm
helloworld
architecture: arm, flags 0x00000112:
EXEC_P, HAS_SYMS, D_PAGED
start address 0x00008325
Program Header:
0x70000001 off 0x00000444 vaddr 0x00008444 paddr 0x00008444 align 2**2
filesz 0x00000008 memsz 0x00000008 flags r--
PHDR off 0x00000034 vaddr 0x00008034 paddr 0x00008034 align 2**2
filesz 0x00000100 memsz 0x00000100 flags r-x
INTERP off 0x00000134 vaddr 0x00008134 paddr 0x00008134 align 2**0
filesz 0x00000019 memsz 0x00000019 flags r--
LOAD off 0x00000000 vaddr 0x00008000 paddr 0x00008000 align 2**15
filesz 0x00000450 memsz 0x00000450 flags r-x
LOAD off 0x00000450 vaddr 0x00010450 paddr 0x00010450 align 2**15
filesz 0x00000124 memsz 0x00000128 flags rw-
DYNAMIC off 0x0000045c vaddr 0x0001045c paddr 0x0001045c align 2**2
filesz 0x000000f0 memsz 0x000000f0 flags rw-
NOTE off 0x00000150 vaddr 0x00008150 paddr 0x00008150 align 2**2
filesz 0x00000044 memsz 0x00000044 flags r--
STACK off 0x00000000 vaddr 0x00000000 paddr 0x00000000 align 2**2
filesz 0x00000000 memsz 0x00000000 flags rwx
Dynamic Section:
NEEDED libc.so.6
INIT 0x000082d1
FINI 0x00008439
INIT_ARRAY 0x00010450
INIT_ARRAYSZ 0x00000004
FINI_ARRAY 0x00010454
FINI_ARRAYSZ 0x00000004
HASH 0x00008194
GNU_HASH 0x000081bc
STRTAB 0x00008238
SYMTAB 0x000081e8
STRSZ 0x00000043
SYMENT 0x00000010
DEBUG 0x00000000
PLTGOT 0x0001054c
PLTRELSZ 0x00000020
PLTREL 0x00000011
JMPREL 0x000082b0
REL 0x000082a8
RELSZ 0x00000008
RELENT 0x00000008
VERNEED 0x00008288
VERNEEDNUM 0x00000001
VERSYM 0x0000827c
Version References:
required from libc.so.6:
0x0d696914 0x00 02 GLIBC_2.4
private flags = 5000002: [Version5 EABI] [has entry point]
Sections:
Idx Name Size VMA LMA File off Algn
0 .interp 00000019 00008134 00008134 00000134 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
1 .note.ABI-tag 00000020 00008150 00008150 00000150 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
2 .note.gnu.build-id 00000024 00008170 00008170 00000170 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
3 .hash 00000028 00008194 00008194 00000194 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
4 .gnu.hash 0000002c 000081bc 000081bc 000001bc 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
5 .dynsym 00000050 000081e8 000081e8 000001e8 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
6 .dynstr 00000043 00008238 00008238 00000238 2**0
CONTENTS, ALLOC, LOAD, READONLY, DATA
7 .gnu.version 0000000a 0000827c 0000827c 0000027c 2**1
CONTENTS, ALLOC, LOAD, READONLY, DATA
8 .gnu.version_r 00000020 00008288 00008288 00000288 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
9 .rel.dyn 00000008 000082a8 000082a8 000002a8 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
10 .rel.plt 00000020 000082b0 000082b0 000002b0 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
11 .init 0000000a 000082d0 000082d0 000002d0 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
12 .plt 00000048 000082dc 000082dc 000002dc 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
13 .text 00000114 00008324 00008324 00000324 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
14 .fini 00000006 00008438 00008438 00000438 2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
15 .rodata 00000004 00008440 00008440 00000440 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
16 .ARM.exidx 00000008 00008444 00008444 00000444 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
17 .eh_frame 00000004 0000844c 0000844c 0000044c 2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
18 .init_array 00000004 00010450 00010450 00000450 2**2
CONTENTS, ALLOC, LOAD, DATA
19 .fini_array 00000004 00010454 00010454 00000454 2**2
CONTENTS, ALLOC, LOAD, DATA
20 .jcr 00000004 00010458 00010458 00000458 2**2
CONTENTS, ALLOC, LOAD, DATA
21 .dynamic 000000f0 0001045c 0001045c 0000045c 2**2
CONTENTS, ALLOC, LOAD, DATA
22 .got 00000020 0001054c 0001054c 0000054c 2**2
CONTENTS, ALLOC, LOAD, DATA
23 .data 00000008 0001056c 0001056c 0000056c 2**2
CONTENTS, ALLOC, LOAD, DATA
24 .bss 00000004 00010574 00010574 00000574 2**0
ALLOC
25 .comment 0000001d 00000000 00000000 00000574 2**0
CONTENTS, READONLY
26 .ARM.attributes 00000031 00000000 00000000 00000591 2**0
CONTENTS, READONLY
SYMBOL TABLE:
00008134 l d .interp 00000000 .interp
00008150 l d .note.ABI-tag 00000000 .note.ABI-tag
00008170 l d .note.gnu.build-id 00000000 .note.gnu.build-id
00008194 l d .hash 00000000 .hash
000081bc l d .gnu.hash 00000000 .gnu.hash
000081e8 l d .dynsym 00000000 .dynsym
00008238 l d .dynstr 00000000 .dynstr
0000827c l d .gnu.version 00000000 .gnu.version
00008288 l d .gnu.version_r 00000000 .gnu.version_r
000082a8 l d .rel.dyn 00000000 .rel.dyn
000082b0 l d .rel.plt 00000000 .rel.plt
000082d0 l d .init 00000000 .init
000082dc l d .plt 00000000 .plt
00008324 l d .text 00000000 .text
00008438 l d .fini 00000000 .fini
00008440 l d .rodata 00000000 .rodata
00008444 l d .ARM.exidx 00000000 .ARM.exidx
0000844c l d .eh_frame 00000000 .eh_frame
00010450 l d .init_array 00000000 .init_array
00010454 l d .fini_array 00000000 .fini_array
00010458 l d .jcr 00000000 .jcr
0001045c l d .dynamic 00000000 .dynamic
0001054c l d .got 00000000 .got
0001056c l d .data 00000000 .data
00010574 l d .bss 00000000 .bss
00000000 l d .comment 00000000 .comment
00000000 l d .ARM.attributes 00000000 .ARM.attributes
0000835c l F .text 00000000 call_gmon_start
00000000 l df *ABS* 00000000 crtstuff.c
00010458 l O .jcr 00000000 __JCR_LIST__
00008374 l F .text 00000000 __do_global_dtors_aux
00010574 l O .bss 00000001 completed.5637
00010454 l O .fini_array 00000000 __do_global_dtors_aux_fini_array_entry
00008384 l F .text 00000000 frame_dummy
00010450 l O .init_array 00000000 __frame_dummy_init_array_entry
000083b8 l .text 00000000 beagle_bone_0
00000000 l df *ABS* 00000000 crtstuff.c
0000844c l O .eh_frame 00000000 __FRAME_END__
00010458 l O .jcr 00000000 __JCR_END__
00010454 l .init_array 00000000 __init_array_end
0001045c l O .dynamic 00000000 _DYNAMIC
00010450 l .init_array 00000000 __init_array_start
0001054c l O .got 00000000 _GLOBAL_OFFSET_TABLE_
00008434 g F .text 00000002 __libc_csu_fini
0001056c w .data 00000000 data_start
000082f0 F *UND* 00000000 printf@@GLIBC_2.4
00010574 g *ABS* 00000000 __bss_start__
00010578 g *ABS* 00000000 _bss_end__
00010574 g *ABS* 00000000 _edata
00008438 g F .fini 00000000 _fini
00010578 g *ABS* 00000000 __bss_end__
0001056c g .data 00000000 __data_start
000082fc F *UND* 00000000 __libc_start_main@@GLIBC_2.4
00000000 w *UND* 00000000 __gmon_start__
00010570 g O .data 00000000 .hidden __dso_handle
00008440 g O .rodata 00000004 _IO_stdin_used
000083f0 g F .text 00000044 __libc_csu_init
00010578 g *ABS* 00000000 _end
00008324 g F .text 00000000 _start
00010578 g *ABS* 00000000 __end__
00010574 g *ABS* 00000000 __bss_start
0000839c g .text 00000000 main
00000000 w *UND* 00000000 _Jv_RegisterClasses
00008318 F *UND* 00000000 abort@@GLIBC_2.4
000082d0 g F .init 00000000 _init
2 Answers
The answer to my question was actually really simple. Since ldr r0, =beagle_bone_0 loads the address of beagle_bone_0 into register 0 I can just manipulate beagle_bone_0 with that address.
Working test code:
.syntax unified
.data
beagle_bone_0: .ascii "Hello, world\n"
.text
.global main
main:
push {ip, lr}
ldr r0, =beagle_bone_0
mov r1, #0x65
strb r1, [r0]
bl printf
pop {ip, pc}
answered on Stack Overflow Dec 7, 2014 by 
schmidt73
schmidt73I ran and debugged your code. The line mov r0, beagle_bone_0 didn't even compile (on my compiler, at least). You want to load in r0 the address of beagle_bone. For this, you should use the adr pseudo-instruction, that is translated by the compiler in a pc-relative move (something like mov r0, [pc, #8]. You cannot use it this way. Probably your compiler translated it into something different.
So, to fix it, just replace the line mov r0, beagle_bone_0 by adr r0, beagle_bone_0.
Also the string was in the .text section which we cannot edit. So, I put beagle_bone_0 in the .data section.
Paul92
Ben AffleckUser contributions licensed under CC BY-SA 3.0