ARM XOR encoder not decoding the whole payload

[peace-maker]

After manually applying #1923 I noticed that the arm xor encoder is broken for longer payloads.

pwntools/pwnlib/encoders/arm/xor.py

Lines 29 to 46 in bd12d18

	decoder = """
	adr r8, payload
	mov r4, #%(length)s
	adr r6, xor_cacheflush
	loop:
	cmp r4, #%(maximum)s
	bxhi r6
	sub r4, r4, #%(length)s
	ldrb r5, [r8, r4]
	eor r5, r5, #%(key)s
	strb r5, [r8, r4]
	add r4, r4, #%(length)s + 1
	b loop
	xor_cacheflush:
	%(cacheflush)s
	payload:
	"""

maximum is hardcoded to 256, so it only decodes maximum - length bytes instead of all of length.
My quick fix was to set maximum = len(raw_bytes)*2, but I'm not sure if that violates some guarantees for the blacklist.

Test:

from pwn import *
context.arch = 'arm'

arm_shellcode = asm(shellcraft.execve('/bin/sh', ['/bin/sh', '-c', 'ls -la'], 0))
arm_shellcode = encoder.encode(arm_shellcode, avoid=b'\x00\n')

exe = ELF.from_bytes(arm_shellcode)
io = process(['/usr/bin/qemu-arm', exe.path])
io.interactive()