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import re
# Match a register name: $eax, $gp0, $orig_eax
REGISTER_REGEX = re.compile(r"([a-z]+[a-z0-9_]+)")
# Hexadacimel number (eg. 0xa)
HEXADECIMAL_REGEX = re.compile(r"0x[0-9a-f]+")
# Make sure that the expression does not contain invalid characters
# Examples:
# (1-2)<<5
# 340&91
EXPR_REGEX = re.compile(r"^[()<>+*/&0-9-]+$")
def replaceHexadecimal(regs):
"""
Convert an hexadecimal number to decimal number (as string).
Callback used by parseExpression().
"""
text = regs.group(0)
if text.startswith("0x"):
text = text[2:]
elif not re.search("[a-f]", text):
return text
value = int(text, 16)
return str(value)
def parseExpression(process, text):
"""
Parse an expression. Syntax:
- "10": decimal number
- "0x10": hexadecimal number
- "eax": register value
- "a+b", "a-b", "a*b", "a/b", "a**b", "a<<b", "a>>b": operators
>>> from ptrace.mockup import FakeProcess
>>> process = FakeProcess()
>>> parseExpression(process, "1+1")
2
>>> process.setreg("eax", 3)
>>> parseExpression(process, "eax*0x10")
48
"""
# Remove spaces and convert to lower case
text = text.strip()
orig_text = text
if " " in text:
raise ValueError("Space are forbidden: %r" % text)
text = text.lower()
def readRegister(regs):
name = regs.group(1)
value = process.getreg(name)
return str(value)
# Replace hexadecimal by decimal
text = HEXADECIMAL_REGEX.sub(replaceHexadecimal, text)
# Replace registers by their value
text = REGISTER_REGEX.sub(readRegister, text)
# Reject invalid characters
if not EXPR_REGEX.match(text):
raise ValueError("Invalid expression: %r" % orig_text)
# Use integer division (a//b) instead of float division (a/b)
text = text.replace("/", "//")
# Finally, evaluate the expression
try:
value = eval(text)
except SyntaxError:
raise ValueError("Invalid expression: %r" % orig_text)
return value
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