415 lines
14 KiB
Python
415 lines
14 KiB
Python
#!/usr/bin/python3
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#
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# Copyright 2009-2010 Canonical, Ltd.
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# Author: Kees Cook <kees@ubuntu.com>
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#
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# This program is free software; you can redistribute it and/or modify it
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# under the terms of the GNU General Public License as published by the
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# Free Software Foundation; either version 2 of the License, or (at your
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# option) any later version. See http://www.gnu.org/copyleft/gpl.html for
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# the full text of the license.
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"""Examine the crash files saved by apport to attempt to determine the cause
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of a segfault. Currently very very simplistic, and only finds commonly
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understood situations for x86/x86_64."""
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# TODO: Address following pylint complaints
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# pylint: disable=invalid-name,missing-class-docstring,missing-function-docstring
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import logging
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import re
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import sys
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class ParseSegv:
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def __init__(self, registers, disassembly, maps):
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self.regs = self.parse_regs(registers)
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self.sp = None
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for reg in ("rsp", "esp"):
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if reg in self.regs:
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self.sp = self.regs[reg]
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(self.line, self.pc, self.insn, self.src, self.dest) = self.parse_disassembly(
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disassembly
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)
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self.stack_vma = None
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self.maps = self.parse_maps(maps)
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def find_vma(self, addr):
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for vma in self.maps:
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if vma["start"] <= addr < vma["end"]:
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return vma
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return None
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def parse_maps(self, maps_str):
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maps = []
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for line in maps_str.splitlines():
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items = line.strip().split()
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if len(items) < 4:
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raise ValueError(f"Cannot parse maps line: {line.strip()}")
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span, perms = items[0:2]
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if len(items) == 5:
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name = None
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else:
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name = items[5]
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start, end = [int(x, 16) for x in span.split("-")]
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if name == "[stack]":
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self.stack_vma = len(maps)
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maps.append({"start": start, "end": end, "perms": perms, "name": name})
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logging.debug(
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"start: %s, end: %s, perms: %s, name: %s", start, end, perms, name
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)
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return maps
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@staticmethod
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def parse_regs(reg_str):
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regs = {}
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for line in reg_str.splitlines():
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reg, hexvalue = line.split()[0:2]
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regs[reg] = int(hexvalue, 16)
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logging.debug("%s:0x%08x", reg, regs[reg])
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return regs
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def parse_disassembly(self, disassembly):
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# TODO: Split into smaller functions/methods
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# pylint: disable=too-many-branches
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if not self.regs:
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raise ValueError("Registers not loaded yet!?")
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lines = disassembly.splitlines()
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# Throw away possible 'Dump' gdb report line
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if len(lines) > 0 and lines[0].startswith("Dump"):
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lines.pop(0)
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if len(lines) < 1:
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raise ValueError("Failed to load empty disassembly")
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line = lines[0].strip()
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# Drop GDB 7.1's leading $pc mark
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if line.startswith("=>"):
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line = line[2:].strip()
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logging.debug(line)
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pc_str = line.split()[0]
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if pc_str.startswith("0x"):
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pc = int(pc_str.split(":")[0], 16)
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else:
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# Could not identify this instruction line
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raise ValueError(
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f'Could not parse PC "{pc_str}" from disassembly line: {line}'
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)
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logging.debug("pc: 0x%08x", pc)
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full_insn_str = line.split(":", 1)[1].strip()
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# Handle invalid memory
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if "Cannot access memory at address" in full_insn_str or (
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full_insn_str == "" and len(lines) == 1
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):
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return line, pc, None, None, None
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# Handle wrapped lines
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if full_insn_str == "" and lines[1].startswith(" "):
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line = f"{line} {lines[1].strip()}"
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full_insn_str = line.split(":", 1)[1].strip()
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insn_parts = full_insn_str.split()
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# Drop call target names "call 0xb7a805af <_Unwind_Find_FDE@plt+111>"
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if insn_parts[-1].endswith(">") and insn_parts[-1].startswith("<"):
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insn_parts.pop(-1)
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# Attempt to find arguments
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args_str = ""
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if len(insn_parts) > 1:
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args_str = insn_parts.pop(-1)
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# Assume remainder is the insn itself
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insn = " ".join(insn_parts)
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logging.debug("insn: %s", insn)
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args = []
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src = None
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dest = None
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if args_str == "":
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# Could not find insn args
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args = None
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else:
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logging.debug('args: "%s"', args_str)
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for m in re.finditer(r"([^,\(]*(\(:?[^\)]+\))*)", args_str):
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if len(m.group(0)):
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args.append(m.group(0))
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if len(args) > 0:
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src = args[0]
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logging.debug("src: %s", src)
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if len(args) > 1:
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dest = args[1]
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logging.debug("dest: %s", dest)
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# Set up possible implicit memory destinations (stack actions)
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if insn in {"push", "pop", "pushl", "popl", "call", "callq", "ret", "retq"}:
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for reg in ("rsp", "esp"):
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if reg in self.regs:
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dest = f"(%{reg})"
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break
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return line, pc, insn, src, dest
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def validate_vma(self, perm, addr, name):
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perm_name = {
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"x": ["executable", "executing"],
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"r": ["readable", "reading"],
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"w": ["writable", "writing"],
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}
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vma = self.find_vma(addr)
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if vma is None:
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alarmist = "unknown"
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if addr < 65536:
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alarmist = "NULL"
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return (
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False,
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f"{name} (0x{addr:08x}) not located in a known VMA region"
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f" (needed {perm_name[perm][0]} region)!",
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f"{perm_name[perm][1]} {alarmist} VMA",
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)
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if perm not in vma["perms"]:
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alarmist = ""
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if perm == "x":
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if "w" in vma["perms"]:
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alarmist = "writable "
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else:
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alarmist = "non-writable "
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short = f"{perm_name[perm][1]} {alarmist}VMA {vma['name']}"
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return (
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False,
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f"{name} (0x{addr:08x}) in non-{perm_name[perm][0]} VMA"
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f" region: 0x{vma['start']:08x}-0x{vma['end']:08x}"
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f" {vma['perms']} {vma['name']}",
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short,
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)
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return (True, f"{name} (0x{addr:08x}) ok", f"{perm_name[perm][1]} ok")
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def register_value(self, reg):
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reg_orig = reg
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mask = 0
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if reg.startswith("%"):
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reg = reg[1:]
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if reg in self.regs:
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return self.regs[reg]
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if len(reg) == 2 and reg.endswith("l"):
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mask |= 0xFF00
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reg = f"{reg[0]}x"
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if reg in self.regs:
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return self.regs[reg] & ~mask
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if len(reg) == 2 and reg.endswith("x"):
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mask |= 0xFFFF0000
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reg = f"e{reg}"
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if reg in self.regs:
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return self.regs[reg] & ~mask
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if len(reg) == 3 and reg.startswith("e"):
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mask |= 0xFFFFFFFF00000000
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reg = f"r{reg[1:]}"
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if reg in self.regs:
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return self.regs[reg] & ~mask
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raise ValueError(f"Could not resolve register '{reg_orig}'")
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def calculate_arg(self, arg):
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# TODO: Split into smaller functions/methods
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# pylint: disable=too-many-branches
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# Check for and pre-remove segment offset
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segment = 0
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if arg.startswith("%") and ":" in arg:
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parts = arg.split(":", 1)
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segment = self.regs[parts[0][1:]]
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arg = parts[1]
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# Handle standard offsets
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parts = arg.split("(")
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offset = parts[0]
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# Handle negative signs
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sign = 1
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if offset.startswith("-"):
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sign = -1
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offset = offset[1:]
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# Skip call target dereferences
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if offset.startswith("*"):
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offset = offset[1:]
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if len(offset) > 0:
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if offset.startswith("%"):
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# Handle the *%REG case
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add = self.regs[offset[1:]]
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else:
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if not offset.startswith("0x"):
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raise ValueError(f"Unknown offset literal: {parts[0]}")
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add = int(offset[2:], 16) * sign
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else:
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add = 0
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def _reg_val(self, text, val=0):
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if text.startswith("%"):
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val = self.regs[text[1:]]
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elif text == "":
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val = 0
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else:
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val = int(text)
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return val
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# (%ebx, %ecx, 4) style
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value = 0
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if len(parts) > 1:
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parens = parts[1][0:-1]
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reg_list = parens.split(",")
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base = 0
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if len(reg_list) > 0:
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base = _reg_val(self, reg_list[0], base)
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index = 0
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if len(reg_list) > 1:
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index = _reg_val(self, reg_list[1], index)
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scale = 1
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if len(reg_list) > 2:
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scale = _reg_val(self, reg_list[2], scale)
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value = base + index * scale
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value = segment + value + add
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if "esp" in self.regs:
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# 32bit
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return value % 0x100000000
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# 64bit
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return value % 0x10000000000000000
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def report(self):
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# TODO: Split into smaller functions/methods
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# pylint: disable=too-many-branches,too-many-statements
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understood = False
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reason = []
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details = [f"Segfault happened at: {self.line}"]
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# Verify PC is in an executable region
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valid, out, short = self.validate_vma("x", self.pc, "PC")
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details.append(out)
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if not valid:
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reason.append(short)
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understood = True
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if self.insn in {"lea", "leal"}:
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# Short-circuit for instructions that do not cause vma access
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details.append(f"insn ({self.insn}) does not access VMA")
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else:
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# Verify source is readable
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if self.src:
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if (
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":" not in self.src
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and (self.src[0] in {"%", "$", "*"})
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and not self.src.startswith("*%")
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):
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details.append(f'source "{self.src}" ok')
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else:
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addr = self.calculate_arg(self.src)
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valid, out, short = self.validate_vma(
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"r", addr, f'source "{self.src}"'
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)
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details.append(out)
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if not valid:
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reason.append(short)
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understood = True
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# Verify destination is writable
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if self.dest:
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if ":" not in self.dest and (self.dest[0] in {"%", "$", "*"}):
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details.append(f'destination "{self.dest}" ok')
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else:
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addr = self.calculate_arg(self.dest)
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valid, out, short = self.validate_vma(
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"w", addr, f'destination "{self.dest}"'
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)
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details.append(out)
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if not valid:
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reason.append(short)
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understood = True
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# Handle I/O port operations
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if self.insn in {"out", "in"} and not understood:
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msg = (
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f"disallowed I/O port operation"
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f" on port {self.register_value(self.src)}"
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)
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reason.append(msg)
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details.append(msg)
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understood = True
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# Note position of SP with regard to "[stack]" VMA
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if self.sp is not None:
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if self.stack_vma is not None:
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if self.sp < self.maps[self.stack_vma]["start"]:
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details.append("Stack memory exhausted (SP below stack segment)")
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if self.sp >= self.maps[self.stack_vma]["end"]:
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details.append("Stack pointer not within stack segment")
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if not understood:
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valid, out, short = self.validate_vma("r", self.sp, "SP")
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details.append(out)
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if not valid:
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reason.append(short)
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understood = True
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if not understood:
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vma = self.find_vma(self.pc)
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msg = "Reason could not be automatically determined."
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if vma and (vma["name"] == "[vdso]" or vma["name"] == "[vsyscall]"):
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msg += " (Unhandled exception in kernel code?)"
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reason.append(msg)
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details.append(msg)
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return understood, "\n".join(reason), "\n".join(details)
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def add_info(report):
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# Only interested in segmentation faults...
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if report.get("Signal", "0") != "11":
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return
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needed = ["Signal", "Architecture", "Disassembly", "ProcMaps", "Registers"]
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for field in needed:
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if field not in report:
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report["SegvAnalysis"] = f'Skipped: missing required field "{field}"'
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return
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# Only run on segv for x86 and x86_64...
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if not report["Architecture"] in {"i386", "amd64"}:
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return
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try:
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segv = ParseSegv(report["Registers"], report["Disassembly"], report["ProcMaps"])
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understood, reason, details = segv.report()
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if understood:
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report["SegvReason"] = reason
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report["SegvAnalysis"] = details
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except Exception as error: # pylint: disable=broad-except
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report["SegvAnalysis"] = f"Failure: {str(error)}"
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# pylint: disable-next=missing-function-docstring
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def main():
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if len(sys.argv) != 4 or sys.argv[1] in {"-h", "--help"}:
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print("To run self-test, run without any arguments (or with -v)")
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print("To do stand-alone crash parsing:")
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print(f" Usage: {sys.argv[0]} Registers.txt Disassembly.txt ProcMaps.txt")
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sys.exit(0)
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with open(sys.argv[1], encoding="utf-8") as registers_file:
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registers = registers_file.read()
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with open(sys.argv[2], encoding="utf-8") as disassembly_file:
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disassembly = disassembly_file.read()
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with open(sys.argv[3], encoding="utf-8") as maps_file:
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maps = maps_file.read()
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segv = ParseSegv(registers, disassembly, maps)
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understood, reason, details = segv.report()
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print(f"{reason}\n\n{details}")
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rc = 0
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if not understood:
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rc = 1
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sys.exit(rc)
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if __name__ == "__main__":
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main()
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