Minimal Stub for remote debugging

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Minimal Stub for remote debugging

• Minheng Tan
• Columbia University

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My project - debugger stub

• My GDBServer debugger stub.
• Runs on Red Hat Linux, x86
• Provides minimum command support(but facilitates
  all debugging requirements)
• Speaks Remote Serial Protocol (RSP) over tcp/ip
• Debugs most applications running Linux.

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Debuggers

• MSDev
• Windbg
• dbx
• gdb

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Remote Debugging
Chip
Machine A
Debugger
Program
Stub
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Remote Debugging continued
Read register 3, Read memory at 0x338828, Write
CC at 0x380280, Continue program.
Machine A
Debugger
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Remote Debugging continued
Register 3 is 0x75939ff3, Memory content at
0x338828 is 0x094833, Memory content
written, Program resumed execution.
Chip
Program
Stub
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Remote Serial Protocol

• Request/Reply protocol
• ASCII encoding
• Packet based.
• Simple to parse, implement, extend.
• Runs on almost all communication medium

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RSP commands implemented

• g read all register
• G write all register
• m read memory from a memory at specific
  address
• M write data to memory at specific address
• ? Get last signal(what happened to the
  program)

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RSP commands implementscontinued

• s step the program. Make the debugged
  program execute 1 instruction and relinquish
  control.
• c continue the program. Resume the debugged
  program and wait until it stop on a breakpoint,
  bus error, access violation, etc

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Implement read register

• buf malloc (regset-gtsize)
• res ptrace (PTRACE_GETREGS, childpid, 0, buf)

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Implement write register

• regset-gtfill_function (buf)
• res ptrace (PTRACE_SETREGS, childpid, 0, (int)
  buf)

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Implement read memory

• i 0
• while (startAddr lt endAddr)
• bufferi ptrace(PTRACE_PEEKTEXT,
  childpid, startAddr, 0 )
• startAddrsizeof(PTRACE_XFER_TYPE)

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Implement write memory

• i 0
• while ( startAddr lt endAddr )
• ptrace (PTRACE_POKETEXT, childpid, startAddr,
  bufferi)
• StartAddrsizeof(PTRACE_XFER_TYPE)

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Implement Step/Continue

• ptrace (PTRACE_CONT, childpid, 1, 0)
• ptrace (PTRACE_SINGLESTEP, childpid, 1, 0)

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Summary

• Minimum commands implemented
• Packet based remote serial protocol.
• Debugger uses the bare minimum stub to implement
  big things.