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Minimal Stub for remote debugging

Developed by Minheng Tan at Columbia University, this remote debugging stub runs on Red Hat Linux (x86) and provides essential command support, facilitating all necessary debugging actions. It utilizes the Remote Serial Protocol (RSP) over TCP/IP, enabling it to debug a wide range of applications on Linux. The stub supports commands for reading/writing registers and memory, stepping through code, and continuing program execution. With a simple packet-based structure for easy implementation and extension, it ensures effective remote debugging for various applications.

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Minimal Stub for remote debugging

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  1. Minimal Stub for remote debugging Minheng Tan Columbia University

  2. 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.

  3. Debuggers • MSDev • Windbg • dbx • gdb

  4. Remote Debugging Chip Machine A Debugger Program Stub

  5. Remote Debugging …continued Read register 3, Read memory at 0x338828, Write “CC” at 0x380280, Continue program. Machine A Debugger

  6. Remote Debugging …continued Register 3 is 0x75939ff3, Memory content at 0x338828 is 0x094833, Memory content written, Program resumed execution. Chip Program Stub

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

  8. 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)

  9. RSP commands implements…continued • “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…

  10. Implement read register • buf = malloc (regset->size); • res = ptrace (PTRACE_GETREGS, childpid, 0, buf);

  11. Implement write register • regset->fill_function (buf); • res = ptrace (PTRACE_SETREGS, childpid, 0, (int) buf);

  12. Implement read memory • i = 0; • while (startAddr <= endAddr) { • buffer[i++] = ptrace(PTRACE_PEEKTEXT, childpid, startAddr, 0 ); • startAddr+=sizeof(PTRACE_XFER_TYPE); • }

  13. Implement write memory • i = 0; • while ( startAddr <= endAddr ) { • ptrace (PTRACE_POKETEXT, childpid, startAddr, buffer[i++]); • StartAddr+=sizeof(PTRACE_XFER_TYPE); • }

  14. Implement Step/Continue • ptrace (PTRACE_CONT, childpid, 1, 0); • ptrace (PTRACE_SINGLESTEP, childpid, 1, 0);

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

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