[ { "id": "conditional", "title": "Conditional breakpoint: stop at the bad sample", "steps": [ { "command": "break check_output if actual != expected", "output": "Breakpoint 1 at 0x1560: file /lab/debug_lab.cpp, line 18.\n", "caption": "Skip correct samples automatically", "duration_ms": 4500 }, { "command": "run", "output": "[Thread debugging using libthread_db enabled]\nUsing host libthread_db library \"/lib/aarch64-linux-gnu/libthread_db.so.1\".\n\nBreakpoint 1, check_output (tick=tick@entry=3, expected=expected@entry=20, actual=-20) at /lab/debug_lab.cpp:18\n18\t asm volatile(\"\" : : \"r\"(tick), \"r\"(expected), \"r\"(actual) : \"memory\");\n", "caption": "First bad result: tick 3, expected 20, actual -20", "duration_ms": 4500 }, { "command": "info args\nbt 3", "output": "tick = 3\nexpected = 20\nactual = -20\n#0 check_output (tick=tick@entry=3, expected=expected@entry=20, actual=-20) at /lab/debug_lab.cpp:18\n#1 0x0000aaaaaaaa15b0 in conditional_case () at /lab/debug_lab.cpp:31\n#2 0x0000aaaaaaaa1954 in main (argc=2, argv=) at /lab/debug_lab.cpp:84\n", "caption": "Inspect values and the caller", "duration_ms": 4500 }, { "command": "up\nlist", "output": "#1 0x0000aaaaaaaa15b0 in conditional_case () at /lab/debug_lab.cpp:31\n31\t check_output(tick, 20, actual);\n26\t\n27\tNOINLINE void conditional_case() {\n28\t for (int tick = 0; tick < 6; ++tick) {\n29\t const int demand = 10;\n30\t const int actual = apply_gain(tick, demand);\n31\t check_output(tick, 20, actual);\n32\t }\n33\t}\n34\t\n35\tNOINLINE void watch_checkpoint() { asm volatile(\"\" ::: \"memory\"); }\n", "caption": "Trace the incorrect result back to apply_gain", "duration_ms": 4500 } ], "checks": [ "tick=3, expected=20, actual=-20" ], "status": "passed" }, { "id": "watchpoint", "title": "Hardware watchpoint: find the unexpected writer", "steps": [ { "command": "break watch_checkpoint\nrun\nwatch -l account.balance", "output": "Breakpoint 1 at 0x15c8: file /lab/debug_lab.cpp, line 35.\n[Thread debugging using libthread_db enabled]\nUsing host libthread_db library \"/lib/aarch64-linux-gnu/libthread_db.so.1\".\n\nBreakpoint 1, watch_checkpoint () at /lab/debug_lab.cpp:35\n35\tNOINLINE void watch_checkpoint() { asm volatile(\"\" ::: \"memory\"); }\nHardware watchpoint 2: -location account.balance\n", "caption": "Watch memory, not a guessed source line", "duration_ms": 4500 }, { "command": "continue", "output": "\nHardware watchpoint 2: -location account.balance\n\nOld value = 100\nNew value = 125\n0x0000aaaaaaaa15dc in apply_deposit (amount=amount@entry=25) at /lab/debug_lab.cpp:36\n36\tNOINLINE void apply_deposit(int amount) { account.balance += amount; }\n", "caption": "A valid deposit changes 100 to 125", "duration_ms": 4500 }, { "command": "continue", "output": "\nHardware watchpoint 2: -location account.balance\n\nOld value = 125\nNew value = -999\n0x0000aaaaaaaa15ec in buggy_fee () at /lab/debug_lab.cpp:37\n37\tNOINLINE void buggy_fee() { account.balance = -999; } // Wrong business write.\n", "caption": "The next writer changes 125 to -999", "duration_ms": 4500 }, { "command": "bt 3\nprint account.balance", "output": "#0 0x0000aaaaaaaa15ec in buggy_fee () at /lab/debug_lab.cpp:37\n#1 0x0000aaaaaaaa1608 in watchpoint_case () at /lab/debug_lab.cpp:41\n#2 0x0000aaaaaaaa1960 in main (argc=2, argv=) at /lab/debug_lab.cpp:85\n$1 = -999\n", "caption": "buggy_fee is the responsible writer", "duration_ms": 4500 } ], "checks": [ "hardware watchpoint selected", "legitimate deposit reaches 125", "bad write reaches -999", "stack identifies buggy_fee" ], "status": "passed" }, { "id": "threads", "title": "Thread stacks: diagnose a missing notification", "steps": [ { "command": "break threads_checkpoint\nrun", "output": "Breakpoint 1 at 0x1610: file /lab/debug_lab.cpp, line 56.\n[Thread debugging using libthread_db enabled]\nUsing host libthread_db library \"/lib/aarch64-linux-gnu/libthread_db.so.1\".\n[New Thread 0xfffff7b0f100 (LWP 66)]\n[New Thread 0xfffff72ff100 (LWP 67)]\n\nThread 1 \"debug_lab\" hit Breakpoint 1, threads_checkpoint () at /lab/debug_lab.cpp:56\n56\tNOINLINE void threads_checkpoint() { asm volatile(\"\" ::: \"memory\"); }\n", "caption": "Two workers are waiting; main reaches a known checkpoint", "duration_ms": 4500 }, { "command": "info threads", "output": " Id Target Id Frame \n* 1 Thread 0xfffff7ff1a20 (LWP 64) \"debug_lab\" threads_checkpoint () at /lab/debug_lab.cpp:56\n 2 Thread 0xfffff7b0f100 (LWP 66) \"debug_lab\" 0x0000fffff7c2cba8 in ?? () from /lib/aarch64-linux-gnu/libc.so.6\n 3 Thread 0xfffff72ff100 (LWP 67) \"debug_lab\" 0x0000fffff7c2cba8 in ?? () from /lib/aarch64-linux-gnu/libc.so.6\n", "caption": "Three threads, not just the selected main thread", "duration_ms": 4500 }, { "command": "thread apply all bt 8", "output": "\nThread 3 (Thread 0xfffff72ff100 (LWP 67) \"debug_lab\"):\n#0 0x0000fffff7c2cba8 in ?? () from /lib/aarch64-linux-gnu/libc.so.6\n#1 0x0000fffff7c2f6c4 in pthread_cond_wait () from /lib/aarch64-linux-gnu/libc.so.6\n#2 0x0000aaaaaaaa149c in std::condition_variable::wait > (__p=, __lock=@0xfffff72fe7b8: {_M_device = 0xaaaaaaab3030 , _M_owns = true}, this=0xaaaaaaab3068 ) at /usr/include/c++/11/condition_variable:103\n#3 wait_for_job (worker_id=2) at /lab/debug_lab.cpp:52\n#4 0x0000aaaaaaaa19d0 in std::__invoke_impl (__f=) at /usr/include/c++/11/bits/invoke.h:61\n#5 std::__invoke (__fn=) at /usr/include/c++/11/bits/invoke.h:96\n#6 std::thread::_Invoker >::_M_invoke<0ul, 1ul> (this=) at /usr/include/c++/11/bits/std_thread.h:259\n#7 std::thread::_Invoker >::operator() (this=) at /usr/include/c++/11/bits/std_thread.h:266\n\nThread 2 (Thread 0xfffff7b0f100 (LWP 66) \"debug_lab\"):\n#0 0x0000fffff7c2cba8 in ?? () from /lib/aarch64-linux-gnu/libc.so.6\n#1 0x0000fffff7c2f6c4 in pthread_cond_wait () from /lib/aarch64-linux-gnu/libc.so.6\n#2 0x0000aaaaaaaa149c in std::condition_variable::wait > (__p=, __lock=@0xfffff7b0e7b8: {_M_device = 0xaaaaaaab3030 , _M_owns = true}, this=0xaaaaaaab3068 ) at /usr/include/c++/11/condition_variable:103\n#3 wait_for_job (worker_id=1) at /lab/debug_lab.cpp:52\n#4 0x0000aaaaaaaa19d0 in std::__invoke_impl (__f=) at /usr/include/c++/11/bits/invoke.h:61\n#5 std::__invoke (__fn=) at /usr/include/c++/11/bits/invoke.h:96\n#6 std::thread::_Invoker >::_M_invoke<0ul, 1ul> (this=) at /usr/include/c++/11/bits/std_thread.h:259\n#7 std::thread::_Invoker >::operator() (this=) at /usr/include/c++/11/bits/std_thread.h:266\n\nThread 1 (Thread 0xfffff7ff1a20 (LWP 64) \"debug_lab\"):\n#0 threads_checkpoint () at /lab/debug_lab.cpp:56\n#1 0x0000aaaaaaaa1828 in threads_case () at /lab/debug_lab.cpp:62\n#2 0x0000aaaaaaaa196c in main (argc=2, argv=) at /lab/debug_lab.cpp:86\n", "caption": "Inspect every stack before guessing a deadlock", "duration_ms": 6500 }, { "command": "print waiting_workers\nprint release_workers", "output": "$1 = std::atomic = { 2 }\n$2 = false\n", "caption": "The release predicate is still false", "duration_ms": 4500 } ], "checks": [ "three live threads", "both workers are in wait_for_job", "release_workers is false" ], "status": "passed" }, { "id": "exception", "title": "C++ catchpoint: stop where an exception is thrown", "steps": [ { "command": "catch throw", "output": "Catchpoint 1 (throw)\n", "caption": "Catch the throw before application code swallows it", "duration_ms": 4500 }, { "command": "run", "output": "[Thread debugging using libthread_db enabled]\nUsing host libthread_db library \"/lib/aarch64-linux-gnu/libthread_db.so.1\".\n\nCatchpoint 1 (exception thrown), 0x0000fffff7e32e4c in __cxa_throw () from /lib/aarch64-linux-gnu/libstdc++.so.6\n", "caption": "Stop in the C++ runtime throw path", "duration_ms": 4500 }, { "command": "bt 4\nup\ninfo args", "output": "#0 0x0000fffff7e32e4c in __cxa_throw () from /lib/aarch64-linux-gnu/libstdc++.so.6\n#1 0x0000aaaaaaaa1658 in parse_duration (seconds=seconds@entry=-1) at /lab/debug_lab.cpp:70\n#2 0x0000aaaaaaaa167c in exception_case () at /lab/debug_lab.cpp:75\n#3 0x0000aaaaaaaa1978 in main (argc=2, argv=) at /lab/debug_lab.cpp:87\n#1 0x0000aaaaaaaa1658 in parse_duration (seconds=seconds@entry=-1) at /lab/debug_lab.cpp:70\n70\t if (seconds < 0) throw std::runtime_error(\"negative trajectory duration\");\nseconds = -1\n", "caption": "Select the user frame: seconds is -1", "duration_ms": 4500 }, { "command": "continue", "output": "[Inferior 1 (process 78) exited normally]\n", "caption": "The application catches it and exits normally", "duration_ms": 4500 } ], "checks": [ "catch throw fired", "throw origin contains seconds=-1", "inferior exited" ], "status": "passed" }, { "id": "core", "title": "Core debugging: inspect a crash after the process is gone", "steps": [ { "command": "run", "output": "[Thread debugging using libthread_db enabled]\nUsing host libthread_db library \"/lib/aarch64-linux-gnu/libthread_db.so.1\".\n\nProgram received signal SIGSEGV, Segmentation fault.\nstore_target (target=target@entry=0x0, command=command@entry=47) at /lab/debug_lab.cpp:79\n79\tNOINLINE void store_target(int* target, int command) { *target = command; }\n", "caption": "SIGSEGV: inspect before changing anything", "duration_ms": 4500 }, { "command": "bt 3\ninfo args", "output": "#0 store_target (target=target@entry=0x0, command=command@entry=47) at /lab/debug_lab.cpp:79\n#1 0x0000aaaaaaaa16b8 in core_case () at /lab/debug_lab.cpp:80\n#2 0x0000aaaaaaaa1948 in main (argc=2, argv=) at /lab/debug_lab.cpp:88\ntarget = 0x0\ncommand = 47\n", "caption": "The bad target is null; the command is 47", "duration_ms": 4500 }, { "command": "generate-core-file /work/out/crash.core\nkill", "output": "warning: target file /proc/90/cmdline contained unexpected null characters\nSaved corefile /work/out/crash.core\n[Inferior 1 (process 90) killed]\n", "caption": "Save process memory and terminate this test process", "duration_ms": 4500 }, { "command": "core-file /work/out/crash.core\nbt 3\ninfo args", "output": "[New LWP 90]\n[Thread debugging using libthread_db enabled]\nUsing host libthread_db library \"/lib/aarch64-linux-gnu/libthread_db.so.1\".\nCore was generated by `/work/debug_lab core'.\nProgram terminated with signal SIGSEGV, Segmentation fault.\n#0 store_target (target=target@entry=0x0, command=command@entry=47) at /lab/debug_lab.cpp:79\n79\tNOINLINE void store_target(int* target, int command) { *target = command; }\n#0 store_target (target=target@entry=0x0, command=command@entry=47) at /lab/debug_lab.cpp:79\n#1 0x0000aaaaaaaa16b8 in core_case () at /lab/debug_lab.cpp:80\n#2 0x0000aaaaaaaa1948 in main (argc=2, argv=) at /lab/debug_lab.cpp:88\ntarget = 0x0\ncommand = 47\n", "caption": "Reload the snapshot with the same executable", "duration_ms": 6500 } ], "checks": [ "null target crash", "nonempty core produced", "reloaded core preserves arguments" ], "status": "passed" }, { "id": "python", "title": "GDB Python: a programmable breakpoint filter", "steps": [ { "command": "source /lab/python_breakpoint.py", "output": "Breakpoint 1 at 0x1560: file /lab/debug_lab.cpp, line 18.\n", "caption": "Load a breakpoint class with a Python stop() predicate", "duration_ms": 4500 }, { "command": "run", "output": "[Thread debugging using libthread_db enabled]\nUsing host libthread_db library \"/lib/aarch64-linux-gnu/libthread_db.so.1\".\nPython filter accepted: tick=3, actual=-20\n\nBreakpoint 1, check_output (tick=tick@entry=3, expected=expected@entry=20, actual=-20) at /lab/debug_lab.cpp:18\n18\t asm volatile(\"\" : : \"r\"(tick), \"r\"(expected), \"r\"(actual) : \"memory\");\n", "caption": "Only a negative result is accepted by the Python filter", "duration_ms": 4500 }, { "command": "info args\nbt 3", "output": "tick = 3\nexpected = 20\nactual = -20\n#0 check_output (tick=tick@entry=3, expected=expected@entry=20, actual=-20) at /lab/debug_lab.cpp:18\n#1 0x0000aaaaaaaa15b0 in conditional_case () at /lab/debug_lab.cpp:31\n#2 0x0000aaaaaaaa1954 in main (argc=2, argv=) at /lab/debug_lab.cpp:84\n", "caption": "GDB remains interactive at the selected anomaly", "duration_ms": 4500 } ], "checks": [ "Python predicate accepted bad result", "Python filter stopped at tick 3" ], "status": "passed" }, { "id": "reverse", "title": "Reverse debugging: test support before relying on it", "steps": [ { "command": "break reverse_begin\nbreak reverse_end\nrun\nfinish", "output": "Breakpoint 1 at 0x714: file /lab/reverse_demo.cpp, line 3.\nBreakpoint 2 at 0x71c: file /lab/reverse_demo.cpp, line 4.\n[Thread debugging using libthread_db enabled]\nUsing host libthread_db library \"/lib/aarch64-linux-gnu/libthread_db.so.1\".\n\nBreakpoint 1, reverse_begin () at /lab/reverse_demo.cpp:3\n3\t__attribute__((noinline)) void reverse_begin() { asm volatile(\"\" ::: \"memory\"); }\nmain () at /lab/reverse_demo.cpp:7\n7\t counter = 1;\n", "caption": "Start recording before the two integer writes", "duration_ms": 4500 }, { "command": "record full", "output": "", "caption": "Enable software process recording", "duration_ms": 4500 }, { "command": "continue\nprint counter", "output": "\nBreakpoint 2, reverse_end () at /lab/reverse_demo.cpp:4\n4\t__attribute__((noinline)) void reverse_end() { asm volatile(\"\" ::: \"memory\"); }\n$1 = 2\n", "caption": "Forward execution reaches counter=2", "duration_ms": 4500 }, { "command": "reverse-next\nprint counter", "output": "main () at /lab/reverse_demo.cpp:9\n9\t reverse_end();\n$2 = 2\n", "caption": "Rewind one source step", "duration_ms": 4500 }, { "command": "reverse-next\nprint counter", "output": "8\t counter = 2;\n$3 = 1\n", "caption": "Rewind one source step", "duration_ms": 4500 } ], "checks": [ "forward counter=2", "reverse execution restores counter=1" ], "status": "passed" } ]