Linux Applications Debugging Techniques/The debugger

Preparations Someday some hard to reproduce issue will be found on a production machine. Typically, such a machine is difficult to access, has no development environment and nothing can be installed on it. At most it will have gdb, but very likely not. Typically also, the heaviest user will be the one to find the issue and typically still, the heaviest user is the one being the bigger -- money-wise. And that issue has to be root cause diagnosed and fixed. Unless the application has been prepared for this forensic gathering moment, not much can be done to diagnose where the issue is. Thus, preparations should start with compilation: Have a "symbol server" and make sure it is reacheable. Compile on the symbol server. Compile the release with debugging symbols. Strip them if you do not want to ship them but keep them. Ship gdbserver with the application for remote debugging. These preparations will allow one to: Debug the application running on any machine, including machines where there is no gdb installed. Debug from any machine that has network visibility to the symbol server. Also, think beforehand how would you debug on the machine: Embed a breakpoint in the code, at hard of reach places of interest, then Start the application Attach to it with the debugger Wait until the breakpoint is hit The "symbol server" One way to easily reach the right code from within the debugger is to build the binaries within an auto-mounted folder, each build in its own sub-folder. The same auto-mount share should be accessible from the machine you are debugging on. Debian Install autofs In /etc/auto.master uncomment the line: /net -hosts In /etc/exports export the folder: # Build directory: rw for localhost, read-only for any other host /home/amelinte/projects/lpt/lpt localhost(rw,sync,no_subtree_check) *(ro,root_squash,no_subtree_check) As root: restart autofs & nfs and export the build share: /etc/init.d/autofs restart /etc/init.d/nfs-kernel-server restart /usr/sbin/exportfs -a Redhat Export the folder: edit /etc/exports As root (RedHat): service autofs start Finally, build the binaries within the automounted directory on the build machine (here the build machine is bear): cd /net/bear/home/amelinte/projects/lpt/lpt.destructor make clean make Notice the filename path that is resolved with the symbol information: [0x413464] lpt::stack::call_stack<40ul>::call_stack(bool)+0x52 At /net/bear/home/amelinte/projects/lpt/lpt.destructor/lpt/include/lpt/call_stack.hpp:74 In binaries/bin/stacktest If the symbols have been stripped from the binaries, point gdb to the folders where the symbols are with the debug-file-directory directive. References Debugging Information in Separate Files RedHat debuginfod symbol server gdb 10.1 See also debuginfod Source Code To point the debugger to the source files: (gdb) set substitute-path /from/path1 /to/path1 (gdb) set substitute-path /from/path2 /to/path2 Remote debugging On the machine where the application runs (appmachine): If gdbserver is not present, copy it over. Start the application. Start gdbserver: gdbserver gdbmachine:2345 --attach program On gdbmachine: At the gdb prompt, enter: target remote appmachine:2345 Sometimes you may have to tunnel over ssh: On gdbmachine: ssh -L 5432:appmachine:2345 user@appmachine At the gdb prompt: target remote localhost:5432 References GDB Tunneling Attaching to a process Find out the PID of the process, then: (gdb) attach 1045 Attaching to process 1045 Reading symbols from /usr/lib64/firefox-3.0.18/firefox...(no debugging symbols found)...done. Reading symbols from /lib64/libpthread.so.0...(no debugging symbols found)...done. [Thread debugging using libthread_db enabled] [New Thread 0x448b4940 (LWP 1063)] [New Thread 0x428b0940 (LWP 1054)] .... (gdb) detach Debugging programs that spawn multiple children set detach-on-fork off see "all-stop" vs "non-stop" modes in the GDB documentation and their related settings Embedding breakpoints in the source On x86 platforms: #define EMBEDDED_BREAKPOINT asm volatile ("int3;") Or a more elaborate one: #define EMBEDDED_BREAKPOINT \ asm("0:" \ ".pushsection embed-breakpoints;" \ ".quad 0b;" \ ".popsection;") This will break into the debugger on hard to reach conditions: if (cpuload > 3.1416 && started > 111*MINS*AGO && whatever-dynamic-condition) { EMBEDDED_BREAKPOINT; } References http://mainisusuallyafunction.blogspot.com/2012/01/embedding-gdb-breakpoints-in-c-source.html Execute commands on breakpoint b bigFoot commands bt continue end Break on each function call set logging on set confirm off rbreak . Data breakpoints (watchpoints) Watchpoints can be implemented either in software either in hardware if the CPU supports it. Typically on an Intel processor there are eight debug registers out of which only four can be used for hardware breakpoints and this limits the number of watchpoints system wide. Install a condition on a hardware watchpoint so that only accesses are stopped that increase the value of the variable: watch -location i set var $prev = i command $bpnum if $prev < i printf "increase prev %d %d \n", $prev, i set var $prev = i else printf "decrease prev %d %d \n", $prev, i set var $prev = i continue end end Breakpoints conditional on caller This requires gdb 7.9 or later, configured with python support: (gdb) break funcA if $_caller_is("funcB") The text user interface thumb|GDB TUI GDB features a text user interface for code, disassembler and registers. For instance: Ctrl-x 1 will show the code pane Ctrl-x a will hide the TUI panes None of the GUI interfaces to gdb (Qt Creator stands out for being intuitive and easy to use) can offer access to all of the gdb functionality. curses gdb offers an improved TUI. A comprehensive list of debugger GUIs is available here. Reverse debugging As an example, reverse debugging is a functionality no GUI offers access to: (gdb) l 1 /* 1 */ int var; 2 /* 2 */ int main (void) { 3 /* 3 */ int i; for (i=0;i<100;i++) 4 /* 4 */ var=i/10; 5 /* 5 */ return 0; } (gdb) start (gdb) record (gdb) adv 5 main () at history.c:5 5 /* 5 */ return 0; } (gdb) watch var Hardware watchpoint 2: var (gdb) reverse-continue Continuing. Hardware watchpoint 2: var Old value = 9 New value = 8 0x00000000004004c3 in main () at history.c:4 4 /* 4 */ var=i/10; (gdb) p i $1 = 90 (gdb) reverse-continue Continuing. Hardware watchpoint 2: var Old value = 8 New value = 7 0x00000000004004c3 in main () at history.c:4 4 /* 4 */ var=i/10; (gdb) p i $2 = 80 See also rr & gdbgui Register watch You can watch registers. Note this will force the debugger to single step the debugged program and it will run very slowly: (gdb) watch $ebp References http://sourceware.org/gdb/onlinedocs/gdb/TUI.html http://sourceware.org/gdb/news/reversible.html .gdbinit As a note, in upcoming gdb releases, .gdbinit will be replaced by gdb-gdb.gdb: gdb-gdb.gdb ^ ^ ^ | | |---- It's a gdb script. | | If it were Python this would be .py. | | | --------- "-gdb" is a gdb convention, it's the suffix added to a file | for auxiliary support. | E.g., gdb will auto-load libstdc++.so-gdb.py (version elided) | which contains the std c++ pretty-printers. | ------------- This init script is for the program named "gdb". If this were for readelf the script would be named readelf-gdb.gdb. C++ support Canned gdb macros gdb STL support STL macros (and more) Mangling gdb might need a bit of guidance with C++11 binaries: (gdb) set cp-abi gnu-v3 (gdb) set language c++ (gdb) maintenance demangle _ZNSt16nested_exceptionD2Ev std::nested_exception::~nested_exception() Templates One can eventually use templight to debug and profile templates. Getting the subclass from the base class pointer (gdb) python print(x.type) SuperClass * (gdb) python print(x.dynamic_type) SubClass * References GDB Architecture Overview gdb & lib debugging support https://sourceware.org/gdb/onlinedocs/gdb/JIT-Interface.html https://sourceware.org/gdb/current/onlinedocs/gdb/Unwinding-Frames-in-Python.html drgn debugger Python-based frontend gdb in depth A collection of gdb Python extensions pl:Programowanie_w_systemie_UNIX/gdb