What is it? The /proc/.../core_pattern is set to just "core", which will drop a core dump file called "core" in the current directory. I hope anyone searching for gdb examples finds the full output I've shared to be useful, as well as the various caveats I discussed along the way. List gdb command categories: help category: List gdb commands of category: help … cscope is a text-based source code browser from Bell Labs in the 1980's.

But it covers the basics and could serve as a tutorial of sorts, bearing in mind there's a lot more to gdb than I used here. break


As an experiment and to explore a possible workaround, I'll modify memory of the running process to avoid the set_curterm() of zero. *" shows it's a shared library, which might have a man page, website, package description, etc.

For purpose of this tutorial, we will see few commands which are commonly used.

One advantage of using tracers is that they don't pause the target process, like gdb does (although that doesn't matter for this cachetop.py example). It felt clumsy and limited. Syntax: gdb [-help] [-nx] [-q] [-batch] [-cd=dir] [-f] [-b bps] [-tty=dev] [-s symfile] [-e prog] [-se prog] [-c core] [-x cmds] [-d dir] [prog[core|procID]] Example: In next section, you will do your first debugging session and will learn how to use GDB. I'll start with some instruction stepping. I can step through each by running c (short for continue). For example, here's the full listing for breakpoints: This helps to illustrate how many capabilities gdb has, and how few I needed to use in this example. I could also use an external tracer to grab data and stack traces on segfault events. The "??" I'd like a core dump to debug this. I can set a breakpoint on doupdate+289, then single-step through each instruction to see how the registers are set and change. This is libncursesw, and I don't have debug info installed (Ubuntu): Good, those versions match. gdb For the above example with a program named main, the command becomes gdb main Setting Breakpoints. Along with the macros, this code is not that easy to follow... Ok, who actually sets cur_term? Now I'll run gdb with the target program location (using shell substitution, "`", although you should specify the full path unless you're sure that will work), and the core dump file: The last two lines are especially interesting: it tells us it's a segmentation fault in the doupdate() function from the libncursesw library. At this point I double checked that I had the right debug info version, and re-ran the application to segfault it in a live gdb session. d N - Deletes breakpoint number N When I first used gdb (years ago), I really didn't like it. Same place. The answer (perhaps unfortunately) is Here's a reminder: I'd also like to see a stack trace for arg1=0x0 invocation, but this ftrace tool doesn't support stack traces yet. But it gives 1 year as result for any value of balance or target. If that were hundreds of invocations, then I'd use a conditional breakpoint. Ok, so what's CUR? The options are documented in the Linux kernel source, under Documentation/sysctl/kernel.txt. Feature sets vary between debuggers, but gdb may be the most powerful text-based debugger nowadays, with lldb catching up. You will have to execute command on gdb shell. If you're wondering how I knew the %di register was the first argument, then it comes from the AMD64/x86_64 ABI (and the assumption that this compiled library is ABI compliant). This isn't a particularly interesting or exotic issue, it's just a routine gdb debugging session.

The python debug packages have added other capabilities to gdb.

(I'll get back to this in section 15.). That's worth a quick web search in case it's a well-known issue.

"'s, but not hugely more helpful, yet.

Maybe %rax would be interesting, but it's been set to zero by the prior instruction, so we can't see it in the core dump register state. The stack trace looks a bit different: we aren't really in doupdate(), but ClrBlank(), which has been inlined in ClrUpdate(), and inlined in doupdate(). But the commands and procedures I used to debug it were mostly routine: viewing stack traces, checking registers, setting breakpoints, stepping, and browsing source. I'll print the stack trace so that we can see who was setting curterm to 0. So this ends up running "python cachetop.py". GDB is command line utility. Where is set_curterm()? I'm a little frustrated with finding "gdb examples" online that show the commands but not their output. It was added to bcc as a workaround. My hope is that by not executing it, it won't set the global curterm to 0x0.

I've found conditionals don't work on pending breakpoints, at least on this gdb version.

I'll check the state of the registers next. Now we can look at the python backtrace: It's identifying where in our Python code we were executing that hit the segfault. Given this is ncurses, is our TERM environment set to something odd? It's worth showing how this looks in the gdb text user interface (TUI), which I haven't used that much but was inspired after seeing Greg's talk. Before you learn to how to use GDB, it would be good idea to get to know some GDB commands.

(A segfault would be due to a memory dereference, which in C would be a->b or *a, but in this case it's just "back_color_erase", which looks like it's accessing an ordinary variable and not dereferencing memory.). So we're segfaulting on "if (back_color_erase)"? The good news is that this is even possible!

Zero is unlikely a valid address, and this type of segfault is a common software bug: dereferencing an uninitialized or NULL pointer.

Printing register state using i r (short for info registers): Well, %rsi is zero. Looks like wgetch()->wrefresh()->doupdate().
Search for documentation on "adding new GDB commands in Python", as they can be written in Python. Ok, at this breakpoint we can see that set_curterm() is being invoked with a termp=0x0 argument, thanks to debuginfo for that information. You can double check if zero is valid using i proc m (short for info proc mappings): The first valid virtual address is 0x400000. GDB offers a big list of commands, however the following commands are the ones used most frequently: b main - Puts a breakpoint at the beginning of the program. Maybe that isn't being called? I tried setting that to vt100 and running the program, but it hit the same segfault. Using the first option in cscope: I added the highlighting. But at least we've found how cur_term is set: via set_curterm(). cscope -bqR builds the lookup database. Reading these four instructions: it looks like it's pulling something from the stack into %rax, then dereferencing %rax into %rsi, the setting %eax to zero (the xor is an optimization, instead of doing a mov of $0), and then we dereference %rsi with an offset, although we know %rsi is zero. If you have a modern IDE that you prefer, use that instead.

I took a quick look but didn't find a single common cause. The line at which you want the program to temporarily stop is called the breakpoint. Here is program which is buggy in its behavior. I'll step one instruction (si, short for stepi) then inspect registers: Another clue. The bcc trace.py tool should have a switch for printing user stack traces, since the kernel now has BPF stack capabilities as of Linux 4.6, although at the time of writing we haven't added this switch yet. That will be ok if it's only called a few times, but if it's called a few thousand times I'll want a different approach. That doesn't seem possible. However, we've hit another, also with an argument of zero. This section will demonstrate how to use GDB commands by going through example. I should really have used gdb breakpoints on set_curterm() to start with, but I hope that was an interesting detour through ftrace and BPF.

The llvm compiler? Another advantage is that I can trace a few events or a few thousand just as easily. I'll start by disassembling the function we segfaulted in, doupdate(): Output truncated. I could run the program live in gdb to inspect the issue. Another crash. I'll start a python session again, to show this from the beginning: Now I'll set a breakpoint on doupdate as before, but once it's hit, I'll enable recording, then continue the program and let it crash.

WARNING: see previous warning, which also applies here. %rdi is now populated, so those registers look ok to continue. It works by playing back register state from our recording.

I had to highlight in bold the line of code I think is taking effect there. At this point I can reverse-step through lines or instructions. Now I'll rerun gdb and view the stack trace: No more "?? It worked! The last time it was passed zero, which sounds like it could be the problem. For purpose of this tutorial, we will see few commands which are commonly used.

This program is writtten to compute no. (They could have at least capitalized it, as is a common style with #define's.). I'll move back in time two instructions, then print registers: So, back to finding the "cur_term" clue. Anything below that is invalid, and if referenced, will trigger a segmentation fault. While it might look like I've written comprehensive tour of gdb, I really haven't: there's a lot more to gdb.

You also aren't expected to read through all this: I've enumerated each step so you can browse them and find ones of interest. Again, this is just a hacky experiment: The screen goes blank and pauses...then redraws: I'd been posting debugging output to github, especially since the lead BPF engineer, Alexei Starovoitov, is also well versed in llvm internals, and the root cause seemed to be a bug in llvm.

Best case, your application crashes immediately, and you realize your mistake. In that case you'll likely see a single valid frame, then a small number of bogus addresses. (A core dump is a copy of process memory – the name coming from the era of magnetic core memory – and can be investigated using a debugger.).

Searching for back_color_erase definition: Oh, a #define. Great!

I've posted instructions for running it on Ubuntu Xenial. It's complaining about no Python source. That will be ok for now, but I'll show how to set this up for a global location: You can customize that core_pattern further; eg, %h for hostname and %t for time of dump. This tool helps to debug the programs written in C, C++, Ada, Fortran, etc. So, more macros. gdb has improved a lot since then, as have my gdb skills, and I now see it as a powerful modern debugger.

Start running program until a breakpoint or end of program, Set a breakpoint at the begining of function "fun", Set a breakpoint at line number N of source file currently executing, Set a breakpoint at line number N of file "file.c", Continues/Resumes running the program until the next breakpoint or end of program, Runs until the current function is finished, Prints the current value of the variable "var".

gdb is the GNU Debugger, the standard debugger on Linux. First, I need to launch gdb so that we're executing the program live: Now to set the breakpoint using b (short for break): Oops. However, when I tested it, it hit a segfault: Note that it says "Segmentation fault" and not "Segmentation fault (core dumped)".

GDB is command line utility. Maybe I'll post some more gdb sessions when I get a chance, especially for other runtimes like Java. whenever a watched variable’s value is modified. More source code browsing using cscope, this time in llvm. Trying to trace set_curterm() in libtinfo: That works.

(I could also have just typed "disas" and it would have defaulted to doupdate.).
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gdb commands with example

autor: | Říj 14, 2020 | Uncategorized


I'll cover using gdb for this in a moment, but I can't help trying the uprobe tool from my perf-tools collection, which uses Linux ftrace and uprobes. I already can guess what libncursesw is for, but if that were foreign to you, then being under "/lib" and ending in ".so. (And we should get that llvm bug fixed.).

What is it? The /proc/.../core_pattern is set to just "core", which will drop a core dump file called "core" in the current directory. I hope anyone searching for gdb examples finds the full output I've shared to be useful, as well as the various caveats I discussed along the way. List gdb command categories: help category: List gdb commands of category: help … cscope is a text-based source code browser from Bell Labs in the 1980's.

But it covers the basics and could serve as a tutorial of sorts, bearing in mind there's a lot more to gdb than I used here. break


As an experiment and to explore a possible workaround, I'll modify memory of the running process to avoid the set_curterm() of zero. *" shows it's a shared library, which might have a man page, website, package description, etc.

For purpose of this tutorial, we will see few commands which are commonly used.

One advantage of using tracers is that they don't pause the target process, like gdb does (although that doesn't matter for this cachetop.py example). It felt clumsy and limited. Syntax: gdb [-help] [-nx] [-q] [-batch] [-cd=dir] [-f] [-b bps] [-tty=dev] [-s symfile] [-e prog] [-se prog] [-c core] [-x cmds] [-d dir] [prog[core|procID]] Example: In next section, you will do your first debugging session and will learn how to use GDB. I'll start with some instruction stepping. I can step through each by running c (short for continue). For example, here's the full listing for breakpoints: This helps to illustrate how many capabilities gdb has, and how few I needed to use in this example. I could also use an external tracer to grab data and stack traces on segfault events. The "??" I'd like a core dump to debug this. I can set a breakpoint on doupdate+289, then single-step through each instruction to see how the registers are set and change. This is libncursesw, and I don't have debug info installed (Ubuntu): Good, those versions match. gdb For the above example with a program named main, the command becomes gdb main Setting Breakpoints. Along with the macros, this code is not that easy to follow... Ok, who actually sets cur_term? Now I'll run gdb with the target program location (using shell substitution, "`", although you should specify the full path unless you're sure that will work), and the core dump file: The last two lines are especially interesting: it tells us it's a segmentation fault in the doupdate() function from the libncursesw library. At this point I double checked that I had the right debug info version, and re-ran the application to segfault it in a live gdb session. d N - Deletes breakpoint number N When I first used gdb (years ago), I really didn't like it. Same place. The answer (perhaps unfortunately) is Here's a reminder: I'd also like to see a stack trace for arg1=0x0 invocation, but this ftrace tool doesn't support stack traces yet. But it gives 1 year as result for any value of balance or target. If that were hundreds of invocations, then I'd use a conditional breakpoint. Ok, so what's CUR? The options are documented in the Linux kernel source, under Documentation/sysctl/kernel.txt. Feature sets vary between debuggers, but gdb may be the most powerful text-based debugger nowadays, with lldb catching up. You will have to execute command on gdb shell. If you're wondering how I knew the %di register was the first argument, then it comes from the AMD64/x86_64 ABI (and the assumption that this compiled library is ABI compliant). This isn't a particularly interesting or exotic issue, it's just a routine gdb debugging session.

The python debug packages have added other capabilities to gdb.

(I'll get back to this in section 15.). That's worth a quick web search in case it's a well-known issue.

"'s, but not hugely more helpful, yet.

Maybe %rax would be interesting, but it's been set to zero by the prior instruction, so we can't see it in the core dump register state. The stack trace looks a bit different: we aren't really in doupdate(), but ClrBlank(), which has been inlined in ClrUpdate(), and inlined in doupdate(). But the commands and procedures I used to debug it were mostly routine: viewing stack traces, checking registers, setting breakpoints, stepping, and browsing source. I'll print the stack trace so that we can see who was setting curterm to 0. So this ends up running "python cachetop.py". GDB is command line utility. Where is set_curterm()? I'm a little frustrated with finding "gdb examples" online that show the commands but not their output. It was added to bcc as a workaround. My hope is that by not executing it, it won't set the global curterm to 0x0.

I've found conditionals don't work on pending breakpoints, at least on this gdb version.

I'll check the state of the registers next. Now we can look at the python backtrace: It's identifying where in our Python code we were executing that hit the segfault. Given this is ncurses, is our TERM environment set to something odd? It's worth showing how this looks in the gdb text user interface (TUI), which I haven't used that much but was inspired after seeing Greg's talk. Before you learn to how to use GDB, it would be good idea to get to know some GDB commands.

(A segfault would be due to a memory dereference, which in C would be a->b or *a, but in this case it's just "back_color_erase", which looks like it's accessing an ordinary variable and not dereferencing memory.). So we're segfaulting on "if (back_color_erase)"? The good news is that this is even possible!

Zero is unlikely a valid address, and this type of segfault is a common software bug: dereferencing an uninitialized or NULL pointer.

Printing register state using i r (short for info registers): Well, %rsi is zero. Looks like wgetch()->wrefresh()->doupdate().
Search for documentation on "adding new GDB commands in Python", as they can be written in Python. Ok, at this breakpoint we can see that set_curterm() is being invoked with a termp=0x0 argument, thanks to debuginfo for that information. You can double check if zero is valid using i proc m (short for info proc mappings): The first valid virtual address is 0x400000. GDB offers a big list of commands, however the following commands are the ones used most frequently: b main - Puts a breakpoint at the beginning of the program. Maybe that isn't being called? I tried setting that to vt100 and running the program, but it hit the same segfault. Using the first option in cscope: I added the highlighting. But at least we've found how cur_term is set: via set_curterm(). cscope -bqR builds the lookup database. Reading these four instructions: it looks like it's pulling something from the stack into %rax, then dereferencing %rax into %rsi, the setting %eax to zero (the xor is an optimization, instead of doing a mov of $0), and then we dereference %rsi with an offset, although we know %rsi is zero. If you have a modern IDE that you prefer, use that instead.

I took a quick look but didn't find a single common cause. The line at which you want the program to temporarily stop is called the breakpoint. Here is program which is buggy in its behavior. I'll step one instruction (si, short for stepi) then inspect registers: Another clue. The bcc trace.py tool should have a switch for printing user stack traces, since the kernel now has BPF stack capabilities as of Linux 4.6, although at the time of writing we haven't added this switch yet. That will be ok if it's only called a few times, but if it's called a few thousand times I'll want a different approach. That doesn't seem possible. However, we've hit another, also with an argument of zero. This section will demonstrate how to use GDB commands by going through example. I should really have used gdb breakpoints on set_curterm() to start with, but I hope that was an interesting detour through ftrace and BPF.

The llvm compiler? Another advantage is that I can trace a few events or a few thousand just as easily. I'll start by disassembling the function we segfaulted in, doupdate(): Output truncated. I could run the program live in gdb to inspect the issue. Another crash. I'll start a python session again, to show this from the beginning: Now I'll set a breakpoint on doupdate as before, but once it's hit, I'll enable recording, then continue the program and let it crash.

WARNING: see previous warning, which also applies here. %rdi is now populated, so those registers look ok to continue. It works by playing back register state from our recording.

I had to highlight in bold the line of code I think is taking effect there. At this point I can reverse-step through lines or instructions. Now I'll rerun gdb and view the stack trace: No more "?? It worked! The last time it was passed zero, which sounds like it could be the problem. For purpose of this tutorial, we will see few commands which are commonly used.

This program is writtten to compute no. (They could have at least capitalized it, as is a common style with #define's.). I'll move back in time two instructions, then print registers: So, back to finding the "cur_term" clue. Anything below that is invalid, and if referenced, will trigger a segmentation fault. While it might look like I've written comprehensive tour of gdb, I really haven't: there's a lot more to gdb.

You also aren't expected to read through all this: I've enumerated each step so you can browse them and find ones of interest. Again, this is just a hacky experiment: The screen goes blank and pauses...then redraws: I'd been posting debugging output to github, especially since the lead BPF engineer, Alexei Starovoitov, is also well versed in llvm internals, and the root cause seemed to be a bug in llvm.

Best case, your application crashes immediately, and you realize your mistake. In that case you'll likely see a single valid frame, then a small number of bogus addresses. (A core dump is a copy of process memory – the name coming from the era of magnetic core memory – and can be investigated using a debugger.).

Searching for back_color_erase definition: Oh, a #define. Great!

I've posted instructions for running it on Ubuntu Xenial. It's complaining about no Python source. That will be ok for now, but I'll show how to set this up for a global location: You can customize that core_pattern further; eg, %h for hostname and %t for time of dump. This tool helps to debug the programs written in C, C++, Ada, Fortran, etc. So, more macros. gdb has improved a lot since then, as have my gdb skills, and I now see it as a powerful modern debugger.

Start running program until a breakpoint or end of program, Set a breakpoint at the begining of function "fun", Set a breakpoint at line number N of source file currently executing, Set a breakpoint at line number N of file "file.c", Continues/Resumes running the program until the next breakpoint or end of program, Runs until the current function is finished, Prints the current value of the variable "var".

gdb is the GNU Debugger, the standard debugger on Linux. First, I need to launch gdb so that we're executing the program live: Now to set the breakpoint using b (short for break): Oops. However, when I tested it, it hit a segfault: Note that it says "Segmentation fault" and not "Segmentation fault (core dumped)".

GDB is command line utility. Maybe I'll post some more gdb sessions when I get a chance, especially for other runtimes like Java. whenever a watched variable’s value is modified. More source code browsing using cscope, this time in llvm. Trying to trace set_curterm() in libtinfo: That works.

(I could also have just typed "disas" and it would have defaulted to doupdate.).

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