This week at work I spent all week trying to debug a segfault. I’d never done this before, and some of the basic things involved (get a core dump! find the line number that segfaulted!) took me a long time to figure out. So here’s a blog post explaining how to do those things!
At the end of this blog post, you should know how to go from “oh no my program is segfaulting and I have no idea what is happening” to “well I know what its stack / line number was when it segfaulted at at least!“.
* a C++ vtable pointer that got corrupted and is pointing to the wrong place, which causes the program to try to execute some memory that isn’t executable
This “C++ vtable pointer” thing is what was happening to my segfaulting program. I might explain that in a future blog post because I didn’t know any C++ at the beginning of this week and this vtable lookup thing was a new way for a program to segfault that I didn’t know about.
But I wanted also wanted to do a more in-depth investigation and find out more than just what valgrind was telling me! So I wanted to get a core dump and explore it.
When your program segfaults, the Linux kernel will sometimes write a core dump to disk. When I originally tried to get a core dump, I was pretty frustrated for a long time because – Linux wasn’t writing a core dump!! Where was my core dump????
`ulimit -c`sets themaximum size of a core dump. It’s often set to 0, which means that the kernel won’t write core dumps at all. It’s in kilobytes. ulimits areper process– you can see a process’s limits by running`cat /proc/PID/limit`
The kernel uses thesoft limit(in this case, “max core file size = 0”) when deciding how big of a core file to write. You can increase the soft limit up to the hard limit using the`ulimit`shell builtin (`ulimit -c unlimited`!)
Kernel parameters are a way to setglobalsettings on your system. You can get a list of every kernel parameter by running`sysctl -a`, or use`sysctl kernel.core_pattern`to look at the`kernel.core_pattern`setting specifically.
It’s important to know that`kernel.core_pattern`is a global settings – it’s good to be a little careful about changing it because it’s possible that other systems depend on it being set a certain way.
This caused me a lot of confusion (what is this apport thing and what is it doing with my core dumps??) so here’s what I learned about this:
* Ubuntu uses a system called “apport” to report crashes in apt packages
* Setting`kernel.core_pattern=|/usr/share/apport/apport %p %s %c %d %P`means that core dumps will be piped to`apport`
* apport has logs in /var/log/apport.log
* apport by default will ignore crashes from binaries that aren’t part of an Ubuntu packages
I ended up just overriding this Apport business and setting`kernel.core_pattern`to`sysctl -w kernel.core_pattern=/tmp/core-%e.%p.%h.%t`because I was on a dev machine, I didn’t care whether Apport was working on not, and I didn’t feel like trying to convince Apport to give me my core dumps.
### So you have a core dump. Now what?
Okay, now we know about ulimits and`kernel.core_pattern`and you have actually have a core dump file on disk in`/tmp`. Amazing! Now what??? We still don’t know why the program segfaulted!
The next step is to open the core file with`gdb`and get a backtrace.
### Getting a backtrace from gdb
You can open a core file with gdb like this:
```
$ gdb -c my_core_file
```
Next, we want to know what the stack was when the program crashed. Running`bt`at the gdb prompt will give you a backtrace. In my case gdb hadn’t loaded symbols for the binary, so it was just like`??????`. Luckily, loading symbols fixed it.
Here’s how to load debugging symbols.
```
symbol-file /path/to/my/binary
sharedlibrary
```
This loads symbols from the binary and from any shared libraries the binary uses. Once I did that, gdb gave me a beautiful stack trace with line numbers when I ran`bt`!!!
If you want this to work, the binary should be compiled with debugging symbols. Having line numbers in your stack traces is extremely helpful when trying to figure out why a program crashed :)
### look at the stack for every thread
Here’s how to get the stack for every thread in gdb!
```
thread apply all bt full
```
### gdb + core dumps = amazing
If you have a core dump & debugging symbols and gdb, you are in an amazing situation!! You can go up and down the call stack, print out variables, and poke around in memory to see what happened. It’s the best.
If you are still working on being a gdb wizard, you can also just print out the stack trace with`bt`and that’s okay :)
### ASAN
Another path to figuring out your segfault is to do one compile the program with AddressSanitizer (“ASAN”) (`$CC -fsanitize=address`) and run it. I’m not going to discuss that in this post because this is already pretty long and anyway in my case the segfault disappeared with ASAN turned on for some reason, possibly because the ASAN build used a different memory allocator (system malloc instead of tcmalloc).
I might write about ASAN more in the future if I ever get it to work :)
### getting a stack trace from a core dump is pretty approachable!
This blog post sounds like a lot and I was pretty confused when I was doing it but really there aren’t all that many steps to getting a stack trace out of a segfaulting program:
1. try valgrind
if that doesn’t work, or if you want to have a core dump to investigate:
1. make sure the binary is compiled with debugging symbols
2. set`ulimit`and`kernel.core_pattern`correctly
3. run the program
4. open your core dump with`gdb`, load the symbols, and run`bt`
5. try to figure out what happened!!
I was able using gdb to figure out that there was a C++ vtable entry that is pointing to some corrupt memory, which was somewhat helpful and helped me feel like I understood C++ a bit better. Maybe we’ll talk more about how to use gdb to figure things out another day!
