Skip to main content

Dive into a go program memory with GDB

In a previous blog post, I took a look at how to enumerate all the syscalls and even their arguments using tools such as eBPF. That left me pondering and craving to learn more about how memory is mapped and what do simple variables look like in the memory. What is behind all those memory addresses you can see in the stack traces?

I do have an intuitive sense of that. Sure, I have seen blog posts and talks about the topic, taken a look at heap dumps in a hunt for memory leaks but I wonder does it make any sense to look at the memory in a language/runtime agnostic manner. Probably not, but hey, could be exciting.

To find out, I created a simple program that simply prints out the contents of a few variables 

I try to make the outputs depend on the runtime environment to avoid any unexpected compiler optimizations. I want to make sure the memory will be allocated at runtime.

I run the code in my trusty Digitalocean VM with "no hang-up" and attach the GNU Project debugger (GDB) while the program is asleep to take a core dump. It seems to be one of the easiest ways to read the program state from memory. Sure, I could use no tools to copy and parse the virtual memory files under /proc/PID/maps yet GDB is more straightforward and is not really anything that runtime specific. It's no pprof.

While the gomemory app is waiting in the background, I run GDB to obtain a core dump. The go program outputs the variables and their addresses so I can try to find those from the core dump

I can examine the core file with the GDB CLI tools or alternatively I could hexdump it. The CLI way is easier though and I can reliably find the memory contents. I don't have to wrap my head around any endianness confusion.

List of files inside the core dump and the executed binary

I can access a specific memory address with the X <address> command. The go program output shows the addresses so it is easy to see their contents. Let's take a look at the memory locations which the program printed to the console.

Memory address and its contents

Cool! At first glance, the first variable does not make any sense, so it must be a reference to some other location. The same goes for the second one which was supposed to contain a string /home/juho/memexp. 1a044 is no sensible ASCII code, it is not a text string so once again it is a pointer to another location. The last one is the actual contents of the variable. 51 in hex is 81 in decimal so the memory location indeed contains the random integer generated by the program.

So what are those referenced addresses? I can use GDB to inspect the memory addresses and even read a range of addresses but I decided, after all, to take a hex dump from the virtual memory map. I ran the app again and took a hex dump of the address space file under /proc/PID/maps from the 0xc0 to 0xc4 range. There we can find the 1a040 address.


So it refers to the PWD env var, interesting!

How about that second one?


Indeed that referenced address contains the printed string. It looks like it is not in the stack frame memory but resides in the heap. The go compiler must have decided it is supposed to be there.

What did this exercise help me understand? Well, firstly I would not try to debug any real-life go application like this. If the only thing I had from a go app is a core dump, I would look into something like delve. That was something I knew already though. Secondly, it is pretty easy actually to read virtual memory. After all, everything is just a file.



Labels:

Comments

Post a Comment

Popular posts from this blog

I'm not a passionate developer

A family friend of mine is an airlane pilot. A dream job for most, right? As a child, I certainly thought so. Now that I can have grown-up talks with him, I have discovered a more accurate description of his profession. He says that the truth about the job is that it is boring. To me, that is not that surprising. Airplanes are cool and all, but when you are in the middle of the Atlantic sitting next to the colleague you have been talking to past five years, how stimulating can that be? When he says the job is boring, it is not a bad kind of boring. It is a very specific boring. The "boring" you would want as a passenger. Uneventful.  Yet, he loves his job. According to him, an experienced pilot is most pleased when each and every tiny thing in the flight plan - goes according to plan. Passengers in the cabin of an expert pilot sit in the comfort of not even noticing who is flying. As someone employed in a field where being boring is not exactly in high demand, this sounds pro...
Post a Comment
Read more

Bird is causing high CPU on my macOS

There is no lack of people complaining about MacOS Tahoe, mostly about rounded corners and inconsistent design decisions. I have not paid that much attentention to that, but there is one mac bug i have paid attention to. It hasn't been a visual or ux but rather few system processes pegging the CPU. trustd , alongside with ecosystemd and ecosystemanalyticsd all reported high CPU usage. I can't exactly recall when this started, it might have predated my Tahoe upgrade but anyway, the trio of processes all had high CPU usage. Sure, it may have been the virtual efficiency cores and whether it affeced battery life or slowed down other processes i don't know to be hon...
Post a Comment
Read more

Ousterhout's law

Back in the day, everyone was using Winamp. It is a music player with the user interface of a mixing console. The bloody thing had an equalizer on the front page! As an amateur music producer, I know that EQ is a powerful tool but making changes that sound good is difficult, to say the least. Mixing engineers spend considerable effort with the artist to balance the frequency ranges to arrive at the desired musical outcome. I bet the 15-year-old me butchered a lot of songs with the thing. Now we are using Spotify, a player with basically a search bar and a play button. I ran into something called Ousterhout's Law on the  Operating Systems: Three Easy Pieces book . Here is a quote from the book TIP: AVOID VOO-DOO CONSTANTS (OUSTERHOUT’S LAW) Avoiding voo-doo constants is a good idea whenever possible. Unfortunately, as in the example above, it is often difficult. One could try to make the system learn a good value, but that too is not straightforward. The frequent result: a configura...
Post a Comment
Read more