98 lines
3.4 KiB
Markdown
98 lines
3.4 KiB
Markdown
---
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date: 2012-03-29T00:00:00-05:00
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title: "GDB and SystemTap probes -- part 1"
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tags: [gdb, fedora-planet, systemtap, howto, en_us, english, free-software]
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---
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After a long time, here we are again :-).
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With this post I will start to talk about the integration between GDB
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and SystemTap. This is something that [Tom
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Tromey](http://tromey.com/blog/) and I did during the last year. The
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patch is being reviewed as I write this post, and I expect to see it
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checked-in in the next few days/weeks. But let's get our hands dirty...
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SystemTap Userspace Probes
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--------------------------
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You probably use (or have at least heard of) SystemTap, and maybe you
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think the tool is only useful for kernel inspections. If that's your
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case, I have a good news: you're wrong! You can actually use SystemTap
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to inspect userspace applications too, by using what we call **SDT
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probes**, or *S*tatic *D*efined *T*racing probes. This is a very cheap
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and easy way to include probes in your application, and you can even
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specify arguments to those probes.
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In order to use the probes (see an example below), you must include the
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`<sys/sdt.h>` header file in your source code. If you are using Fedora
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systems, you can obtain this header file by installing the package
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`systemtap-sdt-devel`, version equal or greater than `1.4`.
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Here's a simple example of an application with a one-argument probe:
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```c
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#include <sys/sdt.h>
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int
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main (int argc, char *argv[])
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{
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int a = 10;
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STAP_PROBE1 (test_program, my_probe, a);
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return 0;
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}
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```
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As you can see, this is a very simple program with one probe, which
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contains one argument. You can now compile the program:
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```console
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$ gcc test_program.c -o test_program
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```
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Now you must be thinking: "*Wait, wait... Didn't you just forget to link
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this program against some SystemTap-specific library or something?*" And
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my answer is **no**. One of the spetacular things about this
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`<sys/sdt.h>` header is that it does not have any dependencies at all!
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As Tom said in [his blog post](http://tromey.com/blog/?p=687), this is
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"*a virtuoso display of ELF and GCC asm wizardy*".
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If you want to make sure your probe was inserted in the binary, you can
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use `readelf` command:
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```console
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$ readelf -x .note.stapsdt ./test_program
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Hex dump of section '.note.stapsdt':
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0x00000000 08000000 3a000000 03000000 73746170 ....:.......stap
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0x00000010 73647400 86044000 00000000 88054000 sdt...@.......@.
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0x00000020 00000000 00000000 00000000 74657374 ............test
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0x00000030 5f70726f 6772616d 006d795f 70726f62 _program.my_prob
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0x00000040 65002d34 402d3428 25726270 29000000 e.-4@-4(%rbp)...
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```
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*(I will think about writing an explanation on how the probes are laid
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out on the binary, but for now you just have to care if you actually
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see an output from this* `readelf` *command.)*
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You can also use SystemTap to perform this verification:
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```console
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$ stap -L 'process("./test_program").mark("*")'
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process("./test_program").mark("my_probe") $arg1:long
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```
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So far, so good. If you see an output like the one above, it means your
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probe is correctly inserted. You could obviously use SystemTap to
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inspect this probe, but I won't do this right now because this is not
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the purpose of this post.
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For now, we have learned how to:
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1. Include an `SDT probe` in our source code, and compile it;
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2. Verify if the probe was correctly inserted.
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In the next post, I will talk about the GDB support that allows you to
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inspect, print arguments, and gather other information about
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`SDT probes`. I hope you like it!
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