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Fixing DTrace libdir dependency resolution

February 18, 2011

As a part of the work Bryan, Dave, Brendan, and I have been doing to instrument Node, Bryan wrote a translator file. One of the nice things about DTrace is that you can define the arguments to a probe to be a structure and simply access them like a C struct. The way that this all works is via a translator for USDT probes which embeds knowledge of how to look up members of a probes struct. Unfortunately, there is currently a bug in how we handle dependencies for translators, but we also have a fix for that problem. But before I get there, let’s show an example of he use of translators. Let’s say that I want to get the method for every HTTP request that is coming in with node. I can use the following DTrace snippet:

# dtrace -n 'node*:::http-server-request{ printf("%s", args[0]->method); }'
dtrace: description 'node*:::http-server-request' matched 6 probes
0 3705 _ZN4node26DTRACE_HTTP_SERVER_REQUESTERKN2v89ArgumentsE:http-server-request GET
0 3705 _ZN4node26DTRACE_HTTP_SERVER_REQUESTERKN2v89ArgumentsE:http-server-request GET
0 3705 _ZN4node26DTRACE_HTTP_SERVER_REQUESTERKN2v89ArgumentsE:http-server-request GET
0 3705 _ZN4node26DTRACE_HTTP_SERVER_REQUESTERKN2v89ArgumentsE:http-server-request GET
0 3705 _ZN4node26DTRACE_HTTP_SERVER_REQUESTERKN2v89ArgumentsE:http-server-request GET
0 3705 _ZN4node26DTRACE_HTTP_SERVER_REQUESTERKN2v89ArgumentsE:http-server-request GET
0 3705 _ZN4node26DTRACE_HTTP_SERVER_REQUESTERKN2v89ArgumentsE:http-server-request GET
0 3705 _ZN4node26DTRACE_HTTP_SERVER_REQUESTERKN2v89ArgumentsE:http-server-request GET

The DTrace probes themselves specify a specific argument type. If that translator argument is a struct, we can define an operation to transform the probe argument and get out a specific value. Generally this involves a lot of use of copyin and copyinstr and is quite messy o write, but once written makes all scripts that leverage it much more powerful. These translators is that you can express the dependencies between them. In this case, the Node translator relies on the procfs translator. Thus for this to work, you need to have procfs.d which ships with SunOS.

Now, the big question to ask is how does DTrace know where to look for these translators. By default DTrace looks in /usr/lib/dtrace. However, you can also tell DTrace to look in an additional location by using the -Lpath flag to dtrace(1M). So if you wanted to keep your node.d provider in say /opt/rm/lib/dtrace, we could run the above command as:

# dtrace -L/opt/rm/lib/dtrace -n 'node*:::http-server-request{ printf("%s", args[0]->method); }'

Now, unfortunately there is a bug in this; however, I have a fix for it. Without this fix when trying to resolve library dependencies, DTrace only looks for the file in the same directory. So it would try and look for the dependency procfs.d in /opt/rm/lib/dtrace/procfs.d. However, we don’t have it there, we have it in /usr/lib/dtrace.

Here is a patch against Illumos that fixes this issue. Please note that the patch there is released under the CDDL and copyrighted by Joyent. It causes us to search our entire library path to try and find the first occurrence of a file with the name we’re looking for. So in our case above, we will find that proc.d is in /usr/lib/dtrace and resolve the dependency correctly. Hopefully we’ll see this fix make it upstream into Illumos before too long, as well as, Mac OS X and FreeBSD.

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