OPP Scripting

La Brea allows you to inject scripts into an existing application without recompiling.

Initially, this was built to slow down specific parts of an application programatically. It's great for simulating a slow network, disk, or other types of things within a process.

Implemented Injection Points

The following patterns are available for injection:

before_fname

Invoked before the function fname is called. The arguments to be given to fname are provided.

These functions should not return values.

after_fname

Invoked after the function fname has completed. Both the return value and the original arguments are provided (in that order). This is invoked before returning control back to the application, but you have little opportuntity to do anything particularly destructive here.

These functions should not return values.

around_fname

Invoked instead of fname. This allows for ridiculous magic. Since it's a bit complex, I'll cover it in more detail here.

First, the arguments are as follows:

1. A reference to the original function.
2. Any additional arguments required by that function.

The original function can be invoked using the invoke function in lua and that function can pass along whatever parameters may be interesting. For example, if you'd like to truncate all reads so that the application never reads more than 64 bytes, you can do the following:

function around_read(f, d, buf, size)
    if size > 64 then
        size = 64
    end
    return labrea.invoke(f, d, buf, size)
end

Alternatively, you can just cause an error as shown in the random errors example.

Usage

First, write a script to slow stuff down. For example, if you'd like to slow random seeks down, you can create a script that looks like the following:

function before_lseek(fd, offset, whence)
   if math.random(1, 100) == 13 then
      io.write(string.format("Slowing down a seek on fd=%d to %d (%d)\n",
                             fd, offset, whence))
      labrea.usleep(1000000)
   end
end

Save that in a file, say, /tmp/slowseek.lua and then use the labrea as follows:

labrea /tmp/slowseek.lua myprogram -and -its -arguments