Article from Issue 214/2018

Graham looks at Cutter, Thunar 1.8.0, usbfd, Linuxbrew, Uniread, GSequencer, fkill, and more!

Visual disassembler


Reverse engineering, outside of those who actually do it, has a slightly bad name, because it's often associated with stealing algorithms from proprietary software or enabling game piracy. But it's actually a huge and legitimate area of research, as it enables old software to carry on running, old hardware to be emulated, or old bugs to be circumvented and performance investigated. It's also absolutely fascinating and a great way to learn about how your operating system actually works without the training wheels of an API or hardware abstraction. Unfortunately, the days when you could load a disassembler into your CPU's MMU are gone, and you can no longer realistically splice nop commands into an execution stack.

But there are tools that make the almost infinite complexity of modern operating systems and CPUs manageable, and one of these is the command-line tool radare2. Radare2 will take your Linux binary and turn it into a stream of assembler, letting you work out exactly what happens when a binary is executed. Of course, you need to understand assembler, your hardware, and the various subsystems a binary takes to produce output, but that's where radare2 will try and help, splitting the assembler into functions, libraries, and symbols to help you make better sense of what's happening.

The only problem with radare2 is that it's only slightly less complex than the infinite complexity of modern operating systems and CPUs. Fortunately, radare2 has just found itself with a shiny new GUI called Cutter, to help bring it's 1337 skills to the great unwashed mouse clickers. After launch, you simply select a binary and you're done. Something locally compiled will reveal the most useful information, but you can try anything. Cutter will then spend a little time analyzing the binary, the results of which are shown in a "sections" ring chart that brilliantly shows the components detected within the binary. A similar graphical representation of the binary is also shown as a horizontal bar chart at the top of the window, with each slice showing the use of sequential memory. But the main part of the Qt-configurable window is the disassembler itself, showing the assembly mnemonics, usually helpfully annotated to show loops and jumps, the names of registers, and function names. This is where you can follow the track of execution through your binary. Cutter helps with this, too, as you can click on jump or branch instructions to see where the code runs next.


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