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.

The disassembler is augmented with various other tabs to help you make sense of the code. There's a hex viewer, for instance, so you can see the raw binary contents of the file. There's also an excellent pseudo-code generator that will turn the assembler in a function into something where loops and structures are demarcated by curly brackets – it feels a lot like C, where the keywords have been replaced with assembler. Strings, entry points, and imports can be investigated in the same way, and you can search directly for either a flag name or an address. It's still complicated, of course, but even if you have no prior reverse engineering experience, Cutter still helps you get some usable knowledge out of almost any binary, even if it's just a greater understanding of how many subsystems there are in Linux and how many CPU cycles are potentially wasted drawing drop shadows.

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1 Functions: Cutter can detect when a chunk of code is independent and therefore a function. 2 Binary map: See where things are in the binary file you're viewing. 3 Search: Look through the binary for both flags and a specific address. 4 Overview: See the details for where you are and where you're going next. 5 Ring chart: Analysis breaks up the executable into parts; this chart shows how each part is proportionately used. 6 Details info: Create pseudo code, see a visual representation of the current function, and view the hex. 7 Sections: The annotated version of the ring chart. 8 Disassembly: Here's the code being run by your binary.

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