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One of the most important things to remember in reverse engineering is a core difference between static analysis and dynamic analysis. As many already know, static analysis suffers from the path explosion problem, which is impossible to solve even in the most basic way without at least a partial emulation.
Thus many professional reverse engineering tools use code emulation while performing an analysis of binary code, and radare2 is no difference here.
For partial emulation (or imprecise full emulation) radare2 uses its own intermediate language and virtual machine.
Radare2 supports this kind of partial emulation for all platforms that implement ESIL uplifting (x86/x86_64, ARM, arm64, MIPS, powerpc, sparc, AVR, 8051, Gameboy, ...).
One of the most common usages of such emulation is to calculate indirect jumps and conditional jumps.
To see the ESIL representation of the program one can use the ao command or enable the asm.esil configuration variable, to check if the program uplifted correctly, and to grasp how ESIL works:
To manually setup the ESIL imprecise emulation you need to run this command sequence:
aei to initialize ESIL VM
aeim to initialize ESIL VM memory (stack)
aeip to set the initial ESIL VM IP (instruction pointer)
a sequence of aer commands to set the initial register values.
While performing emulation, please remember, that ESIL VM cannot emulate external calls or system calls, along with SIMD instructions. Thus the most common scenario is to emulate only a small chunk of the code, like encryption/decryption, unpacking or calculating something.
After we successfully set up the ESIL VM we can interact with it like with a usual debugging mode. Commands interface for ESIL VM is almost identical to the debugging one:
aes to step (or s key in visual mode)
aesi to step over the function calls
aesu <address> to step until some specified address
aesue <ESIL expression> to step until some specified ESIL expression met
aec to continue until break (Ctrl-C), this one is rarely used though, due to the omnipresence
of external calls
aecu <address> to continue until some specified address
In visual mode, all of the debugging hotkeys will work also in ESIL emulation mode.
Along with usual emulation, there is a possibility to record and replay mode:
aets to list all current ESIL R&R sessions
aets+ to create a new one
aesb to step back in the current ESIL R&R session
Apart from the manual emulation mode, it can be used automatically in the analysis loop. For example, the aaaa command performs the ESIL emulation stage along with others. To disable or enable its usage you can use anal.esil configuration variable. There is one more important option, though setting it might be quite dangerous, especially in the case of malware - emu.write which allows ESIL VM to modify memory. Sometimes it is required though, especially in the process of deobfuscating or unpacking code.
To show the process of emulation you can set asm.emu variable, which will show calculated register and memory values in disassembly comments:
Note here likely comments, which indicates that ESIL emulation predicted for particular conditional jump to happen.
Apart from the basic ESIL VM setup, you can change the behavior with other options located in emu. and esil. configuration namespaces.
For manipulating ESIL working with memory and stack you can use the following options:
esil.stack to enable or disable temporary stack for asm.emu mode
esil.stack.addr to set stack address in ESIL VM (like aeim command)
esil.stack.size to set stack size in ESIL VM (like aeim command)
esil.stack.depth limits the number of PUSH operations into the stack
esil.romem specifies read-only access to the ESIL memory
esil.fillstack and esil.stack.pattern allows you to use a various pattern for filling ESIL VM
stack upon initialization
esil.nonull when set stops ESIL execution upon NULL pointer read or write.
More about this operation mode you can read in chapter.