How can DBT handle self-modifying code, dynamic loading, and indirect branches?

Dynamic binary translation (DBT) is a technique that allows a processor to execute code that was originally written for a different architecture. DBT can improve performance, compatibility, and security of software applications, but it also faces some challenges. In this article, we will explore how DBT can handle three common scenarios that require special attention: self-modifying code, dynamic loading, and indirect branches.

1 Self-modifying code

Self-modifying code is code that changes itself during execution, either by writing to its own memory or by generating new code on the fly. Self-modifying code can be used for optimization, obfuscation, or encryption purposes, but it poses a problem for DBT. If the DBT system translates the original code before it is modified, it may end up executing outdated or incorrect instructions. To avoid this, the DBT system needs to detect and invalidate any translated code that is affected by self-modification, and re-translate it when needed. This can be done by using various mechanisms, such as checksums, timestamps, or memory protection.

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Blaine Gaither

Computer Performance Modeling and Analysis
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Simple emulators, that translate instructions one at a time, can detect code modification by simply noting that the write data for an instruction is in a region occupied by code. It also can determine if control is being passed to a Data region. Simple emulators are not fast. More complex emulators for code that is self modifying must contain as list of code regions. If a code region is modified, that region must be recompiled on the fly before use. Transferring control to a data region should result in the Simple emulation of the instructions found in the data region. If emulating a loader where data becomes code, special considerations must taken.

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2 Dynamic loading

Dynamic loading is the process of loading and linking additional code modules into an application at run time, rather than at compile time. Dynamic loading can be used for modularity, extensibility, or plug-in functionality, but it also introduces some complexity for DBT. If the DBT system translates the code modules before they are loaded, it may not know the correct addresses or dependencies of the modules. If the DBT system translates the code modules after they are loaded, it may incur some overhead or latency. To cope with this, the DBT system needs to monitor and update the address space and the symbol table of the application, and resolve any relocation or binding issues.

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3 Indirect branches

Indirect branches are instructions that jump to a target address that is not known at compile time, but is determined at run time by a register or a memory location. Indirect branches can be used for function pointers, virtual methods, or switch statements, but they also create some uncertainty for DBT. If the DBT system translates the code before the target address is known, it may have to generate multiple translations for different possible targets, or use a slow interpreter or a fallback mechanism. If the DBT system translates the code after the target address is known, it may have to perform frequent translations or cache lookups. To optimize this, the DBT system needs to predict and cache the most likely target addresses, and use fast and accurate lookup methods.

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