Flexible and Efficient Memory Object Metadata
Compiler-based tools can protect software from attack and find bugs within programs. To support programs written in type-unsafe languages such as C, such tools need to add code into a program that must, at run-time, take a pointer into a memory object and locate metadata for that memory object. Current methods of locating metadata are either flexible (supporting metadata of varying sizes) at the expense of speed and scalability or are fast (e.g., by using shadow tables) at the cost of flexibility (metadata is small and must always be the same size).
This paper presents a new method of attaching metadata to memory objects, named Padding Area MetaData (PAMD), that is both flexible and efficient. Metadata can be any size, and different memory objects can have different sized metadata. While flexible, the algorithm for finding the metadata given a pointer into the memory object takes constant time. Our method extends Baggy Bounds with Accurate Checking (BBAC) which attaches constant-sized metadata to memory objects for performing precise dynamic bounds checks. Our design supports variable-sized metadata, and our implementation supports larger programs.
We evaluated the performance and scalability of PAMD using dynamic bounds checking as an exemplar of our method. Our results show that our method adds at most 33% overhead to an identical dynamic bounds checking tool that trades precision for performance by using a simple shadow table. Our results also show that our method, while having the same flexibility as splay trees, performs significantly faster and scales better as a program allocates more memory.
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