In
computing, shadow memory is a technique used to track and store information on
computer memory
In computing, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and digital electronic devices. The term ''memory'' is often synonymous with the term ''primary storage ...
used by a
program during its execution. Shadow memory consists of shadow bytes that map to individual bits or one or more bytes in main memory. These shadow bytes are typically invisible to the original program and are used to record information about the original piece of data.
Technique
The technique is utilized by memory-error checkers that can store information on which parts of memory have been allocated to the program being checked. This shadow memory is then used for detecting and reporting incorrect accesses of memory, even though the program may not be crashing due to a
segmentation fault
In computing, a segmentation fault (often shortened to segfault) or access violation is a fault, or failure condition, raised by hardware with memory protection, notifying an operating system (OS) the software has attempted to access a restricte ...
or similar. An error checker may also store additional information on memory such as which bits have been defined and which ones do not.
Memcheck
Valgrind () is a programming tool for memory debugging, memory leak detection, and profiling.
Valgrind was originally designed to be a free memory debugging tool for Linux on x86, but has since evolved to become a generic framework for creatin ...
, part of the
Valgrind
Valgrind () is a programming tool for memory debugging, memory leak detection, and profiling.
Valgrind was originally designed to be a free memory debugging tool for Linux on x86, but has since evolved to become a generic framework for creatin ...
suite, uses this to detect undefined behavior resulting from acting on or printing undefined memory values.
Use of shadow memory is however not limited to memory-error checkers, as what information is stored in these shadow bytes is not fixed. It is for instance used by
ThreadSanitizer, a
data race detector.
Shadow memory can be both implemented and used a lot of different ways, and have different performance characteristics. Memcheck for instance tracks values with
bit
The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represented ...
precision, while
AddressSanitizer, part of the
clang
Clang is a compiler front end for the C, C++, Objective-C, and Objective-C++ programming languages, as well as the OpenMP, OpenCL, RenderScript, CUDA, and HIP frameworks. It acts as a drop-in replacement for the GNU Compiler Collection (GCC) ...
compiler, is comparatively very fast. Memcheck, like all Valgrind tools, uses
binary translation
In computing, binary translation is a form of binary recompilation where sequences of instructions are translated from a ''source'' instruction set to the ''target'' instruction set. In some cases such as instruction set simulation, the target ...
and
instrumentation
Instrumentation a collective term for measuring instruments that are used for indicating, measuring and recording physical quantities. The term has its origins in the art and science of scientific instrument-making.
Instrumentation can refer to ...
to run code manipulating the shadow memory corresponding to program memory use. AddressSanitizer on the other hand is created on compile-time and inserts error-checking code inline into a program during compilation. Its shadow-memory implementation uses a huge reservation of virtual memory for its shadow memory, giving very different performance characteristics.
References
General
*{{Cite book, first1=N. , last1=Nethercote , first2=J. , last2=Seward , s2cid=10263496 , year=2007 , title=How to shadow every byte of memory used by a program. In Proceedings of the 3rd international Conference on Virtual Execution Environments (San Diego, California, USA) , publication-date=June 13–15, 2007 , work=VEE '07 , publisher=ACM New York , pages=65–74 , doi=10.1145/1254810.1254820, chapter=How to shadow every byte of memory used by a program , isbn=9781595936301 , citeseerx=10.1.1.643.7117
* http://research.google.com/pubs/pub37752.html
Memory management