Guard byte
Encyclopedia
A guard byte is a part of computer program's memory that helps software developers find buffer overflows while developing the program.
is compiled
for debugging
, all memory allocations are prefixed and postfixed by guard bytes. Special memory allocation routines
may then perform additional tasks to determine unwanted read and write attempts outside the allocated memory. These extra bytes help to detect that the program is writing into (or even reading from) inappropriate memory areas, potentially causing buffer overflow
s. In case of accessing these bytes by the program's algorithm, the programmer is warned with information assisting him to locate the problem.
Checking for the inappropriate access to the guard bytes may be done in two ways:
The first way is possible only with a debugger
that handles such breakpoints, but significantly increases the chance of locating the problem. The second way does not require any debuggers or special environments
and can be done even on other computers, but the programmer is alerted about the overflow only upon the deallocation, which is sometimes quite late.
Because guard bytes require additional code to be executed and additional memory to be allocated, they are used only when the program is compiled for debugging. When compiled as a release, guard bytes are not used at all, neither the routines working with them.
of 100 bytes of memory while debugging. The system memory allocating routine
will allocate 108 bytes instead, adding 4 leading and trailing guard bytes, and return
a pointer
shifted by the 4 leading guard bytes to the right, hiding them from the programmer. The programmer should then work with the received pointer without the knowledge of the presence of the guard bytes.
If the programmer's algorithm writes right outside the assigned space, it will overwrite the guard bytes. Later, upon deallocation, the deallocating routine will check, whether the guard bytes are modified and reports an error if appropriate.
Principle
When a programComputer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
is compiled
Compiler
A compiler is a computer program that transforms source code written in a programming language into another computer language...
for debugging
Debugging
Debugging is a methodical process of finding and reducing the number of bugs, or defects, in a computer program or a piece of electronic hardware, thus making it behave as expected. Debugging tends to be harder when various subsystems are tightly coupled, as changes in one may cause bugs to emerge...
, all memory allocations are prefixed and postfixed by guard bytes. Special memory allocation routines
Subroutine
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
may then perform additional tasks to determine unwanted read and write attempts outside the allocated memory. These extra bytes help to detect that the program is writing into (or even reading from) inappropriate memory areas, potentially causing buffer overflow
Buffer overflow
In computer security and programming, a buffer overflow, or buffer overrun, is an anomaly where a program, while writing data to a buffer, overruns the buffer's boundary and overwrites adjacent memory. This is a special case of violation of memory safety....
s. In case of accessing these bytes by the program's algorithm, the programmer is warned with information assisting him to locate the problem.
Checking for the inappropriate access to the guard bytes may be done in two ways:
- by setting a memory breakpointBreakpointIn software development, a breakpoint is an intentional stopping or pausing place in a program, put in place for debugging purposes. It is also sometimes simply referred to as a pause....
on a condition of write and/or read to those bytes, or - by pre-initializing the guard bytes with specific values and checking the values upon deallocation.
The first way is possible only with a debugger
Debugger
A debugger or debugging tool is a computer program that is used to test and debug other programs . The code to be examined might alternatively be running on an instruction set simulator , a technique that allows great power in its ability to halt when specific conditions are encountered but which...
that handles such breakpoints, but significantly increases the chance of locating the problem. The second way does not require any debuggers or special environments
Integrated development environment
An integrated development environment is a software application that provides comprehensive facilities to computer programmers for software development...
and can be done even on other computers, but the programmer is alerted about the overflow only upon the deallocation, which is sometimes quite late.
Because guard bytes require additional code to be executed and additional memory to be allocated, they are used only when the program is compiled for debugging. When compiled as a release, guard bytes are not used at all, neither the routines working with them.
Example
A programmer wants to allocate a bufferBuffer (computer science)
In computer science, a buffer is a region of a physical memory storage used to temporarily hold data while it is being moved from one place to another. Typically, the data is stored in a buffer as it is retrieved from an input device or just before it is sent to an output device...
of 100 bytes of memory while debugging. The system memory allocating routine
Subroutine
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
will allocate 108 bytes instead, adding 4 leading and trailing guard bytes, and return
Return statement
In computer programming, a return statement causes execution to leave the current subroutine and resume at the point in the code immediately after where the subroutine was called, known as its return address. The return address is saved, usually on the process's call stack, as part of the operation...
a pointer
Data pointer
In computer science, a pointer is a programming language data type whose value refers directly to another value stored elsewhere in the computer memory using its address...
shifted by the 4 leading guard bytes to the right, hiding them from the programmer. The programmer should then work with the received pointer without the knowledge of the presence of the guard bytes.
If the programmer's algorithm writes right outside the assigned space, it will overwrite the guard bytes. Later, upon deallocation, the deallocating routine will check, whether the guard bytes are modified and reports an error if appropriate.