Page History

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The type {{size_t}} generally covers the entire address space.  \[[TR 24731-1|AA. C References#ISO/IEC TR 24731-1-2007]\] introduces a new type {{rsize_t}}, defined to be {{size_t}} but explicitly used to hold the size of a single object.  In code that documents this purpose by using the type {{rsize_t}}, the size of an object can be checked to verify that it is no larger than {{RSIZE_MAX}}, the maximum size of a normal single object, which provides additional input validation for library functions.  See \[[STR00-A. Use TR 24731 for remediation of existing string manipulation code]\] for additional discussion of TR 24731-1.

Any variable that is used to represent the size of an object, including integer values used as sizes, indices, loop counters, and lengths, should be declared as rsize_t if available, otherwise size_t if not.

Non-Compliant Code Example

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Under the same assumption, if {{size_t}} is represented by a greater number of bits than {{int}}, {{that is, {{sizeof(size_t) > sizeof(int)}}, the same behavior occurs for values of {{n <= UINT_MAX}}. For values of {{n > UINT_MAX}}, all of memory within {{\[INT_MIN, INT_MAX\]}} from the beginning of the output buffer areis overwritten in an infinite loop.  This is because the expression {{\++i}} will wrap around to zero before the condition {{i < n}} ever evaluates to false.

Note that in a preemptive multithreaded program, only one thread is in the infinite loop, so it is still significant that out-of-bounds memory is changed.

Therefore, So even under the most restrictive of assumptions, there are serious problems with the program.   Undefined behavior gives license for the implementation to do anything at all, which could be far worse.

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Declaring i to be of type rsize_t eliminates the possible integer overflow condition (in this example).  Also, the argument n is changed to be of type rsize_t to document additional validation in the form of a check against RSIZE_MAX.

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In this non-compliant code example, an integer overflow is specifically checked for by checking if whether length + 1 == 0 (that is, integer wrap has occurred). If the test passes, a wrapper to malloc() is called to allocate the appropriate data block (this is a common idiom). In a program compiled using an ILP32 compiler, this code runs as expected, but in an LP64 environment, an integer overflow can occur , because length is now a 64-bit value. The result of the expression, however, is truncated to 32 - bits when passed as an argument to alloc(), because it takes an unsigned int argument.

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The improper calculation or manipulation of an object's size can result in exploitable vulnerabilities.

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