Before discussing why strlcpy and strlcat are considered unsafe, it is essential to understand their functionality and purpose. These functions were designed to address buffer overflow issues inherent in standard C string manipulation functions like strcpy and strcat. They attempt to copy or concatenate strings while ensuring the resulting string is valid by appending a null terminator at the end of the destination buffer.
However, despite offering a certain level of safety compared to strcpy and strcat, strlcpy and strlcat are still considered unsafe for the following reasons:
-
Truncation Issues: strlcpy and strlcat accept an additional parameter to limit the number of characters copied or concatenated, which specifies the destination buffer size. If the source string exceeds this limit, the function truncates the source string at the end of the destination buffer. This truncation may cause data loss or logical errors in the program, particularly when other components expect a complete string.
Example: Suppose a buffer for storing a file path has a size limit of 256 bytes. If strlcpy is used to copy a path longer than 255 bytes into this buffer, the path will be truncated, potentially resulting in an invalid file path or incorrect file references.
-
Incorrect Buffer Size Handling: When using strlcpy and strlcat, developers must accurately know and correctly pass the destination buffer size. If an incorrect size is passed due to errors or oversight, even these safety-focused functions can cause buffer overflows or data truncation.
Example: If a developer mistakenly sets the destination buffer size smaller than the actual size—for instance, by passing a value smaller than the true buffer size as the size parameter to strlcat—the function may write beyond the buffer boundary during string concatenation, triggering a buffer overflow.
-
Misunderstanding of Safety: Some developers mistakenly believe that using strlcpy and strlcat completely eliminates all string-related security risks. This misconception can lead to over-reliance on these functions while neglecting more robust security practices, such as advanced data handling techniques or thorough input validation.
In summary, while strlcpy and strlcat are safer than strcpy and strcat, they cannot fully prevent all string operation-related security issues, including data truncation and incorrect buffer size usage. Correct and safe usage requires developers to thoroughly understand the data they process and carefully handle boundary conditions and buffer sizes.