StringLengthFromUser - NtDoc
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// usermode_accessors.h
SIZE_T StringLengthFromUser(
const CHAR *String
);
View the official Windows Driver Kit DDI referenceNo description available.
The StringLengthFromUser function safely calculates the length of a null-terminated string in user-mode memory.
String[in] A pointer to a null-terminated string in user-mode memory.
The function returns the length of the string in characters, not including the terminating null character.
This function provides safe access to calculate the length of a null-terminated string located in user-mode memory. It is designed for use by kernel-mode code that needs to determine the length of strings passed from user-mode applications.
The function ensures that the string address is a valid user-mode address before performing the length calculation. It raises a structured exception if the operation fails, such as when the string address is not a valid user-mode address or is inaccessible.
Unlike standard string functions, this function includes validation to prevent kernel crashes when accessing potentially invalid user-mode pointers. This makes it essential for secure kernel programming when processing user-mode string data.
This function works on all versions of Windows, not just the latest. You need to consume the latest WDK to get the function declaration from the usermode_accessors.h header. You also need the library (umaccess.lib) from the latest WDK. However, the resulting driver will run fine on older versions of Windows.