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NtString

📢 PUBLIC Page 1033:3/3 | edit | chip 2018-04-15 13:20:16
Tags: KernelMode

August 22 2017

I have grown to really hate UNICODE_STRING. It is the source of endless problems because it does some very bad things:

  • The text terminator (null) is not required for the text in Buffer[]. The text is sometimes null terminated.
  • The Length and MaximumLength counts are in bytes while the Buffer[] array is in characters.
  • Length and MaximumLength do not include room for a terminator.

This means the text in Buffer[] cannot be used in any C-string functions without first allocating a new buffer to accommodate an extra terminating character, then copying the text just to add the terminator.

Whoever defined UNICODE_STRING saved a byte while wasting lifetimes in man-hours. Everything would be compatible if only UNICODE_STRINGS were required to include a terminator byte.

After being bitten by this too many times, I decided to take a day and write a wrapper class to manage UNICODE_STRING so that Buffer[] would always be a well-terminated C-string.

NtString interface: class NtString { public: NtString(void); NtString(const char *pSrc); NtString(const WCHAR *pSrc); NtString(const ANSI_STRING &Src); NtString(const UNICODE_STRING &Src); NtString(bool IsArgList, const char *Fmt, ...); NtString(bool IsArgList, const WCHAR *Fmt, ...); ~NtString(void); NTSTATUS Set(const WCHAR *pSrc); NTSTATUS Set(const char *pSrc); void Truncate(UINT nChr); const WCHAR *GetText(UINT nChr=0); UNICODE_STRING &GetUnicode(void); //SHOULD BE CONST, but it causes too much trouble. const WCHAR *Print(const char *Fmt, ...); const WCHAR *Print(const WCHAR *Fmt, ...); const WCHAR *PrintV(const char *Fmt, va_list ArgList); const WCHAR *PrintV(const WCHAR *Fmt, va_list ArgList); private: class ntString *pObj; BYTE Obj[64]; };

A couple notes:

  • The NtString can be constructed using either ansi or Unicode text, but it is always stored internally as Unicode.
  • The arguments to Print() are always assumed to be Unicode even if the format text is ansi.
  • The Buffer for an empty NtString (never been appended) points to a static well-terminated 0-length C-string.
  • The contents of the UNICODE_STRING returned by GetUnicode() should never be modified. GetUnicode() should really return a const reference, but that forces the user to inevitably cast away the const attribute in order to use it.
  • Storage for the text is allocated using MemAlloc().
  • The Print() functions use my own formatter, which has some subtle differences from the RtlString... functions.
  • NtString borrows code from IString in the user-mode Support library, so it behaves similarly.

Print

  • The decimal formats can be prefixed with ',' (comma) to decorate the number with commas. Ie: "1,234,567".
  • The 'p' format will print only the lower 32bits of a 64bit pointer unless prefixed with 'h'.
  • No floating point formats in the kernel version. (yet).
  • The 'S' format will print ansi text as Unicode.


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