[PATCH 0/1] MR8256: include: vsnprintf should always write a null terminator.

[Yuxuan Shui (＠yshui)]

Microsoft's documentation says:
> The `vsnprintf` function always writes a null terminator, even if it truncates the output. When you use `_vsnprintf` and `_vsnwprintf`, the buffer is null-terminated only if there's room at the end (that is, if the number of characters to write is less than count).

For `_UCRT` and `_MSVCR_VER >= 140`, `vsnprintf` calls `__stdio_common_vsprintf` with `_CRT_INTERNAL_PRINTF_STANDARD_SNPRINTF_BEHAVIOR`, and everything works, a null byte is appended, everything is fine.

If not, `vsnprintf` calls `_vsnprintf` directly and no null byte will be added if the buffer is not big enough. This happens in, for example, kernel32. And this breaks `wine_dbg_vsprintf`, which passes result of `vsnprintf` to `strlen` and goes out-of-bound.

In msvcrt.spec, we also have `vsnprintf` as an alias of `_vsnprintf`, I didn't touch that since I don't know if that's deliberate. But this alias _is_ used (by e.g. winecrt0, i think?), if this one is also wrong then it might break other things.

[PATCH 1/1] include: vsnprintf should always write a null terminator.

[Yuxuan Shui]

From: Yuxuan Shui

---
 include/msvcrt/stdio.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/msvcrt/stdio.h b/include/msvcrt/stdio.h
index c402a9a299e..44c2b816c60 100644
--- a/include/msvcrt/stdio.h
+++ b/include/msvcrt/stdio.h
@@ -458,7 +458,11 @@ _ACRTIMP int __cdecl vsprintf_s(char*,size_t,const char*,va_list) __WINE_CRT_PRI
 _ACRTIMP int __cdecl _vsnprintf(char*,size_t,const char*,va_list) __WINE_CRT_PRINTF_ATTR(3, 0);
 static inline int vsnprintf(char *buffer, size_t size, const char *format, va_list args) __WINE_CRT_PRINTF_ATTR(3, 0);
 static inline int vsnprintf(char *buffer, size_t size, const char *format, va_list args)
-{ return _vsnprintf(buffer,size,format,args); }
+{
+    int r = _vsnprintf(buffer,size,format,args);
+    if (r>=size) buffer[size-1] = 0;
+    return r;
+}

_ACRTIMP int __cdecl _snscanf_l(const char*,size_t,const char*,_locale_t,...) __WINE_CRT_SCANF_ATTR(3, 5);
 _ACRTIMP int __cdecl _sscanf_l(const char *,const char*,_locale_t,...) __WINE_CRT_SCANF_ATTR(2, 4);

[Paul Gofman (＠gofman)]

> Microsoft's documentation says:

that should be backed by a test probably.

[Jinoh Kang (＠iamahuman)]

Jinoh Kang (@iamahuman) commented about include/msvcrt/stdio.h:
> _ACRTIMP int __cdecl _vsnprintf(char*,size_t,const char*,va_list) __WINE_CRT_PRINTF_ATTR(3, 0);
>  static inline int vsnprintf(char *buffer, size_t size, const char *format, va_list args) __WINE_CRT_PRINTF_ATTR(3, 0);
>  static inline int vsnprintf(char *buffer, size_t size, const char *format, va_list args)
> -{ return _vsnprintf(buffer,size,format,args); }
> +{
> - int r = _vsnprintf(buffer,size,format,args);
> - if (r>=size) buffer[size-1] = 0;

Missing check `size >= 1`