StarlingX/Security/Banned C Functions
DRAFT
Guidance
Prohibiting the use of banned functions is a good way to remove a significant number of potential code vulnerabilities from C and C++ code. This list is the compiled library of known bad functions that should be removed to reduce vulnerabilities. It is derived from experience with real-world security bugs and focuses primarily on functions that can lead to buffer overruns (reference: msdn).
Specifically, for Starling X, the main guidelines are that:
- Only functions in the standard C runtime library—libc—are mandated
- Unbounded functions are banned unless specifically noted
- Stack allocation functions are banned unless specifically approved by the project core
There is no requirement to retrofit existing upstream code to meet these guidelines. A summary of the policy is provided below.
| Func | Status |
| strcpy, wcscpy | unbounded, banned; use strncpy |
| strncpy | inspect for unterminated/truncated output |
| strcat, wcscat | unbounded, banned; use strncat |
| strncat | inspect for truncated output |
| sprintf, vsprintf | unbounded, banned; use snprintf, vsnprintf |
| snprintf | inspect for result fitting in buffer: snprintf(buf, size, ...) < size |
| vsnprintf | banned except with approval from core. requires detailed inspection to avoid va_list pitfalls. vsnprint() is typically used for custom logging functionality. Given the flexibility of this function, it is easy to mismatch data types pushed on the stack for a va-list function and types pulled from the stack by the function. The core needs to ensure that the format matches the variables passed to avoid mismatches. |
| strtok | unbounded, banned; use strtok_r or strsep |
| strtok_r, strsep | Inspect for terminated input buffer |
| sscanf, vsscanf | unbounded, banned |
| gets | unbounded, banned, use fgets() instead |
| ato* | banned, use equivalent strto* functions |
| *toa | Non-standard, inspect for output buffer length; prefer snprintf |
| strlen, wcslen | banned except static strings; use strnlen with max length constant |
| memcpy, memmove | allowed |
| alloca | banned except with approval of core. requires detailed inspection to avoid stack overflow |
Color Coding
| Allowed w/Inspection | Banned | Banned w/Exceptions |
