Potential fix for code scanning alert no. 1935: Unbounded write

[HaplessIdiot]

Potential fix for https://github.com/supersonic-xserver/ssX/security/code-scanning/1935

In general, to fix unbounded write issues like this, every copy into a heap or stack buffer must be limited by the known size of that buffer, and the destination must always be null-terminated. For string copies, this means replacing strcpy with strncpy or, more robustly, snprintf/strlcpy-style calls that take the buffer size as an argument. The size used must be the same or smaller than what was used when allocating or defining the buffer.

For this specific instance in programs/xdm/auth.c in MakeServerAuthFile, we already allocate d->authFile with malloc(len) where len was computed as strlen(d->clientAuthFile) + 1 when d->clientAuthFile is used. The simplest safe fix is to copy at most len - 1 bytes and explicitly null-terminate. We can do this with strncpy(d->authFile, d->clientAuthFile, len - 1); d->authFile[len - 1] = '\0';. This preserves behavior (the entire string is copied, because len - 1 equals strlen(d->clientAuthFile) here) but makes the sink safe even if the allocation logic is modified later or if len is computed differently. No new headers are needed because <string.h> is already indirectly available in this codebase; if it were not, we would add it.

Concretely:

• In programs/xdm/auth.c, within MakeServerAuthFile, replace the strcpy (d->authFile, d->clientAuthFile); line with a bounded copy using strncpy and forced null-termination, using the existing len variable.
• No other files/snippets need modification to address this particular strcpy sink; the taint-analysis chain through the other files is only to justify untrusted input, not to be changed for this fix.

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