From 75541aa42b073a5f34a7c54eb10989cb81ae23c8 Mon Sep 17 00:00:00 2001
From: Peter Hutterer <peter.hutterer@who-t.net>
Date: Fri, 17 Apr 2026 10:12:31 +1000
Subject: [PATCH] os/log: handle NULL string argument in vpnprintf

Since this function is called from signal handlers (e.g. OsSigHandler
processing SIGSEGV/SIGBUS), a NULL %s argument triggers a recursive fault.

Assisted-by: Claude:claude-claude-opus-4-6
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2200>
---
 os/log.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/os/log.c b/os/log.c
index c0ebf1fff..21b52a94a 100644
--- a/os/log.c
+++ b/os/log.c
@@ -418,7 +418,7 @@ vpnprintf(char *string, int size_in, const char *f, va_list args)
     int f_idx = 0;
     int s_idx = 0;
     int f_len = strlen_sigsafe(f);
-    char *string_arg;
+    const char *string_arg;
     char number[21];
     int p_len;
     int i;
@@ -474,6 +474,8 @@ vpnprintf(char *string, int size_in, const char *f, va_list args)
         switch (f[f_idx]) {
         case 's':
             string_arg = va_arg(args, char*);
+            if (!string_arg)
+                string_arg = "(null)";
 
             for (i = 0; string_arg[i] != 0 && s_idx < size - 1 && s_idx < precision; i++)
                 string[s_idx++] = string_arg[i];