Fix: signed integer overflow when mixing in server seed value.

sysopt.serverseed is a plain int filled across the full positive
range [1, INT_MAX] (the install-time generator uses v % 2147483647
and read_serverseed_file() accepts any val up to INT_MAX), so signed
arithmetic that adds to it can overflow. On a build compiled with
-fsanitize=undefined -fno-sanitize-recover=all this aborts the game
outright; a player hit it at new-game start, where any seed in
(INT_MAX - 8000000, INT_MAX] overflowed. The only real consumer,
coord_hash(), already reads the value as unsigned int, so the
overflow served no functional purpose.

* src/allmain.c: newgame() did sysopt.serverseed += rn2(8000000),
  overflowing int for seeds near INT_MAX and aborting under UBSan at
  new-game start. Compute the sum as unsigned and mask with
  0x7fffffff so the result stays a non-negative int.

* src/shknam.c: the shopkeeper-name seed (int)((long)ubirthday/257L)
  + sysopt.serverseed had the same latent overflow. Promote the
  addition to unsigned and cast back; nseed is used only via % 13
  and % 5 with an existing negative-result guard, so the defined
  wraparound is harmless and the non-overflowing value is unchanged.

Author: k21971

doc/evilhack-changelog.md

@@ -5470,4 +5470,5 @@ The following changes to date are:
 - Fix: aasimar race receiving Angelslayer as a sacrifice gift
 - Gate artifact surge-suppression on weapon skill as well as blessing
 - Aasimar monsters can invoke an aura of light; attribute light deaths
+- Fix: signed integer overflow when mixing in server seed value
 

src/allmain.c

@@ -1250,8 +1250,11 @@ newgame()
     context.next_moloch_offering = 6000; /* give a grace period before
                                           * the first sacrifice */
 
-    /* Extra entropy added to sysopt.serverseed */
-    sysopt.serverseed += rn2(8000000);
+    /* Extra entropy added to sysopt.serverseed; compute the sum as
+       unsigned and mask to a non-negative int so a seed already near
+       INT_MAX cannot overflow */
+    sysopt.serverseed = (int) (((unsigned) sysopt.serverseed
+                                + (unsigned) rn2(8000000)) & 0x7fffffff);
 
     for (i = LOW_PM; i < NUMMONS; i++)
         mvitals[i].mvflags = mons[i].geno & G_NOCORPSE;

src/shknam.c

@@ -514,7 +514,8 @@ const char *const *nlp;
            and restore support which would be necessary for randomization;
            try not to make too many assumptions about time_t's internals;
            use ledger_no rather than depth to keep mine town distinct. */
-        int nseed = (int) ((long) ubirthday / 257L) + sysopt.serverseed;
+        int nseed = (int) ((unsigned) ((long) ubirthday / 257L)
+                           + (unsigned) sysopt.serverseed);
 
         name_wanted = ledger_no(&u.uz) + (nseed % 13) - (nseed % 5);
         if (name_wanted < 0)