mul_u64_u64_div_u64: fix the division-by-zero behavior

The current implementation forces a compile-time 1/0 division, which
generates an undefined instruction (ud2 on x86) rather than a proper
runtime division-by-zero exception.

Change to trigger an actual div-by-0 exception at runtime, consistent with
other division operations.  Use a non-1 dividend to prevent the compiler
from optimizing the division into a comparison.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Nicolas Pitre <[email protected]>
Cc: Biju Das <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Weißschuh <[email protected]>
Cc: Uwe Kleine-König <[email protected]>
Cc: David Laight <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Nicolas Pitre

lib/math/div64.c

@@ -212,12 +212,13 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
 
 #endif
 
-	/* make sure c is not zero, trigger exception otherwise */
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wdiv-by-zero"
-	if (unlikely(c == 0))
-		return 1/0;
-#pragma GCC diagnostic pop
+	/* make sure c is not zero, trigger runtime exception otherwise */
+	if (unlikely(c == 0)) {
+		unsigned long zero = 0;
+
+		OPTIMIZER_HIDE_VAR(zero);
+		return ~0UL/zero;
+	}
 
 	int shift = __builtin_ctzll(c);