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/q246p466-1453-qon9-29so-37105116009q@onlyvoer.pbz
Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
Cc: Biju Das <biju.das.jz@bp.renesas.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Weißschuh <linux@weissschuh.net>
Cc: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Cc: David Laight <david.laight.linux@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/lib/math/div64.c b/lib/math/div64.c
index 5faa29208bdb43fbc6a630a29de07463dfb3aa4e..bf77b9843175e80e038084b26c62ee92b7cd0520 100644 (file)
--- a/lib/math/div64.c
+++ b/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);