linux-kernel - C language lawyers needed

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Message-ID: <adaod3ef82e.fsf@cisco.com>
Date:	Wed, 27 Aug 2008 14:43:05 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	linux-kernel@...r.kernel.org
Subject: C language lawyers needed

Can anyone explain why building the current kernel gives

    drivers/net/mlx4/mcg.c: In function 'mlx4_multicast_attach':
    drivers/net/mlx4/mcg.c:217: warning: integer overflow in expression

(with gcc (Ubuntu 4.3.1-9ubuntu1) 4.3.1 on x86-64), while the patch
below gets rid of the warning?  I can't see what is overflowing in the
expression that is warned out.

MGM_BLCK_LB_BIT is 30, and 1 << 30 is not negative or close to
overflowing.  And MGM_QPN_MASK is 0x00FFFFFF, which is also nowhere near
to integer overflow.

A fairly small test case that I don't understand either is:

unsigned foo(int x)
{
	return (((x & 0xffffff) | (1 << 30)) & 0xff000000) >> 24;
}

just running "gcc -c" (ie no extra warnings enabled) on that produces
the same:

    b.c: In function 'foo':
    b.c:3: warning: integer overflow in expression

I'm sure there's some promotion rule or something that makes sense of
this, but it's a mystery to me...

Thanks,
  Roland

diff --git a/drivers/net/mlx4/mcg.c b/drivers/net/mlx4/mcg.c
index c83f88c..6af5c6d 100644
--- a/drivers/net/mlx4/mcg.c
+++ b/drivers/net/mlx4/mcg.c
@@ -215,7 +215,7 @@ int mlx4_multicast_attach(struct mlx4_dev *dev, struct mlx4_qp *qp, u8 gid[16],
 
 	if (block_mcast_loopback)
 		mgm->qp[members_count++] = cpu_to_be32((qp->qpn & MGM_QPN_MASK) |
-						       (1 << MGM_BLCK_LB_BIT));
+						       (1u << MGM_BLCK_LB_BIT));
 	else
 		mgm->qp[members_count++] = cpu_to_be32(qp->qpn & MGM_QPN_MASK);
 
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