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From:   Richard Fitzgerald <rf@opensource.cirrus.com>
To:     <pmladek@suse.com>, <rostedt@goodmis.org>,
        <sergey.senozhatsky@gmail.com>,
        <andriy.shevchenko@linux.intel.com>, <linux@rasmusvillemoes.dk>,
        <w@1wt.eu>, <lkml@sdf.org>, <davem@davemloft.net>,
        <kuba@kernel.org>
CC:     <netdev@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        <patches@opensource.cirrus.com>,
        Richard Fitzgerald <rf@opensource.cirrus.com>
Subject: [PATCH 2/2] random32: Fix implicit truncation warning in prandom_seed_state()
Date:   Mon, 24 May 2021 16:59:41 +0100
Message-ID: <20210524155941.16376-3-rf@opensource.cirrus.com>
In-Reply-To: <20210524155941.16376-1-rf@opensource.cirrus.com>
References: <20210524155941.16376-1-rf@opensource.cirrus.com>
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sparse generates the following warning:

 include/linux/prandom.h:114:45: sparse: sparse: cast truncates bits from
 constant value

This is because the 64-bit seed value is manipulated and then placed in a
u32, causing an implicit cast and truncation. A forced cast to u32 doesn't
prevent this warning, which is reasonable because a typecast doesn't prove
that truncation was expected.

Logical-AND the value with 0xffffffff to make explicit that truncation to
32-bit is intended.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
---
 include/linux/prandom.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/prandom.h b/include/linux/prandom.h
index bbf4b4ad61df..056d31317e49 100644
--- a/include/linux/prandom.h
+++ b/include/linux/prandom.h
@@ -111,7 +111,7 @@ static inline u32 __seed(u32 x, u32 m)
  */
 static inline void prandom_seed_state(struct rnd_state *state, u64 seed)
 {
-	u32 i = (seed >> 32) ^ (seed << 10) ^ seed;
+	u32 i = ((seed >> 32) ^ (seed << 10) ^ seed) & 0xffffffffUL;
 
 	state->s1 = __seed(i,   2U);
 	state->s2 = __seed(i,   8U);
-- 
2.20.1