linux-kernel - [PATCH 4.9 051/245] random32: Fix implicit truncation warning in prandom_seed

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Date:   Mon, 19 Jul 2021 16:49:53 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, kernel test robot <lkp@...el.com>,
        Richard Fitzgerald <rf@...nsource.cirrus.com>,
        Petr Mladek <pmladek@...e.com>, Sasha Levin <sashal@...nel.org>
Subject: [PATCH 4.9 051/245] random32: Fix implicit truncation warning in prandom_seed_state()

From: Richard Fitzgerald <rf@...nsource.cirrus.com>

[ Upstream commit d327ea15a305024ef0085252fa3657bbb1ce25f5 ]

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@...el.com>
Signed-off-by: Richard Fitzgerald <rf@...nsource.cirrus.com>
Reviewed-by: Petr Mladek <pmladek@...e.com>
Signed-off-by: Petr Mladek <pmladek@...e.com>
Link: https://lore.kernel.org/r/20210525122012.6336-3-rf@opensource.cirrus.com
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 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 cc1e71334e53..e20339c78a84 100644
--- a/include/linux/prandom.h
+++ b/include/linux/prandom.h
@@ -93,7 +93,7 @@ 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);
-- 
2.30.2