linux-kernel - Re: [PATCH v3 03/13] random: invalidate batched entropy after crng init

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Message-ID: <CAHmME9qp5J6yhj+TYtZEhC-snqWB85gdYQ1=-eUysaYT8BkgwQ@mail.gmail.com>
Date:   Wed, 7 Jun 2017 20:16:05 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     kbuild test robot <lkp@...el.com>
Cc:     kbuild-all@...org, "Theodore Ts'o" <tytso@....edu>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-hardening@...ts.openwall.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Miller <davem@...emloft.net>
Subject: Re: [PATCH v3 03/13] random: invalidate batched entropy after crng init

Strange, not all compilers do this warning. Fixing with:

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 12758db..5252690 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -2061,8 +2061,8 @@ static DEFINE_PER_CPU(struct batched_entropy,
batched_entropy_u64);
u64 get_random_u64(void)
{
       u64 ret;
-       bool use_lock = crng_init < 2;
-       unsigned long flags;
+       const bool use_lock = READ_ONCE(crng_init) < 2;
+       unsigned long flags = 0;
       struct batched_entropy *batch;

#if BITS_PER_LONG == 64
@@ -2099,8 +2099,8 @@ static DEFINE_PER_CPU(struct batched_entropy,
batched_entropy_u32);
u32 get_random_u32(void)
{
       u32 ret;
-       bool use_lock = crng_init < 2;
-       unsigned long flags;
+       const bool use_lock = READ_ONCE(crng_init) < 2;
+       unsigned long flags = 0;
       struct batched_entropy *batch;

       if (arch_get_random_int(&ret))

Const, because it's more correct. READ_ONCE to be certain that the
compiler doesn't try to paste the expression into both uses. And
finally flags=0 to shutup the compiler.

If anybody has a better way of approaching this, feel free to chime in.