linux-kernel - [PATCH v2 02/11] timekeeping: use ktime_read_raw_clock() for random_get_entropy() if no get

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Message-Id: <20220410214951.55294-3-Jason@zx2c4.com>
Date:   Sun, 10 Apr 2022 23:49:42 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        tglx@...utronix.de, arnd@...db.de
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Theodore Ts'o <tytso@....edu>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "David S . Miller" <davem@...emloft.net>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin" <hpa@...or.com>, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        John Stultz <john.stultz@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Dinh Nguyen <dinguyen@...nel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-riscv@...ts.infradead.org, sparclinux@...r.kernel.org,
        linux-um@...ts.infradead.org, x86@...nel.org,
        linux-xtensa@...ux-xtensa.org
Subject: [PATCH v2 02/11] timekeeping: use ktime_read_raw_clock() for random_get_entropy() if no get_cycles()

In the event that a given arch does not define get_cycles(), falling
back to the get_cycles() default implementation that returns 0 is really
not the best we can do. Instead, at least calling ktime_read_raw_clock()
would be preferable, because that always needs to return _something_,
even falling back to jiffies eventually. It's not as though
ktime_read_raw_clock() is super high precision or guaranteed to be
entropic, but basically anything that's not zero all the time is better
than returning zero all the time.

Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Theodore Ts'o <tytso@....edu>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 include/linux/timex.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/include/linux/timex.h b/include/linux/timex.h
index 56502b338287..4050f68c34cb 100644
--- a/include/linux/timex.h
+++ b/include/linux/timex.h
@@ -76,8 +76,14 @@ extern u64 ktime_read_raw_clock(void);
  *
  * By default we use get_cycles() for this purpose, but individual
  * architectures may override this in their asm/timex.h header file.
+ * If a given arch does not have get_cycles(), then we fallback to
+ * using sched_clock().
  */
+#ifdef get_cycles
 #define random_get_entropy()	((unsigned long)get_cycles())
+#else
+#define random_get_entropy()	((unsigned long)ktime_read_raw_clock())
+#endif
 #endif
 
 /*
-- 
2.35.1