linux-kernel - [PATCH RFC v1 07/10] arm64: use sched_clock() for random_get

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Date:   Fri,  8 Apr 2022 20:21:42 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        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>,
        Thomas Gleixner <tglx@...utronix.de>,
        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>,
        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 RFC v1 07/10] arm64: use sched_clock() for random_get_entropy() instead of zero

In the event that random_get_entropy() can't access a cycle counter or
similar, falling back to returning 0 is really not the best we can do.
Instead, at least calling sched_clock() would be preferable, because
that always needs to return _something_, even falling back to jiffies
eventually. It's not as though sched_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.

If CONFIG_ARM_ARCH_TIMER=n, then get_cycles() will return 0, so we only
need the fallback code for that case.

Cc: Arnd Bergmann <arnd@...db.de>
Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Will Deacon <will@...nel.org>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 arch/arm64/include/asm/timex.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/arm64/include/asm/timex.h b/arch/arm64/include/asm/timex.h
index cf59ce91b22d..bfebd2e1ce45 100644
--- a/arch/arm64/include/asm/timex.h
+++ b/arch/arm64/include/asm/timex.h
@@ -13,6 +13,15 @@
  */
 #define get_cycles()	arch_timer_read_counter()
 
+#ifndef CONFIG_ARM_ARCH_TIMER
+/*
+ * The default implementation of random_get_entropy() calls get_cycles(),
+ * which will return 0 if CONFIG_ARM_ARCH_TIMER=n, so we fall back to
+ * sched_clock() here. Not a great situation, but better than nothing.
+ */
+#define random_get_entropy() ((unsigned long)sched_clock())
+#endif
+
 #include <asm-generic/timex.h>
 
 #endif
-- 
2.35.1