From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: "Jason A. Donenfeld" Subject: [PATCH RFC v1 05/10] arm: use sched_clock() for random_get_entropy() instead of zero Date: Fri, 8 Apr 2022 20:21:40 +0200 Message-Id: <20220408182145.142506-6-Jason@zx2c4.com> In-Reply-To: <20220408182145.142506-1-Jason@zx2c4.com> References: <20220408182145.142506-1-Jason@zx2c4.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit To: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, arnd@arndb.de Cc: "Jason A. Donenfeld" , Theodore Ts'o , Dominik Brodowski , Russell King , Catalin Marinas , Will Deacon , Geert Uytterhoeven , Thomas Bogendoerfer , Paul Walmsley , Palmer Dabbelt , Albert Ou , "David S . Miller" , Richard Weinberger , Anton Ivanov , Johannes Berg , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H . Peter Anvin" , Chris Zankel , Max Filippov , John Stultz , Stephen Boyd , linux-arm-kernel@lists.infradead.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-riscv@lists.infradead.org, sparclinux@vger.kernel.org, linux-um@lists.infradead.org, x86@kernel.org, linux-xtensa@linux-xtensa.org List-ID: 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. Cc: Arnd Bergmann Cc: Russell King Signed-off-by: Jason A. Donenfeld --- arch/arm/include/asm/timex.h | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/arch/arm/include/asm/timex.h b/arch/arm/include/asm/timex.h index 7c3b3671d6c2..1c51580ee55d 100644 --- a/arch/arm/include/asm/timex.h +++ b/arch/arm/include/asm/timex.h @@ -9,7 +9,18 @@ #ifndef _ASMARM_TIMEX_H #define _ASMARM_TIMEX_H +#include + typedef unsigned long cycles_t; #define get_cycles() ({ cycles_t c; read_current_timer(&c) ? 0 : c; }) +static inline unsigned long random_get_entropy(void) +{ + unsigned long ret = get_cycles(); + if (ret) + return ret; + return sched_clock(); +} +#define random_get_entropy random_get_entropy + #endif -- 2.35.1