[PATCH v2 02/11] timekeeping: use ktime_read_raw_clock() for random_get_entropy() if no get_cycles()

From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org,
	tglx@linutronix.de, arnd@arndb.de
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>,
	Theodore Ts'o <tytso@mit.edu>,
	Dominik Brodowski <linux@dominikbrodowski.net>,
	Russell King <linux@armlinux.org.uk>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	"David S . Miller" <davem@davemloft.net>,
	Richard Weinberger <richard@nod.at>,
	Anton Ivanov <anton.ivanov@cambridgegreys.com>,
	Johannes Berg <johannes@sipsolutions.net>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"H . Peter Anvin" <hpa@zytor.com>,
	Chris Zankel <chris@zankel.net>,
	Max Filippov <jcmvbkbc@gmail.com>,
	John Stultz <john.stultz@linaro.org>,
	Stephen Boyd <sboyd@kernel.org>,
	Dinh Nguyen <dinguyen@kernel.org>,
	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
Subject: [PATCH v2 02/11] timekeeping: use ktime_read_raw_clock() for random_get_entropy() if no get_cycles()
Date: Sun, 10 Apr 2022 23:49:42 +0200	[thread overview]
Message-ID: <20220410214951.55294-3-Jason@zx2c4.com> (raw)
In-Reply-To: <20220410214951.55294-1-Jason@zx2c4.com>

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@linutronix.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.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

