From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: Thomas Gleixner Subject: Re: [PATCH v2 03/11] m68k: use ktime_read_raw_clock() for random_get_entropy() instead of zero In-Reply-To: <20220410214951.55294-4-Jason@zx2c4.com> References: <20220410214951.55294-1-Jason@zx2c4.com> <20220410214951.55294-4-Jason@zx2c4.com> Date: Mon, 11 Apr 2022 10:18:30 +0200 Message-ID: <87sfqkf2y1.ffs@tglx> MIME-Version: 1.0 Content-Type: text/plain To: "Jason A. Donenfeld" , 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 , Ingo Molnar , Borislav Petkov , Dave Hansen , "H . Peter Anvin , Chris Zankel , Max Filippov" , John Stultz , Stephen Boyd , Dinh Nguyen , 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: On Sun, Apr 10 2022 at 23:49, Jason A. Donenfeld wrote: > 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 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 > Cc: Arnd Bergmann > Cc: Geert Uytterhoeven > Signed-off-by: Jason A. Donenfeld > --- > arch/m68k/include/asm/timex.h | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/m68k/include/asm/timex.h b/arch/m68k/include/asm/timex.h > index 6a21d9358280..5351b10e1b18 100644 > --- a/arch/m68k/include/asm/timex.h > +++ b/arch/m68k/include/asm/timex.h > @@ -35,7 +35,7 @@ static inline unsigned long random_get_entropy(void) > { > if (mach_random_get_entropy) > return mach_random_get_entropy(); > - return 0; > + return ktime_read_raw_clock(); I'd rather do something like this in a common header: unsigned long random_get_entropy_fallback(void); and use random_get_entropy_fallback() in the architecture specific files. That way you can encapsulate the fallback implementation in the random code and if it turns out that ktime_read_raw_clock() is a stupid idea or someone has a better idea then you have to change exactly one place and not patch the whole tree again. Thanks, tglx