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Open Source and information security mailing list archives
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Date: Fri, 7 Oct 2022 19:28:59 -0600 From: "Jason A. Donenfeld" <Jason@...c4.com> To: "Darrick J. Wong" <djwong@...nel.org> Cc: linux-kernel@...r.kernel.org, patches@...ts.linux.dev, Andreas Noever <andreas.noever@...il.com>, Andrew Morton <akpm@...ux-foundation.org>, Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, Borislav Petkov <bp@...en8.de>, Catalin Marinas <catalin.marinas@....com>, Christoph Böhmwalder <christoph.boehmwalder@...bit.com>, Christoph Hellwig <hch@....de>, Christophe Leroy <christophe.leroy@...roup.eu>, Daniel Borkmann <daniel@...earbox.net>, Dave Airlie <airlied@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>, "David S . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Florian Westphal <fw@...len.de>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "H . Peter Anvin" <hpa@...or.com>, Heiko Carstens <hca@...ux.ibm.com>, Helge Deller <deller@....de>, Herbert Xu <herbert@...dor.apana.org.au>, Huacai Chen <chenhuacai@...nel.org>, Hugh Dickins <hughd@...gle.com>, Jakub Kicinski <kuba@...nel.org>, "James E . J . Bottomley" <jejb@...ux.ibm.com>, Jan Kara <jack@...e.com>, Jason Gunthorpe <jgg@...pe.ca>, Jens Axboe <axboe@...nel.dk>, Johannes Berg <johannes@...solutions.net>, Jonathan Corbet <corbet@....net>, Jozsef Kadlecsik <kadlec@...filter.org>, KP Singh <kpsingh@...nel.org>, Kees Cook <keescook@...omium.org>, Marco Elver <elver@...gle.com>, Mauro Carvalho Chehab <mchehab@...nel.org>, Michael Ellerman <mpe@...erman.id.au>, Pablo Neira Ayuso <pablo@...filter.org>, Paolo Abeni <pabeni@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Richard Weinberger <richard@....at>, Russell King <linux@...linux.org.uk>, Theodore Ts'o <tytso@....edu>, Thomas Bogendoerfer <tsbogend@...ha.franken.de>, Thomas Gleixner <tglx@...utronix.de>, Thomas Graf <tgraf@...g.ch>, Ulf Hansson <ulf.hansson@...aro.org>, Vignesh Raghavendra <vigneshr@...com>, WANG Xuerui <kernel@...0n.name>, Will Deacon <will@...nel.org>, Yury Norov <yury.norov@...il.com>, dri-devel@...ts.freedesktop.org, kasan-dev@...glegroups.com, kernel-janitors@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, linux-block@...r.kernel.org, linux-crypto@...r.kernel.org, linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-media@...r.kernel.org, linux-mips@...r.kernel.org, linux-mm@...ck.org, linux-mmc@...r.kernel.org, linux-mtd@...ts.infradead.org, linux-nvme@...ts.infradead.org, linux-parisc@...r.kernel.org, linux-rdma@...r.kernel.org, linux-s390@...r.kernel.org, linux-um@...ts.infradead.org, linux-usb@...r.kernel.org, linux-wireless@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, loongarch@...ts.linux.dev, netdev@...r.kernel.org, sparclinux@...r.kernel.org, x86@...nel.org, Jan Kara <jack@...e.cz> Subject: Re: [PATCH v4 2/6] treewide: use prandom_u32_max() when possible On Fri, Oct 07, 2022 at 02:17:22PM -0700, Darrick J. Wong wrote: > On Fri, Oct 07, 2022 at 12:01:03PM -0600, Jason A. Donenfeld wrote: > > Rather than incurring a division or requesting too many random bytes for > > the given range, use the prandom_u32_max() function, which only takes > > the minimum required bytes from the RNG and avoids divisions. > > > > Reviewed-by: Kees Cook <keescook@...omium.org> > > Reviewed-by: KP Singh <kpsingh@...nel.org> > > Reviewed-by: Christoph Böhmwalder <christoph.boehmwalder@...bit.com> # for drbd > > Reviewed-by: Jan Kara <jack@...e.cz> # for ext2, ext4, and sbitmap > > Signed-off-by: Jason A. Donenfeld <Jason@...c4.com> > > --- > > <snip, skip to the xfs part> > > > diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c > > index e2bdf089c0a3..6261599bb389 100644 > > --- a/fs/xfs/libxfs/xfs_alloc.c > > +++ b/fs/xfs/libxfs/xfs_alloc.c > > @@ -1520,7 +1520,7 @@ xfs_alloc_ag_vextent_lastblock( > > > > #ifdef DEBUG > > /* Randomly don't execute the first algorithm. */ > > - if (prandom_u32() & 1) > > + if (prandom_u32_max(2)) > > I wonder if these usecases (picking 0 or 1 randomly) ought to have a > trivial wrapper to make it more obvious that we want boolean semantics: > > static inline bool prandom_bool(void) > { > return prandom_u32_max(2); > } > > if (prandom_bool()) > use_crazy_algorithm(...); > Yea, I've had a lot of similar ideas there. Part of doing this (initial) patchset is to get an intuitive sense of what's actually used and how often. On my list for investigation are a get_random_u32_max() to return uniform numbers by rejection sampling (prandom_u32_max() doesn't do that uniformly) and adding a function for booleans or bits < 8. Possible ideas for the latter include: bool get_random_bool(void); bool get_random_bool(unsigned int probability); bool get_random_bits(u8 bits_less_than_eight); With the core of all of those involving the same batching as the current get_random_u{8,16,32,64}() functions, but also buffering the latest byte and managing how many bits are left in it that haven't been shifted out yet. So API-wise, there are a few ways to go, so hopefully this series will start to give a good picture of what's needed. One thing I've noticed is that most of the prandom_u32_max(2) invocations are in debug or test code, so that doesn't need to be optimized. But kfence does that too in its hot path, so a get_random_bool() function there would in theory lead to an 8x speed-up. But I guess I just have to try some things and see. Anyway, that is a long way to say, I share you curiosity on the matter and I'm looking into it. Jason
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