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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id je12-20020a170903264c00b00176d347e9a7sm11219825plb.233.2022.10.05.21.16.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 05 Oct 2022 21:16:51 -0700 (PDT) Date: Wed, 5 Oct 2022 21:16:50 -0700 From: Kees Cook To: "Jason A. Donenfeld" Cc: linux-kernel@vger.kernel.org, Ajay Singh , Akinobu Mita , Alexandre Torgue , Amitkumar Karwar , Andreas Dilger , Andreas =?iso-8859-1?Q?F=E4rber?= , Andreas Noever , Andrew Lunn , Andrew Morton , Andrii Nakryiko , Andy Gospodarek , Andy Lutomirski , Andy Shevchenko , Anil S Keshavamurthy , Anna Schumaker , Arend van Spriel , Ayush Sawal , Borislav Petkov , Chao Yu , Christoph =?iso-8859-1?Q?B=F6hmwalder?= , Christoph Hellwig , Christophe Leroy , Chuck Lever , Claudiu Beznea , Cong Wang , Dan Williams , Daniel Borkmann , "Darrick J . Wong" , Dave Hansen , David Ahern , "David S . 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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. Yes please! Since this is a treewide patch, it's helpful for (me at least) doing reviews to detail the mechanism of the transformation. e.g. I imagine this could be done with something like Coccinelle and @no_modulo@ expression E; @@ - (prandom_u32() % (E)) + prandom_u32_max(E) > diff --git a/drivers/mtd/ubi/debug.h b/drivers/mtd/ubi/debug.h > index 118248a5d7d4..4236c799a47c 100644 > --- a/drivers/mtd/ubi/debug.h > +++ b/drivers/mtd/ubi/debug.h > @@ -73,7 +73,7 @@ static inline int ubi_dbg_is_bgt_disabled(const struct ubi_device *ubi) > static inline int ubi_dbg_is_bitflip(const struct ubi_device *ubi) > { > if (ubi->dbg.emulate_bitflips) > - return !(prandom_u32() % 200); > + return !(prandom_u32_max(200)); > return 0; > } > Because some looks automated (why the parens?) > @@ -393,14 +387,11 @@ static struct test_driver { > > static void shuffle_array(int *arr, int n) > { > - unsigned int rnd; > int i, j; > > for (i = n - 1; i > 0; i--) { > - rnd = prandom_u32(); > - > /* Cut the range. */ > - j = rnd % i; > + j = prandom_u32_max(i); > > /* Swap indexes. */ > swap(arr[i], arr[j]); And some by hand. :) Reviewed-by: Kees Cook -- Kees Cook