linux-kernel - [PATCH v1 3/5] random: add helpers for random numbers with given floor or range

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Date:   Fri, 21 Oct 2022 21:44:01 -0400
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     linux-kernel@...r.kernel.org
Cc:     "Jason A. Donenfeld" <Jason@...c4.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Christoph Böhmwalder 
        <christoph.boehmwalder@...bit.com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>,
        Richard Weinberger <richard@....at>,
        "Darrick J . Wong" <djwong@...nel.org>,
        SeongJae Park <sj@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Helge Deller <deller@....de>, netdev@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-media@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-mmc@...r.kernel.org, linux-parisc@...r.kernel.org
Subject: [PATCH v1 3/5] random: add helpers for random numbers with given floor or range

Now that we have get_random_u32_below(), it's trivial to make inline
helpers to compute get_random_u32_above() and get_random_u32_between(),
which will help clean up open coded loops and manual computations
throughout the tree.

Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
 include/linux/random.h | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/include/linux/random.h b/include/linux/random.h
index 3a82c0a8bc46..92188a74e50e 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -91,6 +91,30 @@ static inline u32 get_random_u32_below(u32 ceil)
 	}
 }
 
+/*
+ * Returns a random integer in the interval (floor, U32_MAX], with uniform
+ * distribution, suitable for all uses. Fastest when floor is a constant, but
+ * still fast for variable floor as well.
+ */
+static inline u32 get_random_u32_above(u32 floor)
+{
+	BUILD_BUG_ON_MSG(__builtin_constant_p(floor) && floor == U32_MAX,
+			 "get_random_u32_above() must take floor < U32_MAX");
+	return floor + 1 + get_random_u32_below(U32_MAX - floor);
+}
+
+/*
+ * Returns a random integer in the interval [floor, ceil), with uniform
+ * distribution, suitable for all uses. Fastest when floor and ceil are
+ * constant, but still fast for variable floor and ceil as well.
+ */
+static inline u32 get_random_u32_between(u32 floor, u32 ceil)
+{
+	BUILD_BUG_ON_MSG(__builtin_constant_p(floor) && __builtin_constant_p(ceil) &&
+			 floor >= ceil, "get_random_u32_above() must take floor < ceil");
+	return floor + get_random_u32_below(ceil - floor);
+}
+
 /*
  * On 64-bit architectures, protect against non-terminated C string overflows
  * by zeroing out the first byte of the canary; this leaves 56 bits of entropy.
-- 
2.38.1