linux-kernel - [PATCH v2 1/7] random: Simplify API for random address requests

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Message-Id: <20160730154244.403-2-jason@lakedaemon.net>
Date:	Sat, 30 Jul 2016 15:42:38 +0000
From:	Jason Cooper <jason@...edaemon.net>
To:	william.c.roberts@...el.com, Yann Droneaud <ydroneaud@...eya.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	kernel-hardening@...ts.openwall.com
Cc:	linux@....linux.org.uk, akpm@...ux-foundation.org,
	keescook@...omium.org, tytso@....edu, arnd@...db.de,
	gregkh@...uxfoundation.org, catalin.marinas@....com,
	will.deacon@....com, ralf@...ux-mips.org, benh@...nel.crashing.org,
	paulus@...ba.org, mpe@...erman.id.au, davem@...emloft.net,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, viro@...iv.linux.org.uk, nnk@...gle.com,
	jeffv@...gle.com, dcashman@...roid.com,
	Jason Cooper <jason@...edaemon.net>
Subject: [PATCH v2 1/7] random: Simplify API for random address requests

To date, all callers of randomize_range() have set the length to 0, and
check for a zero return value.  For the current callers, the only way
to get zero returned is if end <= start.  Since they are all adding a
constant to the start address, this is unnecessary.

We can remove a bunch of needless checks by simplifying the API to do
just what everyone wants, return an address between [start, start +
range).

While we're here, s/get_random_int/get_random_long/.  No current call
site is adversely affected by get_random_int(), since all current range
requests are < UINT_MAX.  However, we should match caller expectations
to avoid coming up short (ha!) in the future.

All current callers to randomize_range() chose to use the start address
if randomize_range() failed.  Therefore, we simplify things by just
returning the start address on error.

randomize_range() will be removed once all callers have been converted
over to randomize_addr().

Signed-off-by: Jason Cooper <jason@...edaemon.net>
---
Changes from v1:
 - Explicitly mention page_aligned start assumption (Yann Droneaud)
 - pick random pages vice random addresses (Yann Droneaud)
 - catch range=0 last

 drivers/char/random.c  | 28 ++++++++++++++++++++++++++++
 include/linux/random.h |  1 +
 2 files changed, 29 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 0158d3bff7e5..3bedf69546d6 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -1840,6 +1840,34 @@ randomize_range(unsigned long start, unsigned long end, unsigned long len)
 	return PAGE_ALIGN(get_random_int() % range + start);
 }
 
+/**
+ * randomize_addr - Generate a random, page aligned address
+ * @start:	The smallest acceptable address the caller will take.
+ * @range:	The size of the area, starting at @start, within which the
+ *		random address must fall.
+ *
+ * If @start + @range would overflow, @range is capped.
+ *
+ * NOTE: Historical use of randomize_range, which this replaces, presumed that
+ * @start was already page aligned.  This assumption still holds.
+ *
+ * Return: A page aligned address within [start, start + range).  On error,
+ * @start is returned.
+ */
+unsigned long
+randomize_addr(unsigned long start, unsigned long range)
+{
+	if (start > ULONG_MAX - range)
+		range = ULONG_MAX - start;
+
+	range >>= PAGE_SHIFT;
+
+	if (range == 0)
+		return start;
+
+	return start + (get_random_long() % range << PAGE_SHIFT);
+}
+
 /* Interface for in-kernel drivers of true hardware RNGs.
  * Those devices may produce endless random bits and will be throttled
  * when our pool is full.
diff --git a/include/linux/random.h b/include/linux/random.h
index e47e533742b5..f1ca2fa4c071 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -35,6 +35,7 @@ extern const struct file_operations random_fops, urandom_fops;
 unsigned int get_random_int(void);
 unsigned long get_random_long(void);
 unsigned long randomize_range(unsigned long start, unsigned long end, unsigned long len);
+unsigned long randomize_addr(unsigned long start, unsigned long range);
 
 u32 prandom_u32(void);
 void prandom_bytes(void *buf, size_t nbytes);
-- 
2.9.2