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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,
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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
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