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authorJason A. Donenfeld <Jason@zx2c4.com>2022-10-19 23:19:35 -0600
committerJason A. Donenfeld <Jason@zx2c4.com>2022-11-18 02:15:12 +0100
commit7f576b2593a978451416424e75f69ad1e3ae4efe (patch)
treeaec27836d4be841b9aa495e2fb577b06d1f964fb /include/linux/random.h
parente9a688bcb19348862afe30d7c85bc37c4c293471 (diff)
random: add helpers for random numbers with given floor or range
Now that we have get_random_u32_below(), it's nearly trivial to make inline helpers to compute get_random_u32_above() and get_random_u32_inclusive(), which will help clean up open coded loops and manual computations throughout the tree. One snag is that in order to make get_random_u32_inclusive() operate on closed intervals, we have to do some (unlikely) special case handling if get_random_u32_inclusive(0, U32_MAX) is called. The least expensive way of doing this is actually to adjust the slowpath of get_random_u32_below() to have its undefined 0 result just return the output of get_random_u32(). We can make this basically free by calling get_random_u32() before the branch, so that the branch latency gets interleaved. Cc: stable@vger.kernel.org # to ease future backports that use this api Reviewed-by: Kees Cook <keescook@chromium.org> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Diffstat (limited to 'include/linux/random.h')
-rw-r--r--include/linux/random.h25
1 files changed, 25 insertions, 0 deletions
diff --git a/include/linux/random.h b/include/linux/random.h
index 3a82c0a8bc46..bd954ecbef90 100644
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -92,6 +92,31 @@ 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_inclusive(u32 floor, u32 ceil)
+{
+ BUILD_BUG_ON_MSG(__builtin_constant_p(floor) && __builtin_constant_p(ceil) &&
+ (floor > ceil || ceil - floor == U32_MAX),
+ "get_random_u32_inclusive() must take floor <= ceil");
+ return floor + get_random_u32_below(ceil - floor + 1);
+}
+
+/*
* 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.
*/