diff options
author | Jason A. Donenfeld <Jason@zx2c4.com> | 2022-10-20 08:19:35 +0300 |
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committer | Jason A. Donenfeld <Jason@zx2c4.com> | 2022-11-18 04:15:12 +0300 |
commit | 7f576b2593a978451416424e75f69ad1e3ae4efe (patch) | |
tree | aec27836d4be841b9aa495e2fb577b06d1f964fb /drivers | |
parent | e9a688bcb19348862afe30d7c85bc37c4c293471 (diff) | |
download | linux-7f576b2593a978451416424e75f69ad1e3ae4efe.tar.xz |
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 'drivers')
-rw-r--r-- | drivers/char/random.c | 18 |
1 files changed, 17 insertions, 1 deletions
diff --git a/drivers/char/random.c b/drivers/char/random.c index 6f323344d0b9..f5868dddbb61 100644 --- a/drivers/char/random.c +++ b/drivers/char/random.c @@ -161,6 +161,8 @@ EXPORT_SYMBOL(wait_for_random_bytes); * u16 get_random_u16() * u32 get_random_u32() * u32 get_random_u32_below(u32 ceil) + * u32 get_random_u32_above(u32 floor) + * u32 get_random_u32_inclusive(u32 floor, u32 ceil) * u64 get_random_u64() * unsigned long get_random_long() * @@ -522,7 +524,21 @@ u32 __get_random_u32_below(u32 ceil) * of `-ceil % ceil` is analogous to `2^32 % ceil`, but is computable * in 32-bits. */ - u64 mult = (u64)ceil * get_random_u32(); + u32 rand = get_random_u32(); + u64 mult; + + /* + * This function is technically undefined for ceil == 0, and in fact + * for the non-underscored constant version in the header, we build bug + * on that. But for the non-constant case, it's convenient to have that + * evaluate to being a straight call to get_random_u32(), so that + * get_random_u32_inclusive() can work over its whole range without + * undefined behavior. + */ + if (unlikely(!ceil)) + return rand; + + mult = (u64)ceil * rand; if (unlikely((u32)mult < ceil)) { u32 bound = -ceil % ceil; while (unlikely((u32)mult < bound)) |