aleph/lkl_linux
1
0
Fork 0
mirror of https://github.com/lkl/linux.git synced 2025-12-19 16:13:19 +09:00
Code Issues Packages Projects Releases Wiki Activity

random: add helpers for random numbers with given floor or range

Browse Source 
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>
This commit is contained in:
Jason A. Donenfeld
2022-10-19 23:19:35 -06:00
parent e9a688bcb1
commit 7f576b2593
2 changed files with 42 additions and 1 deletions
Download Patch File Download Diff File Expand all files Collapse all files

25
include/linux/random.h
View File

@@ -91,6 +91,31 @@ 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.