1 // Copyright 2014 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 package rand 6 7 import ( 8 "internal/syscall/unix" 9 ) 10 11 func init() { 12 altGetRandom = batched(getRandomLinux, maxGetRandomRead) 13 } 14 15 // maxGetRandomRead is the maximum number of bytes to ask for in one call to the 16 // getrandom() syscall. In linux at most 2^25-1 bytes will be returned per call. 17 // From the manpage 18 // 19 // * When reading from the urandom source, a maximum of 33554431 bytes 20 // is returned by a single call to getrandom() on systems where int 21 // has a size of 32 bits. 22 const maxGetRandomRead = (1 << 25) - 1 23 24 // batched returns a function that calls f to populate a []byte by chunking it 25 // into subslices of, at most, readMax bytes. 26 func batched(f func([]byte) bool, readMax int) func([]byte) bool { 27 return func(buf []byte) bool { 28 for len(buf) > readMax { 29 if !f(buf[:readMax]) { 30 return false 31 } 32 buf = buf[readMax:] 33 } 34 return len(buf) == 0 || f(buf) 35 } 36 } 37 38 // If the kernel is too old (before 3.17) to support the getrandom syscall(), 39 // unix.GetRandom will immediately return ENOSYS and we will then fall back to 40 // reading from /dev/urandom in rand_unix.go. unix.GetRandom caches the ENOSYS 41 // result so we only suffer the syscall overhead once in this case. 42 // If the kernel supports the getrandom() syscall, unix.GetRandom will block 43 // until the kernel has sufficient randomness (as we don't use GRND_NONBLOCK). 44 // In this case, unix.GetRandom will not return an error. 45 func getRandomLinux(p []byte) (ok bool) { 46 n, err := unix.GetRandom(p, 0) 47 return n == len(p) && err == nil 48 } 49
