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Source file src/crypto/rand/rand_linux.go

Documentation: crypto/rand

  // Copyright 2014 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  package rand
  
  import (
  	"internal/syscall/unix"
  )
  
  func init() {
  	altGetRandom = batched(getRandomLinux, maxGetRandomRead)
  }
  
  // maxGetRandomRead is the maximum number of bytes to ask for in one call to the
  // getrandom() syscall. In linux at most 2^25-1 bytes will be returned per call.
  // From the manpage
  //
  //	*  When reading from the urandom source, a maximum of 33554431 bytes
  //	   is returned by a single call to getrandom() on systems where int
  //	   has a size of 32 bits.
  const maxGetRandomRead = (1 << 25) - 1
  
  // batched returns a function that calls f to populate a []byte by chunking it
  // into subslices of, at most, readMax bytes.
  func batched(f func([]byte) bool, readMax int) func([]byte) bool {
  	return func(buf []byte) bool {
  		for len(buf) > readMax {
  			if !f(buf[:readMax]) {
  				return false
  			}
  			buf = buf[readMax:]
  		}
  		return len(buf) == 0 || f(buf)
  	}
  }
  
  // If the kernel is too old (before 3.17) to support the getrandom syscall(),
  // unix.GetRandom will immediately return ENOSYS and we will then fall back to
  // reading from /dev/urandom in rand_unix.go. unix.GetRandom caches the ENOSYS
  // result so we only suffer the syscall overhead once in this case.
  // If the kernel supports the getrandom() syscall, unix.GetRandom will block
  // until the kernel has sufficient randomness (as we don't use GRND_NONBLOCK).
  // In this case, unix.GetRandom will not return an error.
  func getRandomLinux(p []byte) (ok bool) {
  	n, err := unix.GetRandom(p, 0)
  	return n == len(p) && err == nil
  }
  

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