      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174
    175
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200
    201
    202
    203
    204
    205
    206
    207
    208
    209
    210
    211
    212
    213
    214
    215
    216
    217
    218
    219
    220
    221
    222
    223
    224
    225
    226
    227
    228
    229
    230
    231
    232
    233
    234
    235
    236
    237
    238
    239
    240
    241
    242
    243
    244
    245
    246
    247
    248
    249
    250
    251
    252
    253
    254
    255
    256
    257
    258
    259
    260
    261
    262
    263
    264
    265
    266
    267
    268
    269
    270
    271
    272
    273
    274
    275
    276
    277
    278
    279
    280
    281
    282
    283
    284
    285
    286
    287
    288
    289
    290
    291
    292
    293
    294
    295
    296
    297
    298
    299
    300
    301
    302
    303
    304
    305
    306
    307
    308
    309
    310
    311
    312
    313
    314
    315
    316
    317
    318
    319
    320
    321
    322
    323
    324
    325
    326
    327
    328
    329
    330
    331
    332
    333
    334
    335
    336
    337
    338
    339
    340
    341
    342
    343
    344
    345
    346
    347
    348
    349
    350
    351
    352
    353
    354
    355
    356

    // Copyright 2018 Developers of the Rand project.
    // Copyright 2013 The Rust Project Developers.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! A wrapper around another PRNG that reseeds it after it
    //! generates a certain number of random bytes.

    use core::mem::size_of;

    use rand_core::{RngCore, CryptoRng, SeedableRng, Error};
    use rand_core::block::{BlockRngCore, BlockRng};

    /// A wrapper around any PRNG that implements [`BlockRngCore`], that adds the
    /// ability to reseed it.
    ///
    /// `ReseedingRng` reseeds the underlying PRNG in the following cases:
    ///
    /// - On a manual call to [`reseed()`].
    /// - After `clone()`, the clone will be reseeded on first use.
    /// - After a process is forked, the RNG in the child process is reseeded within
    ///   the next few generated values, depending on the block size of the
    ///   underlying PRNG. For ChaCha and Hc128 this is a maximum of
    ///   15 `u32` values before reseeding.
    /// - After the PRNG has generated a configurable number of random bytes.
    ///
    /// # When should reseeding after a fixed number of generated bytes be used?
    ///
    /// Reseeding after a fixed number of generated bytes is never strictly
    /// *necessary*. Cryptographic PRNGs don't have a limited number of bytes they
    /// can output, or at least not a limit reachable in any practical way. There is
    /// no such thing as 'running out of entropy'.
    ///
    /// Occasionally reseeding can be seen as some form of 'security in depth'. Even
    /// if in the future a cryptographic weakness is found in the CSPRNG being used,
    /// or a flaw in the implementation, occasionally reseeding should make
    /// exploiting it much more difficult or even impossible.
    ///
    /// Use [`ReseedingRng::new`] with a `threshold` of `0` to disable reseeding
    /// after a fixed number of generated bytes.
    ///
    /// # Error handling
    ///
    /// Although unlikely, reseeding the wrapped PRNG can fail. `ReseedingRng` will
    /// never panic but try to handle the error intelligently through some
    /// combination of retrying and delaying reseeding until later.
    /// If handling the source error fails `ReseedingRng` will continue generating
    /// data from the wrapped PRNG without reseeding.
    ///
    /// Manually calling [`reseed()`] will not have this retry or delay logic, but
    /// reports the error.
    ///
    /// # Example
    ///
    /// ```
    /// use rand::prelude::*;
    /// use rand_chacha::ChaCha20Core; // Internal part of ChaChaRng that
    ///                              // implements BlockRngCore
    /// use rand::rngs::OsRng;
    /// use rand::rngs::adapter::ReseedingRng;
    ///
    /// let prng = ChaCha20Core::from_entropy();
    /// let mut reseeding_rng = ReseedingRng::new(prng, 0, OsRng);
    ///
    /// println!("{}", reseeding_rng.gen::<u64>());
    ///
    /// let mut cloned_rng = reseeding_rng.clone();
    /// assert!(reseeding_rng.gen::<u64>() != cloned_rng.gen::<u64>());
    /// ```
    ///
    /// [`BlockRngCore`]: rand_core::block::BlockRngCore
    /// [`ReseedingRng::new`]: ReseedingRng::new
    /// [`reseed()`]: ReseedingRng::reseed
    #[derive(Debug)]
    pub struct ReseedingRng<R, Rsdr>(BlockRng<ReseedingCore<R, Rsdr>>)
    where R: BlockRngCore + SeedableRng,
          Rsdr: RngCore;

    impl<R, Rsdr> ReseedingRng<R, Rsdr>
    where R: BlockRngCore + SeedableRng,
          Rsdr: RngCore
    {
        /// Create a new `ReseedingRng` from an existing PRNG, combined with a RNG
        /// to use as reseeder.
        ///
        /// `threshold` sets the number of generated bytes after which to reseed the
        /// PRNG. Set it to zero to never reseed based on the number of generated
        /// values.
        pub fn new(rng: R, threshold: u64, reseeder: Rsdr) -> Self {
            ReseedingRng(BlockRng::new(ReseedingCore::new(rng, threshold, reseeder)))
        }

        /// Reseed the internal PRNG.
        pub fn reseed(&mut self) -> Result<(), Error> {
            self.0.core.reseed()
        }
    }

    // TODO: this should be implemented for any type where the inner type
    // implements RngCore, but we can't specify that because ReseedingCore is private
    impl<R, Rsdr: RngCore> RngCore for ReseedingRng<R, Rsdr>
    where R: BlockRngCore<Item = u32> + SeedableRng,
        <R as BlockRngCore>::Results: AsRef<[u32]> + AsMut<[u32]>
    {
        #[inline(always)]
        fn next_u32(&mut self) -> u32 {
            self.0.next_u32()
        }

        #[inline(always)]
        fn next_u64(&mut self) -> u64 {
            self.0.next_u64()
        }

        fn fill_bytes(&mut self, dest: &mut [u8]) {
            self.0.fill_bytes(dest)
        }

        fn try_fill_bytes(&mut self, dest: &mut [u8]) -> Result<(), Error> {
            self.0.try_fill_bytes(dest)
        }
    }

    impl<R, Rsdr> Clone for ReseedingRng<R, Rsdr>
    where R: BlockRngCore + SeedableRng + Clone,
          Rsdr: RngCore + Clone
    {
        fn clone(&self) -> ReseedingRng<R, Rsdr> {
            // Recreating `BlockRng` seems easier than cloning it and resetting
            // the index.
            ReseedingRng(BlockRng::new(self.0.core.clone()))
        }
    }

    impl<R, Rsdr> CryptoRng for ReseedingRng<R, Rsdr>
    where R: BlockRngCore + SeedableRng + CryptoRng,
          Rsdr: RngCore + CryptoRng {}

    #[derive(Debug)]
    struct ReseedingCore<R, Rsdr> {
        inner: R,
        reseeder: Rsdr,
        threshold: i64,
        bytes_until_reseed: i64,
        fork_counter: usize,
    }

    impl<R, Rsdr> BlockRngCore for ReseedingCore<R, Rsdr>
    where R: BlockRngCore + SeedableRng,
          Rsdr: RngCore
    {
        type Item = <R as BlockRngCore>::Item;
        type Results = <R as BlockRngCore>::Results;

        fn generate(&mut self, results: &mut Self::Results) {
            let global_fork_counter = fork::get_fork_counter();
            if self.bytes_until_reseed <= 0 ||
               self.is_forked(global_fork_counter) {
                // We get better performance by not calling only `reseed` here
                // and continuing with the rest of the function, but by directly
                // returning from a non-inlined function.
                return self.reseed_and_generate(results, global_fork_counter);
            }
            let num_bytes = results.as_ref().len() * size_of::<Self::Item>();
            self.bytes_until_reseed -= num_bytes as i64;
            self.inner.generate(results);
        }
    }

    impl<R, Rsdr> ReseedingCore<R, Rsdr>
    where R: BlockRngCore + SeedableRng,
          Rsdr: RngCore
    {
        /// Create a new `ReseedingCore`.
        fn new(rng: R, threshold: u64, reseeder: Rsdr) -> Self {
            use ::core::i64::MAX;
            fork::register_fork_handler();

            // Because generating more values than `i64::MAX` takes centuries on
            // current hardware, we just clamp to that value.
            // Also we set a threshold of 0, which indicates no limit, to that
            // value.
            let threshold =
                if threshold == 0 { MAX }
                else if threshold <= MAX as u64 { threshold as i64 }
                else { MAX };

            ReseedingCore {
                inner: rng,
                reseeder,
                threshold: threshold as i64,
                bytes_until_reseed: threshold as i64,
                fork_counter: 0,
            }
        }

        /// Reseed the internal PRNG.
        fn reseed(&mut self) -> Result<(), Error> {
            R::from_rng(&mut self.reseeder).map(|result| {
                self.bytes_until_reseed = self.threshold;
                self.inner = result
            })
        }

        fn is_forked(&self, global_fork_counter: usize) -> bool {
            // In theory, on 32-bit platforms, it is possible for
            // `global_fork_counter` to wrap around after ~4e9 forks.
            //
            // This check will detect a fork in the normal case where
            // `fork_counter < global_fork_counter`, and also when the difference
            // between both is greater than `isize::MAX` (wrapped around).
            //
            // It will still fail to detect a fork if there have been more than
            // `isize::MAX` forks, without any reseed in between. Seems unlikely
            // enough.
            (self.fork_counter.wrapping_sub(global_fork_counter) as isize) < 0
        }

        #[inline(never)]
        fn reseed_and_generate(&mut self,
                               results: &mut <Self as BlockRngCore>::Results,
                               global_fork_counter: usize)
        {
            if self.is_forked(global_fork_counter) {
                info!("Fork detected, reseeding RNG");
            } else {
                trace!("Reseeding RNG (periodic reseed)");
            }

            let num_bytes =
                results.as_ref().len() * size_of::<<R as BlockRngCore>::Item>();

            if let Err(e) = self.reseed() {
                warn!("Reseeding RNG failed: {}", e);
                let _ = e;
            }
            self.fork_counter = global_fork_counter;

            self.bytes_until_reseed = self.threshold - num_bytes as i64;
            self.inner.generate(results);
        }
    }

    impl<R, Rsdr> Clone for ReseedingCore<R, Rsdr>
    where R: BlockRngCore + SeedableRng + Clone,
          Rsdr: RngCore + Clone
    {
        fn clone(&self) -> ReseedingCore<R, Rsdr> {
            ReseedingCore {
                inner: self.inner.clone(),
                reseeder: self.reseeder.clone(),
                threshold: self.threshold,
                bytes_until_reseed: 0, // reseed clone on first use
                fork_counter: self.fork_counter,
            }
        }
    }

    impl<R, Rsdr> CryptoRng for ReseedingCore<R, Rsdr>
    where R: BlockRngCore + SeedableRng + CryptoRng,
          Rsdr: RngCore + CryptoRng {}

    #[cfg(all(unix, not(target_os="emscripten")))]
    mod fork {
        use core::sync::atomic::{AtomicUsize, AtomicBool, Ordering};
        #[allow(deprecated)]  // Required for compatibility with Rust < 1.24.
        use core::sync::atomic::{ATOMIC_USIZE_INIT, ATOMIC_BOOL_INIT};

        // Fork protection
        //
        // We implement fork protection on Unix using `pthread_atfork`.
        // When the process is forked, we increment `RESEEDING_RNG_FORK_COUNTER`.
        // Every `ReseedingRng` stores the last known value of the static in
        // `fork_counter`. If the cached `fork_counter` is less than
        // `RESEEDING_RNG_FORK_COUNTER`, it is time to reseed this RNG.
        //
        // If reseeding fails, we don't deal with this by setting a delay, but just
        // don't update `fork_counter`, so a reseed is attempted as soon as
        // possible.

        #[allow(deprecated)]
        static RESEEDING_RNG_FORK_COUNTER: AtomicUsize = ATOMIC_USIZE_INIT;

        pub fn get_fork_counter() -> usize {
            RESEEDING_RNG_FORK_COUNTER.load(Ordering::Relaxed)
        }

        #[allow(deprecated)]
        static FORK_HANDLER_REGISTERED: AtomicBool = ATOMIC_BOOL_INIT;

        extern fn fork_handler() {
            // Note: fetch_add is defined to wrap on overflow
            // (which is what we want).
            RESEEDING_RNG_FORK_COUNTER.fetch_add(1, Ordering::Relaxed);
        }

        pub fn register_fork_handler() {
            if FORK_HANDLER_REGISTERED.load(Ordering::Relaxed) == false {
                unsafe { libc::pthread_atfork(None, None, Some(fork_handler)) };
                FORK_HANDLER_REGISTERED.store(true, Ordering::Relaxed);
            }
        }
    }

    #[cfg(not(all(unix, not(target_os="emscripten"))))]
    mod fork {
        pub fn get_fork_counter() -> usize { 0 }
        pub fn register_fork_handler() {}
    }

    #[cfg(test)]
    mod test {
        use crate::{Rng, SeedableRng};
        use crate::rngs::std::Core;
        use crate::rngs::mock::StepRng;
        use super::ReseedingRng;

        #[test]
        fn test_reseeding() {
            let mut zero = StepRng::new(0, 0);
            let rng = Core::from_rng(&mut zero).unwrap();
            let thresh = 1; // reseed every time the buffer is exhausted
            let mut reseeding = ReseedingRng::new(rng, thresh, zero);

            // RNG buffer size is [u32; 64]
            // Debug is only implemented up to length 32 so use two arrays
            let mut buf = ([0u32; 32], [0u32; 32]);
            reseeding.fill(&mut buf.0);
            reseeding.fill(&mut buf.1);
            let seq = buf;
            for _ in 0..10 {
                reseeding.fill(&mut buf.0);
                reseeding.fill(&mut buf.1);
                assert_eq!(buf, seq);
            }
        }

        #[test]
        fn test_clone_reseeding() {
            let mut zero = StepRng::new(0, 0);
            let rng = Core::from_rng(&mut zero).unwrap();
            let mut rng1 = ReseedingRng::new(rng, 32*4, zero);

            let first: u32 = rng1.gen();
            for _ in 0..10 { let _ = rng1.gen::<u32>(); }

            let mut rng2 = rng1.clone();
            assert_eq!(first, rng2.gen::<u32>());
        }
    }

# Help

## Keyboard Shortcuts

`?`
  Show this help dialog
`S`
  Focus the search field
`↑`
  Move up in search results
`↓`
  Move down in search results
`↹`
  Switch tab
`⏎`
  Go to active search result
`+`
  Expand all sections
`-`
  Collapse all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g., `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g., `vec -> usize` or `* -> vec`)

Search multiple things at once by splitting your query with comma (e.g., `str,u8` or `String,struct:Vec,test`)