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Auto merge of #122240 - RalfJung:miri-addr-reuse, r=oli-obk
miri: add some chance to reuse addresses of previously freed allocations The hope is that this can help us find ABA issues. Unfortunately this needs rustc changes so I can't easily run the regular benchmark suite. I used `src/tools/miri/tests/pass/float_nan.rs` as a substitute: ``` Before: Benchmark 1: ./x.py run miri --stage 0 --args src/tools/miri/tests/pass/float_nan.rs --args --edition=2021 Time (mean ± σ): 9.570 s ± 0.013 s [User: 9.279 s, System: 0.290 s] Range (min … max): 9.561 s … 9.579 s 2 runs After: Benchmark 1: ./x.py run miri --stage 0 --args src/tools/miri/tests/pass/float_nan.rs --args --edition=2021 Time (mean ± σ): 9.698 s ± 0.046 s [User: 9.413 s, System: 0.279 s] Range (min … max): 9.666 s … 9.731 s 2 runs ``` That's a ~1.3% slowdown, which seems fine to me. I have seen a lot of noise in this style of benchmarking so I don't quite trust this anyway; we can make further experiments in the Miri repo after this migrated there. r? `@oli-obk`
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,87 @@ | ||
| //! Manages a pool of addresses that can be reused. | ||
| use rand::Rng; | ||
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| use rustc_target::abi::{Align, Size}; | ||
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| const MAX_POOL_SIZE: usize = 64; | ||
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| // Just use fair coins, until we have evidence that other numbers are better. | ||
| const ADDR_REMEMBER_CHANCE: f64 = 0.5; | ||
| const ADDR_TAKE_CHANCE: f64 = 0.5; | ||
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| /// The pool strikes a balance between exploring more possible executions and making it more likely | ||
| /// to find bugs. The hypothesis is that bugs are more likely to occur when reuse happens for | ||
| /// allocations with the same layout, since that can trigger e.g. ABA issues in a concurrent data | ||
| /// structure. Therefore we only reuse allocations when size and alignment match exactly. | ||
| #[derive(Debug)] | ||
| pub struct ReusePool { | ||
| /// The i-th element in `pool` stores allocations of alignment `2^i`. We store these reusable | ||
| /// allocations as address-size pairs, the list must be sorted by the size. | ||
| /// | ||
| /// Each of these maps has at most MAX_POOL_SIZE elements, and since alignment is limited to | ||
| /// less than 64 different possible value, that bounds the overall size of the pool. | ||
| pool: Vec<Vec<(u64, Size)>>, | ||
| } | ||
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| impl ReusePool { | ||
| pub fn new() -> Self { | ||
| ReusePool { pool: vec![] } | ||
| } | ||
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| fn subpool(&mut self, align: Align) -> &mut Vec<(u64, Size)> { | ||
| let pool_idx: usize = align.bytes().trailing_zeros().try_into().unwrap(); | ||
| if self.pool.len() <= pool_idx { | ||
| self.pool.resize(pool_idx + 1, Vec::new()); | ||
| } | ||
| &mut self.pool[pool_idx] | ||
| } | ||
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| pub fn add_addr(&mut self, rng: &mut impl Rng, addr: u64, size: Size, align: Align) { | ||
| // Let's see if we even want to remember this address. | ||
| if !rng.gen_bool(ADDR_REMEMBER_CHANCE) { | ||
| return; | ||
| } | ||
| // Determine the pool to add this to, and where in the pool to put it. | ||
| let subpool = self.subpool(align); | ||
| let pos = subpool.partition_point(|(_addr, other_size)| *other_size < size); | ||
| // Make sure the pool does not grow too big. | ||
| if subpool.len() >= MAX_POOL_SIZE { | ||
| // Pool full. Replace existing element, or last one if this would be even bigger. | ||
| let clamped_pos = pos.min(subpool.len() - 1); | ||
| subpool[clamped_pos] = (addr, size); | ||
| return; | ||
| } | ||
| // Add address to pool, at the right position. | ||
| subpool.insert(pos, (addr, size)); | ||
| } | ||
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| pub fn take_addr(&mut self, rng: &mut impl Rng, size: Size, align: Align) -> Option<u64> { | ||
| // Determine whether we'll even attempt a reuse. | ||
| if !rng.gen_bool(ADDR_TAKE_CHANCE) { | ||
| return None; | ||
| } | ||
| // Determine the pool to take this from. | ||
| let subpool = self.subpool(align); | ||
| // Let's see if we can find something of the right size. We want to find the full range of | ||
| // such items, beginning with the first, so we can't use `binary_search_by_key`. | ||
| let begin = subpool.partition_point(|(_addr, other_size)| *other_size < size); | ||
| let mut end = begin; | ||
| while let Some((_addr, other_size)) = subpool.get(end) { | ||
| if *other_size != size { | ||
| break; | ||
| } | ||
| end += 1; | ||
| } | ||
| if end == begin { | ||
| // Could not find any item of the right size. | ||
| return None; | ||
| } | ||
| // Pick a random element with the desired size. | ||
| let idx = rng.gen_range(begin..end); | ||
| // Remove it from the pool and return. | ||
| let (chosen_addr, chosen_size) = subpool.remove(idx); | ||
| debug_assert!(chosen_size >= size && chosen_addr % align.bytes() == 0); | ||
| Some(chosen_addr) | ||
| } | ||
| } |
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