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tokio: check page capacity before obtaining base pointer (#4731)

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This doesn't cause any issues in practice because this is a private API
that is only used in ways that cannot trigger UB. Indexing into `slots`
is not sound until after we've asserted that the page is allocated,
since that aliases the first slot which may not be allocated. This PR
also switches to using `as_ptr` to obtain the base pointer for clarity.

Co-authored-by: David Koloski <dkoloski@google.com>
This commit is contained in:
David Koloski 2022-06-01 15:18:06 -04:00 committed by GitHub
parent 925314ba43
commit cc6c2f40cb
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1 changed files with 2 additions and 3 deletions
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tokio/src/util/slab.rs
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@ -551,10 +551,9 @@ impl<T> Slots<T> {
fn index_for(&self, slot: *const Value<T>) -> usize {
use std::mem;
let base = &self.slots[0] as *const _ as usize;
assert!(base != 0, "page is unallocated");
assert_ne!(self.slots.capacity(), 0, "page is unallocated");
let base = self.slots.as_ptr() as usize;
let slot = slot as usize;
let width = mem::size_of::<Slot<T>>();