Prevent repeat slice length overflow - #9819
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alamb
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April 25, 2026 03:48
Dandandan
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Apr 25, 2026
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Thanks (again) @Dandandan |
scovich
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Apr 27, 2026
| .expect("repeated slice byte length overflow"); | ||
| self.len | ||
| .checked_add(repeated_bytes) | ||
| .expect("mutable buffer length overflow"); |
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I don't love introducing panics (tho clearly that's better than UB).
Any possibility to make this API more robust?
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I think it would be fine to add another function like try_repeat_slice_n_times... that returned a Result for those that wanted it
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# Which issue does this PR close? - None. # Rationale for this change MutableBuffer repeated slices used unchecked usize arithmetic when calculating the number of bytes to reserve. In optimized builds, very large repeat counts could wrap the capacity calculation before copying repeated bytes. # What changes are included in this PR? This adds checked arithmetic for repeated slice byte length validation before reserving capacity and copying repeated data. # Are these changes tested? Yes. This adds regression coverage for overflowing repeated slice length calculations. # Are there any user-facing changes? Invalid repeat counts whose requested byte length cannot be represented without overflow now panic consistently. There are no API changes.
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Which issue does this PR close?
Rationale for this change
MutableBuffer repeated slices used unchecked usize arithmetic when calculating the number of bytes to reserve. In optimized builds, very large repeat counts could wrap the capacity calculation before copying repeated bytes.
What changes are included in this PR?
This adds checked arithmetic for repeated slice byte length validation before reserving capacity and copying repeated data.
Are these changes tested?
Yes. This adds regression coverage for overflowing repeated slice length calculations.
Are there any user-facing changes?
Invalid repeat counts whose requested byte length cannot be represented without overflow now panic consistently. There are no API changes.