fix(allocator): fix soundness hole in Allocator::alloc_bytes_start

[overlookmotel]

Fix a bug in Allocator::alloc_bytes_start which could have resulted in UB.

Previously, if called with size: usize::MAX, the rounding up calculation would wrap around so it would not allocate any space in the arena. This PR makes sure such a large size causes a panic as it should.

In practice I don't think there was a real risk of alloc_bytes_start being called with such an absurdly large size, but regardless, we need to handle the edge cases.

[overlookmotel]

• fix(allocator): fix soundness hole in Allocator::alloc_bytes_start #13141 👈 (View in Graphite)
• main

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #13141 will not alter performance

_{Comparing 08-16-fix_allocator_fix_soundness_hole_in_allocator_alloc_bytes_start_ (8264664) with main (3f92fa8)¹}

Summary

✅ 34 untouched benchmarks

Footnotes

1. No successful run was found on main (8264664) during the generation of this report, so 3f92fa8 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[Copilot]

Pull Request Overview

This PR fixes a soundness hole in Allocator::alloc_bytes_start that could lead to undefined behavior when called with extremely large size values. The fix prevents integer overflow during alignment calculations that could result in insufficient memory allocation.

• Replaces potentially overflowing addition with saturating_add to handle edge cases safely
• Adds comprehensive documentation explaining the overflow prevention strategy
• Ensures that oversized allocation requests will be properly rejected by downstream capacity checks

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[overlookmotel]

I don't think anyone has time to review, and I'm the one who knows this code best, so merging.

[overlookmotel]

Merge activity

• Aug 19, 3:08 PM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Aug 19, 3:08 PM UTC: overlookmotel added this pull request to the Graphite merge queue.
• Aug 19, 3:19 PM UTC: Merged by the Graphite merge queue.