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Also, fallocate+chmod is racy: between the two calls, an attacker could open the file, and then keep it open until next reboot. OTOH, mkswap creates the swap file with the correct permissions straight away.

The --size and --file options are relatively new, though: they were added in util-linux v2.40, released in 2024.

> between the two calls, an attacker could open the file, and then keep it open until next reboot.

I feel like if there's a malicious user (or program) on your system with the necessary access for this, you have much bigger problems.

I wrote this years ago before LLMs made it easy to validate guesswork. I’ll update! Good call

LLMs are guesswork. You have to validate them, not the other way around.

Looks like this was added in util-linux 2.40. Works with btrfs correctly too! Added.