LowMem extension: Fix swapfile creation on btrfs; Fix permission race

[dlitz]

Description

This PR includes a fix for the creation of the /swapfile when building with ENABLE_EXTENSIONS=lowmem and a btrfs root filesystem. Specifically, it ensures that the swapfile is created with the No_COW attribute (chattr +C). Otherwise, swapon fails.

This PR also includes a one-line fix (as a separate commit) to set the umask so that there is no moment where /swapfile is world-readable.

How Has This Been Tested?

• BOARD=qemu-uefi-x86 without this patch
• BOARD=qemu-uefi-x86 with this patch
• BOARD=nanopi-r3s-lts without this patch
• BOARD=nanopi-r3s-lts with this patch

Common build options: ./compile.sh RELEASE=trixie BRANCH=edge BUILD_MINIMAL=yes KERNEL_CONFIGURE=no ROOTFS_TYPE=btrfs BTRFS_COMPRESSION=zstd ENABLE_EXTENSIONS=lowmem CLEAN_LEVEL=debs

Notes:

• CLEAN_LEVEL=debs seems to be needed to prevent the build process from including an old, cached copy of the lowmem-mkswap.sh script.
• BTRFS_COMPRESSION=zstd or BTRFS_COMPRESSION=none seems to be needed on nanopi-r3s-lts because u-boot apparently can't properly read /boot/armbianEnv.txt from a zlib-compressed btrfs.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas

Summary by CodeRabbit

• Bug Fixes
  • Strengthened swapfile setup by starting with restrictive permissions (umask 077).
  • Ensured the swapfile target is reset to empty before allocation.
  • Improved behavior on copy-on-write filesystems by attempting to disable CoW on the swapfile (without failing if unsupported).

[coderabbitai]

📝 Walkthrough

Walkthrough

The swapfile setup script now sets a restrictive umask, truncates the target, and attempts to disable copy-on-write before allocation and activation.

Changes

Swapfile initialization hardening

Layer / File(s)	Summary
Prepare swapfile before allocation packages/bsp/armbian-lowmem/lowmem-mkswap.sh	The script applies umask 077, truncates the swapfile, and attempts chattr +C before the existing allocation and activation steps.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• armbian/build#10065: Both PRs modify the swapfile creation workflow, including sizing, setup, and activation.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: improving LowMem swapfile creation on Btrfs and fixing a permissions race.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 3

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@packages/bsp/armbian-lowmem/lowmem-mkswap.sh`:
- Around line 29-35: Update the swapfile creation flow around SWAPFILE_PATH so
failures after truncate, including chattr, fallocate, mkswap, or swapon, remove
the incomplete swapfile before exiting. Use transactional staging or equivalent
validation/repair for existing incomplete files, while preserving valid existing
swapfile handling.
- Around line 34-35: The swapfile setup currently ignores failures from chattr
in the low-memory swap creation flow. Update the chattr +C step after
REAL_SWAPFILE_PATH is resolved to stop immediately with an actionable error when
it fails, before preallocation, mkswap, or swapon proceed; retain normal
continuation when No_COW setup succeeds.
- Around line 32-35: Update the swapfile setup flow to use REAL_SWAPFILE_PATH,
the canonical path resolved by readlink, for free-space checks, df/dirname
lookups, and swapfile allocation operations; retain SWAPFILE_PATH only as the
configurable input and ensure all filesystem calculations target the resolved
path.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

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Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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Run ID: 7f9feebd-0b10-4195-9ab1-9e9715e4f913

📥 Commits

Reviewing files that changed from the base of the PR and between 0538504 and 0aecfa5.

📒 Files selected for processing (1)

• packages/bsp/armbian-lowmem/lowmem-mkswap.sh

[github-actions]

✅ This PR has been reviewed and approved — all set for merge!

[EvilOlaf]

CLEAN_LEVEL=debs seems to be needed to prevent the build process from including an old, cached copy of the lowmem-mkswap.sh script.

So after merge global rootfs recreation seems necessary