[Bubblewrap] Mask denied files with a /dev/null bind instead of tmpfs

[Soham Das (SohamDas2021)]

📖 Description

The Bubblewrap backend masked every denied path with --tmpfs. Mounting a tmpfs over a regular file turns it into an empty directory (bwrap creates the mount point as a dir), changing the object's type and breaking tools that expect a regular file.

This PR classifies each denied path at runtime and masks it with the correct primitive:

• Directories keep --tmpfs (empty, no leak).
• Files (and other non-directory objects) are masked with a read-only bind of /dev/null, preserving a non-directory type with empty content.

This brings Bubblewrap in line with the LXC backend, which already distinguishes files from directories when masking.

🔗 References

🔍 Validation

✅ Checklist

• Signed the Contributor License Agreement
• Linked to an issue
• Updated documentation (if applicable)
• Updated Copilot instructions (if build, architecture, or conventions changed)
• If this PR changes Cargo.lock, the dependency-feed-check check passes (see docs/pull-requests.md)

📋 Issue Type

• Bug fix
• Feature
• Task

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Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR improves Bubblewrap’s denied-path masking behavior so that denying a regular file no longer converts it into an empty directory (a side-effect of --tmpfs creating the mount point as a directory). It aligns Bubblewrap with the LXC backend by masking denied directories and denied non-directories differently to better preserve object type semantics.

Changes:

• Classify denied paths at runtime and mask denied non-directories via --ro-bind /dev/null (directories remain --tmpfs).
• Add unit tests for classified denied-path masking in the Bubblewrap argument builder.
• Add an end-to-end shell test + config to validate denied file vs denied directory masking behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/scripts/run_bwrap_denied_masking_test.sh	New end-to-end test script validating denied file vs denied directory masking semantics.
tests/scripts/run_bwrap_all_tests.sh	Adds the new denied-masking test to the Bubblewrap test suite runner.
tests/configs/bubblewrap_denied_masking.json	New Bubblewrap config fixture used by the denied-masking test.
src/backends/bubblewrap/common/src/bwrap_runner.rs	Classifies denied paths at runtime and passes classification into arg building.
src/backends/bubblewrap/common/src/bwrap_command.rs	Adds build_args_classified and unit tests to mask denied files with /dev/null instead of --tmpfs.

[Gudge (MGudgin)]

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