linux: treats empty path to safe_openat as root

[giuseppe]

When an empty string is provided as the path argument to safe_openat, it is now explicitly treated as "/".

Closes: #1752

Summary by Sourcery

Treat empty paths as root in safe_openat and streamline rootfsfd handling across mounting and device creation code, ensure proper cleanup of the rootfs file descriptor, add helper for validating fd targets, and expand tests for root-level bind and tmpfs mounts and rdt value length checks.

Bug Fixes:

• Treat empty path passed to safe_openat as referring to the root directory of the container.
• Close and reset the stored rootfs file descriptor properly after mounting operations.

Enhancements:

• Consolidate use of rootfsfd by fetching it directly from private_data in various mount and device-creation routines.
• Remove redundant local rootfsfd parameters and simplify do_mounts and libcrun_set_mounts signatures.

Tests:

• Add tests for bind and tmpfs mounts targeting root (‘/’).
• Add a check in Intel RDT tests to verify that get_rdt_value’s return length matches the output string.

Chores:

• Introduce check_fd_is_path helper to verify that an open file descriptor points to a given directory.

[sourcery-ai]

Reviewer's Guide

This PR adds explicit handling for empty path arguments in safe_openat (treating them as root), refactors the create-and-open fallback logic for consistency, streamlines rootfsfd management across mounting routines, and includes new tests for mounting to root (bind and tmpfs) and for RDT value length validation.

Sequence Diagram for Root Filesystem Descriptor Update on Root Mount

sequenceDiagram
    participant DM as do_mount
    participant PD as libcrun_private_data
    participant OS as OperatingSystem

    Note over DM: Called within do_mounts when processing a mount entry
    DM->>DM: Mount target is empty string (e.g., destination: "/")
    DM->>OS: dup(fd_of_new_root_mount_source)
    OS-->>DM: tmp_fd (new rootfs fd)
    DM->>OS: close(PD.rootfsfd) (old rootfs fd)
    DM->>PD: Update rootfsfd = tmp_fd

Loading

File-Level Changes

Change	Details	Files
Handle empty path as root in safe_openat and crun_safe_ensure_at	Added early branch in safe_openat to open rootfs when path is empty Introduced check_fd_is_path to verify the opened fd points to root Extended crun_safe_ensure_at to return an O_PATH fd for empty paths	src/libcrun/utils.c
Refactor create-and-open fallback to use a single return variable	Replaced separate fd variable with a single ret variable in crun_safe_create_and_open_ref_at Unified the initial safe_openat call and fallback path for file/dir creation	src/libcrun/utils.c
Streamline rootfsfd usage and lifecycle in linux.c	Removed local rootfsfd variables and inlined get_private_data(container)->rootfsfd everywhere Updated do_mounts signature to drop rootfsfd parameter Added closing/reset of rootfsfd in cleanup and after mounts are configured	src/libcrun/linux.c
Add tests for mounting to rootfs and RDT value length check	Introduced test_mount_bind_to_rootfs and test_mount_tmpfs_to_rootfs in test_mounts.py Added strlen check for RDT get_rdt_value in tests_libcrun_intelrdt.c	tests/test_mounts.py tests/tests_libcrun_intelrdt.c

Possibly linked issues

• allow bind mount on root directory #1752: PR removes the dead code related to the rootfs null check, addressing the issue.
• allow bind mount on root directory #1752: PR treats empty paths as root for mounts, fixing invalid argument error on root bind mount.

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[sourcery-ai]

Hey @giuseppe - I've reviewed your changes - here's some feedback:

• In test_mounts.py, test_mount_to_root defines mount_opt but never applies it to the config or asserts any outcome—please add the mount to conf['mounts'] (or equivalent) and verify the expected behavior.
• In crun_safe_create_and_open_ref_at, the final safe_openat call’s return value and error context aren’t checked before returning—capture and propagate its error result so failures aren’t swallowed.

Here's what I looked at during the review

• 🟢 General issues: all looks good
• 🟢 Security: all looks good
• 🟡 Testing: 1 issue found
• 🟢 Complexity: all looks good
• 🟢 Documentation: all looks good

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[sourcery-ai]

issue (testing): The mount_opt variable is defined but not added to the configuration passed to run_and_get_output.

Add conf['mounts'] = [mount_opt] before calling run_and_get_output so the test actually mounts to /.

[packit-as-a-service]

TMT tests failed. @containers/packit-build please check.

[sourcery-ai]

Hey @giuseppe - I've reviewed your changes and they look great!

Here's what I looked at during the review

• 🟡 General issues: 1 issue found
• 🟢 Security: all looks good
• 🟡 Testing: 2 issues found
• 🟢 Complexity: all looks good
• 🟢 Documentation: all looks good

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[sourcery-ai]

issue (bug_risk): Potential file descriptor leak on early return

Open the fd into a temporary variable (e.g. via cleanup_close) and assign it to private_data only after all operations succeed, or close/reset it on every error path.

[sourcery-ai]

issue (testing): Add assertions to verify the bind mount to root.

Create a unique file in tmpdir before invoking the container, then inside the container (e.g. via /init or its args) assert that this file appears at / to confirm the bind mount.

[sourcery-ai]

issue (testing): Add assertions to verify the tmpfs mount to root and tmpcopyup.

Assert that /init is present and executable inside the container to confirm tmpcopyup worked after mounting root as tmpfs.

[giuseppe]

@kolyshkin PTAL

[giuseppe]

@flouthoc PTAL

[flouthoc]

LGTM