linux: treats empty path to safe_openat as root #1753

giuseppe · 2025-05-20T14:05:49Z

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 · 2025-05-20T14:05:54Z

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

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

Sourcery is free for open source - if you like our reviews please consider sharing them ✨

_{Help me be more useful! Please click 👍 or 👎 on each comment and I'll use the feedback to improve your reviews.}

sourcery-ai · 2025-05-20T14:11:35Z

tests/test_mounts.py

+    conf = base_config()
+    conf['process']['args'] = ['/init', 'true']
+    add_all_namespaces(conf)
+    mount_opt = {"destination": "/", "type": "tmpfs", "source": "tmpfs", "options": ["tmpcopyup"]}


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 · 2025-05-20T14:18:16Z

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

Signed-off-by: Giuseppe Scrivano <[email protected]>

if `do_open` is used with an empty path, the reopen the `dirpath`. Signed-off-by: Giuseppe Scrivano <[email protected]>

Signed-off-by: Giuseppe Scrivano <[email protected]>

This change ensures that the file descriptor for the rootfs is always sourced directly from the container's private data. This avoids potential stale file descriptor issues that could happen if a local variable were used and the descriptor in the private data was updated elsewhere. Should not introduce any behavior change. Signed-off-by: Giuseppe Scrivano <[email protected]>

If an empty path is used, reopens directly the rootfs so that it can grab a reference to the topmost mount, not the previously open file descriptor. Signed-off-by: Giuseppe Scrivano <[email protected]>

When a mount operation replaces the container's root filesystem ("/"), the existing `rootfsfd` becomes stale. This file descriptor would still point to the old, now overmounted root, potentially causing subsequent filesystem operations within the container setup to fail or target the incorrect filesystem. Closes: containers#1752 Signed-off-by: Giuseppe Scrivano <[email protected]>

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

Sourcery is free for open source - if you like our reviews please consider sharing them ✨

_{Help me be more useful! Please click 👍 or 👎 on each comment and I'll use the feedback to improve your reviews.}

sourcery-ai · 2025-05-21T15:02:22Z

src/libcrun/linux.c

    return crun_make_error (err, errno, "open `%s`", rootfs);

+  get_private_data (container)->rootfsfd = ret;


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 · 2025-05-21T15:02:22Z

tests/test_mounts.py

    out, _ = run_and_get_output(conf, hide_stderr=True, callback_prepare_rootfs=prepare_rootfs)
    if "712" in out:
        return 0
    return -1

+def test_mount_bind_to_rootfs():


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 · 2025-05-21T15:02:23Z

tests/test_mounts.py

+    _, _ = run_and_get_output(conf, hide_stderr=True)
+    return 0
+
+def test_mount_tmpfs_to_rootfs():


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 · 2025-05-22T09:51:39Z

@kolyshkin PTAL

giuseppe · 2025-05-27T08:19:56Z

@flouthoc PTAL

flouthoc

LGTM

sourcery-ai bot approved these changes May 20, 2025

View reviewed changes

giuseppe marked this pull request as draft May 20, 2025 14:17

giuseppe added 3 commits May 20, 2025 16:54

linux: include errno check in UNLIKELY macro

040cb2e

Signed-off-by: Giuseppe Scrivano <[email protected]>

test: fix mount to test

9ddddfb

Signed-off-by: Giuseppe Scrivano <[email protected]>

tests: fix unused variable

372446d

Signed-off-by: Giuseppe Scrivano <[email protected]>

giuseppe force-pushed the fix-opening-root branch 2 times, most recently from b04d295 to 6a4ee79 Compare May 20, 2025 18:53

giuseppe added 5 commits May 20, 2025 21:42

utils: crun_safe_ensure_at opens empty paths

953a8c4

if `do_open` is used with an empty path, the reopen the `dirpath`. Signed-off-by: Giuseppe Scrivano <[email protected]>

linux: store rootfsfd under private data only

e9d159f

Signed-off-by: Giuseppe Scrivano <[email protected]>

linux: safe_openat reopens root

7de03e6

If an empty path is used, reopens directly the rootfs so that it can grab a reference to the topmost mount, not the previously open file descriptor. Signed-off-by: Giuseppe Scrivano <[email protected]>

giuseppe force-pushed the fix-opening-root branch from 6a4ee79 to 64a2e0e Compare May 20, 2025 19:44

giuseppe marked this pull request as ready for review May 21, 2025 14:56

sourcery-ai bot approved these changes May 21, 2025

View reviewed changes

flouthoc approved these changes May 27, 2025

View reviewed changes

flouthoc merged commit b1a71e7 into containers:main May 27, 2025
60 of 61 checks passed

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linux: treats empty path to safe_openat as root #1753

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giuseppe commented May 20, 2025 •

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		return crun_make_error (err, errno, "open `%s`", rootfs);

		get_private_data (container)->rootfsfd = ret;

linux: treats empty path to safe_openat as root #1753

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Summary by Sourcery

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Reviewer's Guide

Sequence Diagram for Root Filesystem Descriptor Update on Root Mount

File-Level Changes

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giuseppe commented May 20, 2025 •

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