cgroup, systemd: validate ebpf is loaded

[giuseppe]

systemd ignores errors with BPFProgram so make sure the ebpf program is installed when requested.

Alternative to #1854

Summary by Sourcery

Validate that systemd has installed the requested eBPF device filter on the cgroup by querying attached programs

Enhancements:

• Add libcrun_ebpf_query_cgroup_progs to list eBPF programs attached to a cgroup
• Introduce verify_ebpf_device_filter_installed to error if no eBPF filter is found on the cgroup
• Invoke eBPF filter verification after creating the systemd cgroup scope when bpf_dev_set is requested

[sourcery-ai]

Reviewer's Guide

The PR adds a mechanism to query attached eBPF programs on a cgroup and integrates a verification step into the systemd cgroup entry path, causing an early failure if a requested eBPF device filter isn’t actually installed.

Sequence diagram for systemd cgroup entry with eBPF device filter verification

sequenceDiagram
    participant Caller
    participant libcrun_cgroup_enter_systemd
    participant verify_ebpf_device_filter_installed
    participant libcrun_ebpf_query_cgroup_progs
    participant cgroup
    Caller->>libcrun_cgroup_enter_systemd: Enter systemd cgroup
    libcrun_cgroup_enter_systemd->>verify_ebpf_device_filter_installed: If bpf_dev_set, verify filter
    verify_ebpf_device_filter_installed->>libcrun_ebpf_query_cgroup_progs: Query attached eBPF programs
    libcrun_ebpf_query_cgroup_progs->>cgroup: Open cgroup path and read programs
    cgroup-->>libcrun_ebpf_query_cgroup_progs: Return program list
    libcrun_ebpf_query_cgroup_progs-->>verify_ebpf_device_filter_installed: Return program count
    verify_ebpf_device_filter_installed-->>libcrun_cgroup_enter_systemd: Return success or error
    libcrun_cgroup_enter_systemd-->>Caller: Return success or error

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Class diagram for new and updated eBPF-related functions

classDiagram
    class libcrun_ebpf_query_cgroup_progs {
        +int libcrun_ebpf_query_cgroup_progs(const char *cgroup_path, uint32_t **progs_out, size_t *n_progs_out, libcrun_error_t *err)
    }
    class verify_ebpf_device_filter_installed {
        +int verify_ebpf_device_filter_installed(const char *cgroup_path, libcrun_error_t *err)
    }
    class libcrun_ebpf_read_program {
        +int libcrun_ebpf_read_program(struct bpf_program **program_ret, const char *path, libcrun_error_t *err)
    }
    class libcrun_ebpf_cmp_programs {
        +bool libcrun_ebpf_cmp_programs(struct bpf_program *program1, struct bpf_program *program2)
    }
    libcrun_ebpf_query_cgroup_progs <|-- verify_ebpf_device_filter_installed

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Class diagram for updated systemd cgroup entry logic

classDiagram
    class libcrun_cgroup_enter_systemd {
        +int libcrun_cgroup_enter_systemd(struct libcrun_cgroup_args *args, ...)
        +calls verify_ebpf_device_filter_installed if bpf_dev_set
    }
    class verify_ebpf_device_filter_installed {
        +int verify_ebpf_device_filter_installed(const char *cgroup_path, libcrun_error_t *err)
    }
    libcrun_cgroup_enter_systemd --> verify_ebpf_device_filter_installed

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File-Level Changes

Change	Details	Files
Introduce a function to list eBPF programs on a cgroup	Added libcrun_ebpf_query_cgroup_progs with HAVE_EBPF guard Opened cgroup path and delegated to read_all_progs Returned an unsupported error when eBPF isn’t enabled	src/libcrun/ebpf.c src/libcrun/ebpf.h
Create helper to verify that systemd installed the eBPF filter	Built full cgroup path Queried the cgroup for attached programs Errored if no programs were found	src/libcrun/cgroup-systemd.c
Invoke verification in systemd cgroup entry path	Check out->bpf_dev_set flag Remove systemd scope suffix from path Call verify_ebpf_device_filter_installed and fail early on error	src/libcrun/cgroup-systemd.c

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[giuseppe]

@sohankunkerkar @kolyshkin PTAL

[sourcery-ai]

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

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[packit-as-a-service]

Ephemeral COPR build failed. @containers/packit-build please check.

[packit-as-a-service]

Ephemeral COPR build failed. @containers/packit-build please check.

[sourcery-ai]

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

• Extract the logic for stripping the systemd scope suffix from the cgroup path into a small helper function to improve readability and reduce duplication.
• In libcrun_ebpf_query_cgroup_progs the HAVE_EBPF disabled path returns an error instead of an empty list; consider returning zero programs (no error) or documenting that callers must handle this specific unsupported‐feature error.

Prompt for AI Agents

Please address the comments from this code review:
## Overall Comments
- Extract the logic for stripping the systemd scope suffix from the cgroup path into a small helper function to improve readability and reduce duplication.
- In libcrun_ebpf_query_cgroup_progs the HAVE_EBPF disabled path returns an error instead of an empty list; consider returning zero programs (no error) or documenting that callers must handle this specific unsupported‐feature error.

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[packit-as-a-service]

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

[sohankunkerkar]

Yeah, this looks much better.
LGTM

[giuseppe]

given the reason in systemd/systemd#38732, this is probably the best we can do.

@kolyshkin what do you think?

[kolyshkin]

LGTM