Re: [PATCH bpf-next 02/16] libbpf: Create arena percpu chunk from the arena percpu ELF section

[bot+bpf-ci]

> libbpf: Create arena percpu chunk from the arena percpu ELF section

TL;DR: Reading the possible-CPUs sysfs file at open time may make
bpf_object_open() fail, breaking 'bpftool gen skeleton' in sandboxes without
/sys; suggests computing nr_chunks lazily in relocate.

> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c

[ ... ]

> @@ -3267,6 +3274,41 @@ static int bpf_object__init_arena_data(struct bpf_object *obj)
>  	return 0;
>  }
>
> +static int init_arena_percpu_data(struct bpf_object *obj, void *percpu_data, size_t percpu_sz)
> +{
> +	static const char *fcpu = "/sys/devices/system/cpu/possible";
> +	bool *mask = NULL;
> +	int possible_cpus;
> +	int err;
> +
> +	if (percpu_sz == 0) {
> +		pr_warn("arena percpu: empty %s section\n", ARENA_PERCPU_SEC);
> +		return -EINVAL;
> +	}
> +
> +	err = parse_cpu_mask_file(fcpu, &mask, &possible_cpus);
> +	if (err) {
> +		pr_warn("arena percpu: failed to get possible cpus: %s\n",
> +			errstr(err));
> +		return err;
> +	}

Can this make bpf_object_open() fail on hosts where
/sys/devices/system/cpu/possible cannot be read?

init_arena_percpu_data() is reached from bpf_object__init_user_btf_maps()
during bpf_object_open(), so opening any object with a .arena.percpu section
now reads this sysfs file.  Until now libbpf only needed the possible CPU
count at load or runtime, through map_set_def_max_entries(), the per-cpu
value size checks and perf_buffer setup.

The value computed here, obj->arena_percpu.nr_chunks, is first used in
bpf_object__relocate(), which runs from bpf_object_prepare():

> +		size_t percpu_sz = obj->arena_percpu.nr_chunks * obj->arena_percpu.chunk_sz;

Open-only consumers such as 'bpftool gen skeleton' and 'bpftool gen
subskeleton' call bpf_object__open_mem() and never load, so they now depend
on the build host's sysfs.  In a build chroot or sandbox without /sys, open
fails with "arena percpu: failed to get possible cpus" and skeleton
generation breaks, even though the CPU count is never used there.

With 'gen skeleton -L', open fails with this error before the intended
-EOPNOTSUPP check in bpf_object__relocate() is reached:

> +		if (obj->gen_loader && obj->arena_percpu.present) {
> +			pr_warn("map '%s': light skeletons don't support arena percpu variables\n",
> +				arena_map->name);
> +			return -EOPNOTSUPP;
> +		}

Would it work to allocate and copy the chunk at open time, and compute
nr_chunks lazily in bpf_object__relocate(), where it is first needed?  No
later commit in the series changes this.

> +
> +	obj->arena_percpu.chunk_sz = roundup(percpu_sz, sysconf(_SC_PAGE_SIZE));
> +	obj->arena_percpu.chunk = calloc(1, obj->arena_percpu.chunk_sz); /* zero padding */
> +	if (!obj->arena_percpu.chunk) {
> +		free(mask);
> +		return -ENOMEM;
> +	}

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/39434884362