Re: [PATCH bpf-next v9 3/9] libbpf: Probe percpu data feature

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The capability probe for percpu data incorrectly emits warnings and returns a negative error code when map creation fails on older kernels, breaking the silent probing expectation and causing double warnings.
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--- Patch [3]: [PATCH bpf-next v9 3/9] libbpf: Probe percpu data feature ---
commit 4bf6930d4ace160491d05ec9880c9cadc9f81bc7
Author: Leon Hwang <leon.hwang@linux.dev>

libbpf: Probe percpu data feature

This commit adds a dedicated feature probe to libbpf for detecting whether
the kernel supports global percpu data. It checks this capability by
attempting to create a simple percpu array map and loading a basic socket
filter program that accesses it.

> diff --git a/tools/lib/bpf/features.c b/tools/lib/bpf/features.c
> index b7e388f99d0bb..ef9581c113035 100644
> --- a/tools/lib/bpf/features.c
> +++ b/tools/lib/bpf/features.c

[ ... ]

> @@ -620,6 +620,38 @@ static int probe_bpf_syscall_common_attrs(int token_fd)

[ ... ]

> +static int probe_kern_percpu_data(int token_fd)
> +{

[ ... ]

> +	map = bpf_map_create(BPF_MAP_TYPE_PERCPU_ARRAY, "libbpf_percpu", sizeof(int), 8, 1,
> +			     &map_opts);
> +	if (map < 0) {
> +		pr_warn("Error in %s(): %s. Couldn't create simple percpu_array map.\n",
> +			__func__, errstr(map));
> +		return map;
> +	}

[Severity: Medium]
Should this probe silently return 0 instead of propagating the negative error
code and printing a warning when bpf_map_create() fails?

When running on older kernels that genuinely lack BPF_MAP_TYPE_PERCPU_ARRAY
support, this code will print the pr_warn() above and return a negative error
code.

Because the caller feat_supported() expects 0 to cleanly indicate a missing
feature, returning a negative value causes feat_supported() to print a second
warning: "Detection of kernel kernel supports percpu data support failed".

Does this unintentionally pollute the logs with double warnings when apps are
safely falling back on older kernels?

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Sashiko AI review · https://sashiko.dev/#/patchset/20260713154024.30851-1-leon.hwang@linux.dev?part=3