Re: [PATCH bpf v1 1/2] bpf: cpumap: propagate underlying error in cpu_map_update_elem()

[Kohei Enju <enjuk@amazon.com>]

On Tue, 2 Dec 2025 17:08:32 -0800, Alexei Starovoitov wrote:

>On Fri, Nov 28, 2025 at 8:05 AM Kohei Enju <enjuk@amazon.com> wrote:
>>
>> After commit 9216477449f3 ("bpf: cpumap: Add the possibility to attach
>> an eBPF program to cpumap"), __cpu_map_entry_alloc() may fail with
>> errors other than -ENOMEM, such as -EBADF or -EINVAL.
>>
>> However, __cpu_map_entry_alloc() returns NULL on all failures, and
>> cpu_map_update_elem() unconditionally converts this NULL into -ENOMEM.
>> As a result, user space always receives -ENOMEM regardless of the actual
>> underlying error.
>>
>> Examples of unexpected behavior:
>>   - Nonexistent fd  : -ENOMEM (should be -EBADF)
>>   - Non-BPF fd      : -ENOMEM (should be -EINVAL)
>>   - Bad attach type : -ENOMEM (should be -EINVAL)
>>
>> Change __cpu_map_entry_alloc() to return ERR_PTR(err) instead of NULL
>> and have cpu_map_update_elem() propagate this error.
>>
>> Fixes: 9216477449f3 ("bpf: cpumap: Add the possibility to attach an eBPF program to cpumap")
>
>The current behavior is what it is. It's not a bug and
>this patch is not a fix. It's probably an ok improvement,
>but since it changes user visible behavior we have to be careful.

Oops, got it.
When I resend, I'll remove the tag and send to bpf-next, not to bpf.

Thank you for taking a look.

>
>I'd like Jesper and/or other cpumap experts to confirm that it's ok.
>

Sure, I'd like to wait for reactions from cpumap experts.