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

[Kohei Enju <enjuk@amazon.com>]

On Wed, 03 Dec 2025 13:31:29 +0100, Toke Høiland-Jørgensen wrote:

>Jesper Dangaard Brouer <hawk@kernel.org> writes:
>
>> On 03/12/2025 11.40, Kohei Enju wrote:
>>> 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.
>>
>> Skimmed the code changes[1] and they look good to me :-)
>
>We have one example of a use of the cpumap programs in xdp-tools, and
>there we just report the error message to the user. I would guess other
>apps would follow the same pattern rather than react to a specific error
>code; especially since there's only one error code being used here.
>
>So I agree, this should be OK to change.
>
>-Toke

Thank you for the clarification, Toke and Jesper.
Since I see no objections so far, I'll work on v2 and resend next week.