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

["Toke Høiland-Jørgensen" <toke@kernel.org>]

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