netdev - Re: [PATCH bpf v1 1/2] bpf: cpumap: propagate underlying error in cpu_map_update

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Message-ID: <87jyz39272.fsf@toke.dk>
Date: Wed, 03 Dec 2025 13:31:29 +0100
From: Toke Høiland-Jørgensen <toke@...nel.org>
To: Jesper Dangaard Brouer <hawk@...nel.org>, Kohei Enju <enjuk@...zon.com>,
 alexei.starovoitov@...il.com
Cc: andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
 daniel@...earbox.net, davem@...emloft.net, eddyz87@...il.com,
 haoluo@...gle.com, john.fastabend@...il.com, jolsa@...nel.org,
 kohei.enju@...il.com, kpsingh@...nel.org, kuba@...nel.org,
 lorenzo@...nel.org, martin.lau@...ux.dev, netdev@...r.kernel.org,
 sdf@...ichev.me, shuah@...nel.org, song@...nel.org,
 yonghong.song@...ux.dev, kernel-team <kernel-team@...udflare.com>
Subject: Re: [PATCH bpf v1 1/2] bpf: cpumap: propagate underlying error in
 cpu_map_update_elem()

Jesper Dangaard Brouer <hawk@...nel.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@...zon.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