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

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Message-ID: <20251203104037.40660-1-enjuk@amazon.com>
Date: Wed, 3 Dec 2025 19:40:34 +0900
From: Kohei Enju <enjuk@...zon.com>
To: <alexei.starovoitov@...il.com>
CC: <andrii@...nel.org>, <ast@...nel.org>, <bpf@...r.kernel.org>,
	<daniel@...earbox.net>, <davem@...emloft.net>, <eddyz87@...il.com>,
	<enjuk@...zon.com>, <haoluo@...gle.com>, <hawk@...nel.org>,
	<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>
Subject: Re: [PATCH bpf v1 1/2] bpf: cpumap: propagate underlying error in cpu_map_update_elem()

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.