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

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Message-ID: <CAADnVQLjw=iv3tDb8UadT_ahm_xuAFSQ6soG-W=eVPEjO_jGZw@mail.gmail.com>
Date: Tue, 2 Dec 2025 17:08:32 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Kohei Enju <enjuk@...zon.com>
Cc: Network Development <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend <john.fastabend@...il.com>, 
	Stanislav Fomichev <sdf@...ichev.me>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, 
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, 
	Lorenzo Bianconi <lorenzo@...nel.org>, Shuah Khan <shuah@...nel.org>, kohei.enju@...il.com
Subject: Re: [PATCH bpf v1 1/2] bpf: cpumap: propagate underlying error in cpu_map_update_elem()

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.

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