linux-kernel - Re: [PATCH bpf-next v11 4/8] bpf: Add BPF_F_CPU and BPF_F_ALL_CPUS flags support for percpu_hash and lru_percpu

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <8e764e95-2a50-4a48-9b89-808334460c95@linux.dev>
Date: Wed, 26 Nov 2025 10:00:44 +0800
From: Leon Hwang <leon.hwang@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
 Andrii Nakryiko <andrii@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Jiri Olsa <jolsa@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
 Song Liu <song@...nel.org>, Eduard <eddyz87@...il.com>,
 Daniel Xu <dxu@...uu.xyz>, Daniel Müller <deso@...teo.net>,
 Martin KaFai Lau <martin.lau@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Shuah Khan <shuah@...nel.org>, Jason Xing <kerneljasonxing@...il.com>,
 Tao Chen <chen.dylane@...ux.dev>, Willem de Bruijn <willemb@...gle.com>,
 Paul Chaignon <paul.chaignon@...il.com>,
 Anton Protopopov <a.s.protopopov@...il.com>,
 Kumar Kartikeya Dwivedi <memxor@...il.com>,
 Mykyta Yatsenko <yatsenko@...a.com>, Tobias Klauser <tklauser@...tanz.ch>,
 kernel-patches-bot@...com, LKML <linux-kernel@...r.kernel.org>,
 "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH bpf-next v11 4/8] bpf: Add BPF_F_CPU and BPF_F_ALL_CPUS
 flags support for percpu_hash and lru_percpu_hash maps



On 26/11/25 07:11, Alexei Starovoitov wrote:
> On Tue, Nov 25, 2025 at 7:00 AM Leon Hwang <leon.hwang@...ux.dev> wrote:
>>

[...]
>> @@ -1342,7 +1360,7 @@ static long __htab_lru_percpu_map_update_elem(struct bpf_map *map, void *key,
>>          * to remove older elem from htab and this removal
>>          * operation will need a bucket lock.
>>          */
>> -       if (map_flags != BPF_EXIST) {
>> +       if (!(map_flags & BPF_EXIST)) {
>>                 l_new = prealloc_lru_pop(htab, key, hash);
>>                 if (!l_new)
>>                         return -ENOMEM;
> 
> It's not in the diff, but this is broken.
> You tried to allow BPF_EXIST combination here, but didn't update
> check_flags(),
> 
> so BPF_[NO]EXIST | BPF_F_CPU combination check_flags() will always
> return 0, so BPF_[NO]EXIST flag will make no difference.
> 
> When you add features, always always add unit tests.
> Patch 8 is not it. It's testing F_CPU. It doesn't check
> that BPF_EXIST | BPF_F_CPU correctly errors when an element doesn't exist.
> 
> v10 was close, but then you decided to add this BPF_EXIST feature
> and did it in a sloppy way. Why ?
> Focus on one thing only. Land it and then do the next one.
> 11 revisions and still no go... it is not a good sign.
> 

Yeah, you're right.

The intention of v11 was solely to address the unstable lru_percpu_hash
map test — not to introduce support for the BPF_EXIST combination.

Given that, the approach in v11 was not the right way to fix the
instability. I'll investigate the underlying cause first and then work
on a better fix.

Thanks,
Leon