linux-kernel - Re: [PATCH bpf-next v3 4/5] bpf: Add support for writing to nf

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Message-ID: <CAP01T76zWax4YSQO5nP2Kt_85JvUPvxTwpOn5Dho6co32r+FBA@mail.gmail.com>
Date:   Sat, 20 Aug 2022 01:52:44 +0200
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Daniel Xu <dxu@...uu.xyz>
Cc:     bpf@...r.kernel.org, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org, pablo@...filter.org, fw@...len.de,
        toke@...nel.org, martin.lau@...ux.dev,
        netfilter-devel@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 4/5] bpf: Add support for writing to nf_conn:mark

On Sat, 20 Aug 2022 at 01:46, Kumar Kartikeya Dwivedi <memxor@...il.com> wrote:
>
> CPU 0                                              CPU 1
> sa = READ_ONCE(nf_ct_bsa);
>
> delete_module("nf_conntrack", ..);
>
> WRITE_ONCE(nf_ct_bsa, NULL);
>                                                          // finishes
> successfully
> if (sa)
>     return sa(...); // oops
>

Ew, I completely screwed it up. Not trying again.

CPU 0 does:
sa = READ_ONCE(nf_ct_bsa);
then CPU 1 does:
delete_module("nf_conntrack", ...);
   WRITE_ONCE(nf_ct_bsa, NULL);
then CPU 0 does:
if (sa) sa(...); // bad