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Date:   Tue, 15 Mar 2022 19:00:17 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Hao Luo <haoluo@google.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
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        KP Singh <kpsingh@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Joe Burton <jevburton.kernel@gmail.com>,
        Tejun Heo <tj@kernel.org>, joshdon@google.com, sdf@google.com,
        bpf@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH bpf-next v1 1/9] bpf: Add mkdir, rmdir, unlink syscalls
 for prog_bpf_syscall
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On Tue, Mar 15, 2022 at 10:27:39AM -0700, Hao Luo wrote:

> Option 1: We can put restrictions on the pathname passed into this
> helper. We can explicitly require the parameter dirfd to be in bpffs
> (we can verify). In addition, we check pathname to be not containing
> any dot or dotdot, so the resolved path will end up inside bpffs,
> therefore won't take ->i_rwsem that is in the callchain of
> cgroup_mkdir().

Won't be enough - mount --bind the parent under itself and there you go...
Sure, you could prohibit mountpoint crossing, etc., but at that point
I'd question the usefulness of pathname resolution in the first place.