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Date:   Mon, 14 Mar 2022 23:10:33 +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>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        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 Mon, Mar 14, 2022 at 10:07:31AM -0700, Hao Luo wrote:
> Hello Al,

> > In which contexts can those be called?
> >
> 
> In a sleepable context. The plan is to introduce a certain tracepoints
> as sleepable, a program that attaches to sleepable tracepoints is
> allowed to call these functions. In particular, the first sleepable
> tracepoint introduced in this patchset is one at the end of
> cgroup_mkdir(). Do you have any advices?

Yes - don't do it, unless you really want a lot of user-triggerable
deadlocks.

Pathname resolution is not locking-agnostic.  In particular, you can't
do it if you are under any ->i_rwsem, whether it's shared or exclusive.
That includes cgroup_mkdir() callchains.  And if the pathname passed
to these functions will have you walk through the parent directory,
you would get screwed (e.g. if the next component happens to be
inexistent, triggering a lookup, which takes ->i_rwsem shared).