linux-kernel - Re: [PATCH bpf-next v1 1/9] bpf: Add mkdir, rmdir, unlink syscalls for prog_bpf

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Message-ID: <Yi/LaZ5id4ZjqFmL@zeniv-ca.linux.org.uk>
Date:   Mon, 14 Mar 2022 23:10:33 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Hao Luo <haoluo@...gle.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Joe Burton <jevburton.kernel@...il.com>,
        Tejun Heo <tj@...nel.org>, joshdon@...gle.com, sdf@...gle.com,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v1 1/9] bpf: Add mkdir, rmdir, unlink syscalls
 for prog_bpf_syscall

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).