From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Hao Luo <haoluo@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
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>, Josh Don <joshdon@google.com>,
Stanislav Fomichev <sdf@google.com>, bpf <bpf@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH bpf-next v1 1/9] bpf: Add mkdir, rmdir, unlink syscalls for prog_bpf_syscall
Date: Tue, 15 Mar 2022 11:59:36 -0700 [thread overview]
Message-ID: <CAADnVQJSMqcv3GPmUDcNKs23veqU-HWQQA5ECqdriMtjTpdv3w@mail.gmail.com> (raw)
In-Reply-To: <CA+khW7jhD0+s9kivrd6PsNEaxmDCewhk_egrsxwdHPZNkubJYA@mail.gmail.com>
On Tue, Mar 15, 2022 at 10:27 AM Hao Luo <haoluo@google.com> wrote:
>
> On Mon, Mar 14, 2022 at 4:12 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > 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).
>
> I'm thinking of two options, let's see if either can work out:
>
> 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().
>
> Option 2: We can avoid pathname resolution entirely. Like above, we
> can adjust the semantics of this helper to be: making an immediate
> directory under the dirfd passed in. In particular, like above, we can
> enforce the dirfd to be in bpffs and pathname to consist of only
> alphabet and numbers. With these restrictions, we call vfs_mkdir() to
> create directories.
>
> Being able to mkdir from bpf has useful use cases, let's try to make
> it happen even with many limitations.
Option 3. delegate vfs_mkdir to a worker and wait in the helper.
next prev parent reply other threads:[~2022-03-15 18:59 UTC|newest]
Thread overview: 54+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-25 23:43 [PATCH bpf-next v1 0/9] Extend cgroup interface with bpf Hao Luo
2022-02-25 23:43 ` [PATCH bpf-next v1 1/9] bpf: Add mkdir, rmdir, unlink syscalls for prog_bpf_syscall Hao Luo
2022-02-27 5:18 ` Kumar Kartikeya Dwivedi
2022-02-28 22:10 ` Hao Luo
2022-03-02 19:34 ` Alexei Starovoitov
2022-03-03 18:50 ` Hao Luo
2022-03-04 18:37 ` Hao Luo
2022-03-05 23:47 ` Alexei Starovoitov
2022-03-08 21:08 ` Hao Luo
2022-03-02 20:55 ` Yonghong Song
2022-03-03 18:56 ` Hao Luo
2022-03-03 19:13 ` Yonghong Song
2022-03-03 19:15 ` Hao Luo
2022-03-12 3:46 ` Al Viro
2022-03-14 17:07 ` Hao Luo
2022-03-14 23:10 ` Al Viro
2022-03-15 17:27 ` Hao Luo
2022-03-15 18:59 ` Alexei Starovoitov [this message]
2022-03-15 19:03 ` Alexei Starovoitov
2022-03-15 19:00 ` Al Viro
2022-03-15 19:47 ` Hao Luo
2022-02-25 23:43 ` [PATCH bpf-next v1 2/9] bpf: Add BPF_OBJ_PIN and BPF_OBJ_GET in the bpf_sys_bpf helper Hao Luo
2022-02-25 23:43 ` [PATCH bpf-next v1 3/9] selftests/bpf: tests mkdir, rmdir, unlink and pin in syscall Hao Luo
2022-02-25 23:43 ` [PATCH bpf-next v1 4/9] bpf: Introduce sleepable tracepoints Hao Luo
2022-03-02 19:41 ` Alexei Starovoitov
2022-03-03 19:37 ` Hao Luo
2022-03-03 19:59 ` Alexei Starovoitov
2022-03-02 21:23 ` Yonghong Song
2022-03-02 21:30 ` Alexei Starovoitov
2022-03-03 1:08 ` Yonghong Song
2022-03-03 2:29 ` Alexei Starovoitov
2022-03-03 19:43 ` Hao Luo
2022-03-03 20:02 ` Alexei Starovoitov
2022-03-03 20:04 ` Alexei Starovoitov
2022-03-03 22:06 ` Hao Luo
2022-02-25 23:43 ` [PATCH bpf-next v1 5/9] cgroup: Sleepable cgroup tracepoints Hao Luo
2022-02-25 23:43 ` [PATCH bpf-next v1 6/9] libbpf: Add sleepable tp_btf Hao Luo
2022-02-25 23:43 ` [PATCH bpf-next v1 7/9] bpf: Lift permission check in __sys_bpf when called from kernel Hao Luo
2022-03-02 20:01 ` Alexei Starovoitov
2022-03-03 19:14 ` Hao Luo
2022-02-25 23:43 ` [PATCH bpf-next v1 8/9] bpf: Introduce cgroup iter Hao Luo
2022-02-26 2:32 ` kernel test robot
2022-02-26 2:32 ` kernel test robot
2022-02-26 2:53 ` kernel test robot
2022-03-02 21:59 ` Yonghong Song
2022-03-03 20:02 ` Hao Luo
2022-03-02 22:45 ` Kumar Kartikeya Dwivedi
2022-03-03 2:03 ` Yonghong Song
2022-03-03 3:03 ` Kumar Kartikeya Dwivedi
2022-03-03 4:00 ` Alexei Starovoitov
2022-03-03 7:33 ` Yonghong Song
2022-03-03 8:13 ` Kumar Kartikeya Dwivedi
2022-03-03 21:52 ` Hao Luo
2022-02-25 23:43 ` [PATCH bpf-next v1 9/9] selftests/bpf: Tests using sleepable tracepoints to monitor cgroup events Hao Luo
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