netdev - Re: [PATCH bpf-next 11/15] bpf: Add bpf_sys

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Message-ID: <YHpeTKV2Y+sjuzbD@zeniv-ca.linux.org.uk>
Date:   Sat, 17 Apr 2021 04:04:28 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 11/15] bpf: Add bpf_sys_close() helper.

On Fri, Apr 16, 2021 at 08:46:05PM -0700, Alexei Starovoitov wrote:
> On Fri, Apr 16, 2021 at 8:42 PM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > On Fri, Apr 16, 2021 at 08:32:20PM -0700, Alexei Starovoitov wrote:
> > > From: Alexei Starovoitov <ast@...nel.org>
> > >
> > > Add bpf_sys_close() helper to be used by the syscall/loader program to close
> > > intermediate FDs and other cleanup.
> >
> > Conditional NAK.  In a lot of contexts close_fd() is very much unsafe.
> > In particular, anything that might call it between fdget() and fdput()
> > is Right Fucking Out(tm).
> > In which contexts can that thing be executed?
> 
> user context only.
> It's not for all of bpf _obviously_.

Let me restate the question: what call chains could lead to bpf_sys_close()?