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

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Message-ID: <CAADnVQLOZ7QL61_XPCSmxDfZ0OHX_pBOmpEWLjSUwqhLm_10Jw@mail.gmail.com>
Date:   Fri, 16 Apr 2021 22:01:43 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
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 9:04 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> 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()?

Already answered. User context only. It's all safe.