linux-kernel - Re: [PATCH v3 4/5] kcmp: add KCMP_FILE_PRIVATE

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Message-ID: <20150930213909.GD23065@smitten>
Date:	Wed, 30 Sep 2015 15:39:09 -0600
From:	Tycho Andersen <tycho.andersen@...onical.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Kees Cook <keescook@...omium.org>,
	Alexei Starovoitov <ast@...nel.org>,
	Will Drewry <wad@...omium.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	"Serge E. Hallyn" <serge.hallyn@...ntu.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v3 4/5] kcmp: add KCMP_FILE_PRIVATE_DATA

On Wed, Sep 30, 2015 at 11:56:25AM -0700, Andy Lutomirski wrote:
> On Wed, Sep 30, 2015 at 11:55 AM, Tycho Andersen
> <tycho.andersen@...onical.com> wrote:
> > On Wed, Sep 30, 2015 at 11:47:05AM -0700, Andy Lutomirski wrote:
> >> On Wed, Sep 30, 2015 at 11:41 AM, Tycho Andersen
> >> <tycho.andersen@...onical.com> wrote:
> >> > On Wed, Sep 30, 2015 at 11:25:41AM -0700, Andy Lutomirski wrote:
> >> >> On Wed, Sep 30, 2015 at 11:13 AM, Tycho Andersen
> >> >> <tycho.andersen@...onical.com> wrote:
> >> >> > This command allows comparing the underling private data of two fds. This
> >> >> > is useful e.g. to find out if a seccomp filter is inherited, since struct
> >> >> > seccomp_filter are unique across tasks and are the private_data seccomp
> >> >> > fds.
> >> >>
> >> >> This is very implementation-specific and may have nasty ABI
> >> >> consequences far outside seccomp.  Let's do something specific to
> >> >> seccomp and/or eBPF.
> >> >
> >> > We could change the name to a less generic KCMP_SECCOMP_FD or
> >> > something, but without some sort of GUID on each struct
> >> > seccomp_filter, the implementation would be effectively the same as it
> >> > is today. Is that enough, or do we need a GUID?
> >> >
> >>
> >> I don't care about the GUID.  I think we should name it
> >> KCMP_SECCOMP_FD and make it only work on seccomp fds.
> >
> > Ok, I can do that.
> >
> >> Alternatively, we could figure out why KCMP_FILE doesn't do the trick
> >> and consider fixing it.  IMO it's really too bad that struct file is
> >> so heavyweight that we can't really just embed one in all kinds of
> >> structures.
> >
> > The problem is that KCMP_FILE compares the file objects themselves,
> > instead of the underlying data. If I ask for a seccomp fd for filter 0
> > twice, I'll have two different file objects and they won't be equal. I
> > suppose we could add some special logic inside KCMP_FILE to compare
> > the underlying data in special cases (seccomp, ebpf, others?), but it
> > seems cleaner to have a separate command as you described above.
> >
> 
> What I meant was that maybe we could get the two requests to actually
> produce the same struct file.  But that could get very messy
> memory-wise.

I see. The attached patch seems to work with KCMP_FILE and doesn't
look too bad if you don't mind the circular references. What do you
think?

Tycho

View attachment "0001-use-exactly-one-seccomp-file-per-filter-object.patch" of type "text/x-diff" (2489 bytes)