# Re: \[PATCH v3 4/5\] kcmp: add KCMP\_FILE\_PRIVATE\_DATA

- From: Tycho Andersen \<hidden\>
- Date: 2015-09-30 21:39:22 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2015/09/4587579
- Thread: https://ratatoskr.run/netdev/2015/09/4587407/t.md

```text
On Wed, Sep 30, 2015 at 11:56:25AM -0700, Andy Lutomirski wrote:
> On Wed, Sep 30, 2015 at 11:55 AM, Tycho Andersen
> [ref] 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
> >> [ref] 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
> >> >> [ref] 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
```

## Attachments

- [0001-use-exactly-one-seccomp-file-per-filter-object.patch](https://ratatoskr.run/linux-api/2015/09/4587579/attachment/0) (text/x-diff, 2488 bytes)