Re: [PATCH v3 4/5] kcmp: add KCMP_FILE_PRIVATE_DATA

[Tycho Andersen <tycho.andersen@canonical.com>]

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@canonical.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@canonical.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.

Tycho