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From:   KP Singh <kpsingh@chromium.org>
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Date:   Fri, 17 Jan 2020 23:16:03 +0100
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc:     open list <linux-kernel@vger.kernel.org>,
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Subject: Re: [PATCH bpf-next v2 08/10] tools/libbpf: Add support for
 BPF_PROG_TYPE_LSM
Message-ID: <20200117221603.GA25978@chromium.org>
References: <20200115171333.28811-1-kpsingh@chromium.org>
 <20200115171333.28811-9-kpsingh@chromium.org>
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 <20200116124955.GD240584@google.com>
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On 16-Jan 11:10, Andrii Nakryiko wrote:
> On Thu, Jan 16, 2020 at 4:49 AM KP Singh <kpsingh@chromium.org> wrote:
> >
> > Thanks for the review Andrii!
> >
> > I will incorporate the fixes in the next revision.
> >
> > On 15-Jan 13:19, Andrii Nakryiko wrote:
> > > On Wed, Jan 15, 2020 at 9:13 AM KP Singh <kpsingh@chromium.org> wrote:
> > > >
> > > > From: KP Singh <kpsingh@google.com>
> > > >
> > > > * Add functionality in libbpf to attach eBPF program to LSM hooks
> > > > * Lookup the index of the LSM hook in security_hook_heads and pass it in
> > > >   attr->lsm_hook_index
> > > >
> > > > Signed-off-by: KP Singh <kpsingh@google.com>
> > > > ---
> > > >  tools/lib/bpf/bpf.c      |   6 +-
> > > >  tools/lib/bpf/bpf.h      |   1 +
> > > >  tools/lib/bpf/libbpf.c   | 143 ++++++++++++++++++++++++++++++++++-----
> > > >  tools/lib/bpf/libbpf.h   |   4 ++
> > > >  tools/lib/bpf/libbpf.map |   3 +
> > > >  5 files changed, 138 insertions(+), 19 deletions(-)
> > > >
> 
> [...]
> 
> > >
> > > > +{
> > > > +       struct btf *btf = bpf_find_kernel_btf();
> > >
> > > ok, it's probably time to do this right. Let's ensure we load kernel
> > > BTF just once, keep it inside bpf_object while we need it and then
> > > release it after successful load. We are at the point where all the
> > > new types of program is loading/releasing kernel BTF for every section
> > > and it starts to feel very wasteful.
> >
> > Sure, will give it a shot in v3.
> 
> thanks!
> 
> [...]
> 
> > >
> > > > +               if (!strcmp(btf__name_by_offset(btf, m->name_off), name))
> > > > +                       return j + 1;
> > >
> > > I looked briefly through kernel-side patch introducing lsm_hook_index,
> > > but it didn't seem to explain why this index needs to be (unnaturally)
> > > 1-based. So asking here first as I'm looking through libbpf changes?
> >
> > The lsm_hook_idx is one-based as it makes it easy to validate the
> > input. If we make it zero-based it's hard to check if the user
> > intended to attach to the LSM hook at index 0 or did not set it.
> 
> Think about providing FDs. 0 is a valid, though rarely
> intended/correct value. Yet we don't make all FD arguments
> artificially 1-based, right? This extra +1/-1 translation just makes
> for more confusing interface, IMO. If user "accidentally" guessed type
> signature of very first hook, well, so be it... If not, BPF verifier
> will politely refuse. Seems like enough protection.

Thanks! I see your point and will update to using the
more-conventional 0-based indexing for the next revision.

- KP

> 
> >
> > We are then up to the verifier to reject the loaded program which
> > may or may not match the signature of the hook at lsm_hook_idx = 0.
> >
> > I will clarify this in the commit log as well.
> >
> 
> [...]