Re: [dm-devel] [PATCH 0/3] block: blk_interposer - Block Layer Interposer

[Sergei Shtepa <sergei.shtepa@veeam.com>]

The 12/15/2020 09:51, Bob Liu wrote:
> Hi Folks,
> 
> On 12/12/20 12:56 AM, Hannes Reinecke wrote:
> > On 12/11/20 5:33 PM, Jens Axboe wrote:
> >> On 12/11/20 9:30 AM, Mike Snitzer wrote:
> >>> While I still think there needs to be a proper _upstream_ consumer of
> >>> blk_interposer as a condition of it going in.. I'll let others make the
> >>> call.
> >>
> >> That's an unequivocal rule.
> >>
> >>> As such, I'll defer to Jens, Christoph and others on whether your
> >>> minimalist blk_interposer hook is acceptable in the near-term.
> >>
> >> I don't think so, we don't do short term bandaids just to plan on
> >> ripping that out when the real functionality is there. IMHO, the dm
> >> approach is the way to go - it provides exactly the functionality that
> >> is needed in an appropriate way, instead of hacking some "interposer"
> >> into the core block layer.
> >>
> > Which is my plan, too.
> > 
> > I'll be working with the Veeam folks to present a joint patchset (including the DM bits) for the next round.
> > 
> 
> Besides the dm approach, do you think Veeam's original requirement is a good
> use case of "block/bpf: add eBPF based block layer IO filtering"?
> https://lwn.net/ml/bpf/20200812163305.545447-1-leah.rumancik@gmail.com/
> 
> Thanks,
> Bob

Hi Bob.
Thank you for your message.

I looked at your patch - it's interesting, but I have a few comments.

1.
#ifdef CONFIG_BPF_IO_FILTER
	struct bpf_prog_array __rcu *progs;
	struct mutex io_filter_lock;
#endif
For DM and blk-snap to work, it is necessary that there is always
the possibility of interception.

2.
We could get rid of the io_filter_lock lock if we do attach/detach while
the queue is stopped. And __rcu for *progs, can not use either.

3.
int io_filter_bpf_run(struct bio *bio)
{
	struct bpf_io_request io_req = {
		.sector_start = bio->bi_iter.bi_sector,
		.sector_cnt = bio_sectors(bio),
		.opf = bio->bi_opf,
	};
	return BPF_PROG_RUN_ARRAY_CHECK(bio->bi_disk->progs, &io_req, BPF_PROG_RUN);
}
Allows to get little information. It will not allow to work with the bios`s data.
blk_interposer allows to get full access to bio.
For use in the DM, we must also be able to add new bio's.

Summary:
For device-mapper purposes, bpf_io_filter is not suitable.
bpf_io_filter in this form is probably convenient to use for monitoring and
studying the block layer.
For the security task, I would suggest making a separate module and using
blk_interposer to intercept bio requests. This will give more flexible
functionality and better performance.

--
Sergei Shtepa
Veeam Software developer.

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