linux-kernel - Re: clean up and streamline probe_kernel

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Date:   Tue, 19 May 2020 07:50:09 +0200
From:   Christoph Hellwig <hch@....de>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Christoph Hellwig <hch@....de>, x86@...nel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-parisc@...r.kernel.org, linux-um@...ts.infradead.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: clean up and streamline probe_kernel_* and friends v2

On Thu, May 14, 2020 at 01:04:38AM +0200, Daniel Borkmann wrote:
> Aside from comments on list, the series looks reasonable to me. For BPF
> the bpf_probe_read() helper would be slightly penalized for probing user
> memory given we now test on copy_from_kernel_nofault() first and if that
> fails only then fall back to copy_from_user_nofault(), but it seems
> small enough that it shouldn't matter too much and aside from that we have
> the newer bpf_probe_read_kernel() and bpf_probe_read_user() anyway that
> BPF progs should use instead, so I think it's okay.
>
> For patch 14 and patch 15, do you roughly know the performance gain with
> the new probe_kernel_read_loop() + arch_kernel_read() approach?

I don't think there should be any measurable difference in performance
for typical use cases.  We'll save the stac/clac pair, but that's it.
The real eason is to avoid that stac/clac pair that opens up a window
for explots, and as a significant enabler for killing of set_fs based 
address limit overrides entirely.