netdev - Re: clean up and streamline probe_kernel

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Date:   Thu, 14 May 2020 01:04:38 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     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>
Cc:     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 5/13/20 6:00 PM, Christoph Hellwig wrote:
> Hi all,
> 
> this series start cleaning up the safe kernel and user memory probing
> helpers in mm/maccess.c, and then allows architectures to implement
> the kernel probing without overriding the address space limit and
> temporarily allowing access to user memory.  It then switches x86
> over to this new mechanism by reusing the unsafe_* uaccess logic.
> 
> This version also switches to the saner copy_{from,to}_kernel_nofault
> naming suggested by Linus.
> 
> I kept the x86 helprs as-is without calling unsage_{get,put}_user as
> that avoids a number of hard to trace casts, and it will still work
> with the asm-goto based version easily.

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?

Thanks,
Daniel