netdev - Re: [PATCH 11/18] maccess: remove strncpy_from

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Date:   Thu, 14 May 2020 01:58:33 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-parisc@...r.kernel.org,
        linux-um <linux-um@...ts.infradead.org>,
        Netdev <netdev@...r.kernel.org>, bpf@...r.kernel.org,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        bgregg@...flix.com
Subject: Re: [PATCH 11/18] maccess: remove strncpy_from_unsafe

On 5/14/20 1:28 AM, Al Viro wrote:
> On Thu, May 14, 2020 at 12:36:28AM +0200, Daniel Borkmann wrote:
> 
>>> So on say s390 TASK_SIZE_USUALLy is (-PAGE_SIZE), which means we'd alway
>>> try the user copy first, which seems odd.
>>>
>>> I'd really like to here from the bpf folks what the expected use case
>>> is here, and if the typical argument is kernel or user memory.
>>
>> It's used for both. Given this is enabled on pretty much all program types, my
>> assumption would be that usage is still more often on kernel memory than user one.
> 
> Then it needs an argument telling it which one to use.  Look at sparc64.
> Or s390.  Or parisc.  Et sodding cetera.
> 
> The underlying model is that the kernel lives in a separate address space.
> Yes, on x86 it's actually sharing the page tables with userland, but that's
> not universal.  The same address can be both a valid userland one _and_
> a valid kernel one.  You need to tell which one do you want.

Yes, see also 6ae08ae3dea2 ("bpf: Add probe_read_{user, kernel} and probe_read_{user,
kernel}_str helpers"), and my other reply wrt bpf_trace_printk() on how to address
this. All I'm trying to say is that both bpf_probe_read() and bpf_trace_printk() do
exist in this form since early [e]bpf days for ~5yrs now and while broken on non-x86
there are a lot of users on x86 for this in the wild, so they need to have a chance
to migrate over to the new facilities before they are fully removed.