Re: [PATCH 11/18] maccess: remove strncpy_from_unsafe

[Daniel Borkmann]

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.