Re: [PATCH bpf-next 08/13] uprobes/x86: Add support to optimize uprobes

[Oleg Nesterov <oleg@redhat.com>]

OK, thanks, I am starting to share your concerns...

Oleg.

On 12/16, David Laight wrote:
>
> From: Oleg Nesterov
> > Sent: 16 December 2024 10:13
> >
> > David,
> >
> > let me say first that my understanding of this magic is very limited,
> > please correct me.
>
> I only (half) understand what the 'magic' has to accomplish and
> some of the pitfalls.
>
> I've copied linux-mm - someone there might know more.
>
> > On 12/16, David Laight wrote:
> > >
> > > It all depends on how hard __replace_page() tries to be atomic.
> > > The page has to change from one backed by the executable to a private
> > > one backed by swap - otherwise you can't write to it.
> >
> > This is what uprobe_write_opcode() does,
>
> And will be enough for single byte changes - they'll be picked up
> at some point after the change.
>
> > > But the problems arise when the instruction prefetch unit has read
> > > part of the 5-byte instruction (it might even only read half a cache
> > > line at a time).
> > > I'm not sure how long the pipeline can sit in that state - but I
> > > can do a memory read of a PCIe address that takes ~3000 clocks.
> > > (And a misaligned AVX-512 read is probably eight 8-byte transfers.)
> > >
> > > So I think you need to force an interrupt while the PTE is invalid.
> > > And that need to be simultaneous on all cpu running that process.
> >
> > __replace_page() does ptep_get_and_clear(old_pte) + flush_tlb_page().
> >
> > That's not enough?
>
> I doubt it. As I understand it.
> The hardware page tables will be shared by all the threads of a process.
> So unless you hard synchronise all the cpu (and flush the TLB) while the
> PTE is being changed there is always the possibility of a cpu picking up
> the new PTE before the IPI that (I presume) flush_tlb_page() generates
> is processed.
> If that happens when the instruction you are patching is part-read into
> the instruction decode buffer then you'll execute a mismatch of the two
> instructions.
>
> I can't remember the outcome of discussions about live-patching kernel
> code - and I'm sure that was aligned 32bit writes.
>
> >
> > > Stopping the process using ptrace would do it.
> >
> > Not an option :/
>
> Thought you'd say that.
>
> 	David
>
> >
> > Oleg.
>
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