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

[David Laight]

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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