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

[Oleg Nesterov]

On 12/15, David Laight wrote:
>
> From: Jiri Olsa
> > The optimized uprobe path
> >
> >   - checks the original instruction is 5-byte nop (plus other checks)
> >   - adds (or uses existing) user space trampoline and overwrites original
> >     instruction (5-byte nop) with call to user space trampoline
> >   - the user space trampoline executes uprobe syscall that calls related uprobe
> >     consumers
> >   - trampoline returns back to next instruction
> ...
>
> How on earth can you safely overwrite a randomly aligned 5 byte instruction
> that might be being prefetched and executed by another thread of the
> same process.

uprobe_write_opcode() doesn't overwrite the instruction in place.

It creates the new page with the same content, overwrites the probed insn in
that page, then calls __replace_page().

Oleg.