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

Oleg Nesterov
2024-12-15T14:14:51Z

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.