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

Jiri Olsa
2024-12-16T08:09:01Z

On Sun, Dec 15, 2024 at 03:14:13PM +0100, Oleg Nesterov wrote:
> 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().

tbh I wasn't completely sure about that as well, I added selftest
in patch #11 trying to hit the issue you described and it seems to
work ok

jirka