Re: [PATCH bpf-next 00/13] uprobes: Add support to optimize usdt probes on x86_64

Jiri Olsa
2024-12-13T13:07:58Z

On Fri, Dec 13, 2024 at 11:51:05AM +0100, Peter Zijlstra wrote:
> On Wed, Dec 11, 2024 at 02:33:49PM +0100, Jiri Olsa wrote:
> > hi,
> > this patchset adds support to optimize usdt probes on top of 5-byte
> > nop instruction.
> >
> > The generic approach (optimize all uprobes) is hard due to emulating
> > possible multiple original instructions and its related issues. The
> > usdt case, which stores 5-byte nop seems much easier, so starting
> > with that.
> >
> > The basic idea is to replace breakpoint exception with syscall which
> > is faster on x86_64. For more details please see changelog of patch 8.
>
> So ideally we'd put a check in the syscall, which verifies it comes from
> one of our trampolines and reject any and all other usage.
>
> The reason to do this is that we can then delete all this code the
> moment it becomes irrelevant without having to worry userspace might be
> 'creative' somewhere.

yes, we do that already in SYSCALL_DEFINE0(uprobe):

/* Allow execution only from uprobe trampolines. */
vma = vma_lookup(current->mm, regs->ip);
if (!vma || vma->vm_private_data != (void *) &tramp_mapping) {
force_sig(SIGILL);
return -1;
}

jirka