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

[Peter Zijlstra]

On Fri, Dec 13, 2024 at 02:07:54PM +0100, Jiri Olsa wrote:
> 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;
>         }

Ah, right I missed that. Doesn't that need more locking through? The
moment vma_lookup() returns that vma can go bad.