Re: [RFC 00/11] uprobes: Add support to optimize usdt probes on x86_64

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Nov 18, 2024 at 10:13:04PM -0800, Andrii Nakryiko wrote:
> On Mon, Nov 18, 2024 at 2:06 AM Mark Rutland <mark.rutland@arm.com> wrote:
> > Yep, on arm64 we definitely can't patch in branches reliably; using BRK
> > (as we do today) is the only reliable option, and it *shouldn't* be
> > slower than a syscall.
> >
> > Looking around, we have a different latent issue with uprobes on arm64
> > in that only certain instructions can be modified while being
> > concurrently executed (in addition to the atomictiy of updating the
> 
> What does this mean for the application in practical terms? Will it
> crash? Or will there be some corruption? Just curious how this can
> manifest.

It can result in a variety of effects including crashes, corruption of
memory, registers, issuing random syscalls, etc.

The ARM ARM (ARM DDI 0487K.a [1]) says in section B2.2.5:

  Concurrent modification and execution of instructions can lead to the
  resulting instruction performing any behavior that can be achieved by
  executing any sequence of instructions that can be executed from the
  same Exception level [...]

Which is to say basically anything might happen, except that this can't
corrupt any state userspace cannot access, and cannot provide a
mechanism to escalate privilege to a higher exception level.

So that's potentially *very bad*, and we're just getting lucky that most
implementations don't happen to do that for most instructions, though
I'm fairly certain there are implementations out there which do exhibit
this behaviour (and it gets more likely as implementations get more
aggressive).

Mark.

[1] https://developer.arm.com/documentation/ddi0487/ka/?lang=en