Re: [RFC][PATCH 1/2] x86: Allow breakpoints to emulate call functions

From: Josh Poimboeuf
Date: Mon May 06 2019 - 11:15:22 EST

On Thu, May 02, 2019 at 11:02:40AM -0700, Linus Torvalds wrote:
> On Thu, May 2, 2019 at 9:21 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > TL;DR, on x86_32 kernel->kernel IRET frames are only 3 entries and do
> > not include ESP/SS, so not only wasn't regs->sp setup, if you changed it
> > it wouldn't be effective and corrupt random stack state.
> Indeed, the 32-bit case for same-RPL exceptions/iret is entirely
> different, and I'd forgotten about that.
> And honestly, this makes the 32-bit case much worse. Now the entry
> stack modifications of int3 suddenly affect not just the entry, but
> every exit too.
> This is _exactly_ the kind of subtle kernel entry/exit code I wanted
> us to avoid.

I actually love this patch (absent the bugs). This is already something
that has been sorely needed for years.

The "struct pt_regs is incomplete on x86-32" thing is a monstrosity
which has long been a source of confusion and bugs. Sure, this patch
adds some complexity to the entry code, but on the other hand it
actually makes it possible to use pt_regs sanely: regs->sp is no longer
uninitialized. So a class of (very non-obvious) bugs is eliminated.

I don't think it would make sense to make this change for int3 only,
because the benefits are global.