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

[Steven Rostedt]

On Tue, 7 May 2019 12:57:15 +0000
David Laight <David.Laight@xxxxxxxxxx> wrote:

> > Only the INT3 thing needs 'the gap', but the far bigger change here is
> > that kernel frames now have a complete pt_regs set and all sorts of
> > horrible crap can go away.  
> 
> I'm not doubting that generating the 'five register' interrupt stack frame
> for faults in kernel space makes life simpler just suggesting that the
> 'emulated call' can be done by emulating the 'iret' rather than generating
> a gap in the stack.

But how would the user put something on the stack? I don't see how
emulating an iret helps here. Can you write some pseudo code to explain
what you mean. I also believe the gap is only added for kernel->kernel
entries.

> 
> > For 32bit 'the gap' happens naturally when building a 5 entry frame. Yes
> > it is possible to build a 5 entry frame on top of the old 3 entry one,
> > but why bother...  
> 
> Presumably there is 'horrid' code to generate the gap in 64bit mode?
> (less horrid than 32bit, but still horrid?)
> Or does it copy the entire pt_regs into a local stack frame and use
> that for the iret?

On x86_64, the gap is only done for int3 and nothing else, thus it is
much less horrid. That's because x86_64 has a sane pt_regs storage for
all exceptions.

-- Steve