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

[David Laight <David.Laight@ACULAB.COM>]

From: Peter Zijlstra
> Sent: 07 May 2019 09:58
...
> +	/*
> +	 * When we're here from kernel mode; the (exception) stack looks like:
> +	 *
> +	 * 4*4(%esp) - <previous context>
> +	 * 3*4(%esp) - flags
> +	 * 2*4(%esp) - cs
> +	 * 1*4(%esp) - ip
> +	 * 0*4(%esp) - orig_eax

Am I right in thinking that this is the only 'INT3' stack frame that
needs to be 'fiddled' with?
And that the 'emulate a call instruction' has verified that is the case??
So the %cs is always the kernel %cs.

If the 'call target' address is saved in a per-cpu location it ought
to be possible to get the code that returns from the INT3 with the
call target address (or zero) in %ax.
If non-zero, instead of 'pop %ax; iret' execute:
	xchg %eax, 4(%esp)   - swap function address and callers ip
	push 12(%esp)        - old flags
	mov  14(%esp),%eax   - callers address over flags
	popf                 - enables interrupts (etc)
	pop  %eax
	retf                 - Jump to called function and remove %cs

Nothing else needs to be moved.

	David

	

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