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SubjectRe: [RFC][PATCH 1/3] x86_64: Add gap to int3 to allow for call emulation
On Tue, May 07, 2019 at 01:42:28PM -0400, Steven Rostedt wrote:
> From: Josh Poimboeuf <jpoimboe@redhat.com>
>
> To allow an int3 handler to emulate a call instruction, it must be able to
> push a return address onto the stack. Add a gap to the stack to allow the
> int3 handler to push the return address and change the return from int3 to
> jump straight to the emulated called function target.
>
> Link: http://lkml.kernel.org/r/20181130183917.hxmti5josgq4clti@treble
> Link: http://lkml.kernel.org/r/20190502162133.GX2623@hirez.programming.kicks-ass.net
>
> [
> Note, this is needed to allow Live Kernel Patching to not miss calling a
> patched function when tracing is enabled. -- Steven Rostedt
> ]
>
> Cc: stable@vger.kernel.org
> Fixes: b700e7f03df5 ("livepatch: kernel: add support for live patching")
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
> arch/x86/entry/entry_64.S | 18 ++++++++++++++++--
> 1 file changed, 16 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 1f0efdb7b629..00df6b135ab1 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -879,7 +879,7 @@ apicinterrupt IRQ_WORK_VECTOR irq_work_interrupt smp_irq_work_interrupt
> * @paranoid == 2 is special: the stub will never switch stacks. This is for
> * #DF: if the thread stack is somehow unusable, we'll still get a useful OOPS.
> */
> -.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1
> +.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1 create_gap=0
> ENTRY(\sym)
> UNWIND_HINT_IRET_REGS offset=\has_error_code*8
>
> @@ -899,6 +899,20 @@ ENTRY(\sym)
> jnz .Lfrom_usermode_switch_stack_\@
> .endif
>
> + .if \create_gap == 1
> + /*
> + * If coming from kernel space, create a 6-word gap to allow the static
> + * call #BP handler to emulate a call instruction.

Might as well refer to it as the int3 handler, since that's what the
rest of the code calls it. Also, no static calls yet :-) So:

s/static call #BP handler/int3 handler/

--
Josh

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