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

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 3 May 2019 11:29:59 +0200
Peter Zijlstra <peterz@infradead.org> wrote:


> OMG, WTF, ARGH... That code is fsck'ing horrible. I'd almost argue to
> always do the INT3 thing, just to avoid games like that.

Hehe, that's almost the exact same thoughts I had when seeing this
code ;-)

> 
> That said; for normal traps &regs->sp is indeed the previous context --
> if it doesn't fall off the stack. Your hack detects the regular INT3
> frame. Howver if regs->sp has been modified (int3_emulate_push, for
> example) your detectoring comes unstuck.

Yep. I realized the issue as well. But wanted to make sure this did
work when sp wasn't changed.

> 
> Now, it is rather unlikely these two code paths interact, but just to be
> safe, something like so might be more reliable:
> 
> 
> diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
> index 4b8ee05dd6ad..aceaad0cc9a9 100644
> --- a/arch/x86/kernel/ptrace.c
> +++ b/arch/x86/kernel/ptrace.c
> @@ -163,6 +163,9 @@ static inline bool invalid_selector(u16 value)
>   * stack pointer we fall back to regs as stack if no previous stack
>   * exists.
>   *
> + * There is a special case for INT3, there we construct a full pt_regs
> + * environment. We can detect this case by a high bit in regs->cs
> + *
>   * This is valid only for kernel mode traps.
>   */
>  unsigned long kernel_stack_pointer(struct pt_regs *regs)
> @@ -171,6 +174,9 @@ unsigned long kernel_stack_pointer(struct pt_regs *regs)
>  	unsigned long sp = (unsigned long)&regs->sp;
>  	u32 *prev_esp;
>  
> +	if (regs->__csh & (1 << 13)) /* test CS_FROM_INT3 */
> +		return regs->sp;
> +

Thanks, I was looking into doing something like this (setting a flag in
the int3 code), but didn't have the time to see the best way to do this.

I'll add this version of the code and run it through my tests.

-- Steve

>  	if (context == (sp & ~(THREAD_SIZE - 1)))
>  		return sp;
>  
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -388,6 +388,7 @@
>  
>  #define CS_FROM_ENTRY_STACK	(1 << 31)
>  #define CS_FROM_USER_CR3	(1 << 30)
> +#define CS_FROM_INT3		(1 << 29)
>  
>  .macro SWITCH_TO_KERNEL_STACK
>  
> @@ -1515,6 +1516,9 @@ ENTRY(int3)
>  
>  	add	$16, 12(%esp) # point sp back at the previous context
>  
> +	andl	$0x0000ffff, 4(%esp)
> +	orl	$CS_FROM_INT3, 4(%esp)
> +
>  	pushl	$-1				# orig_eax; mark as interrupt
>  
>  	SAVE_ALL