Re: [PATCH 2/4] x86/kprobes: Fix frame pointer annotations

[Peter Zijlstra]

On Thu, May 09, 2019 at 11:01:06PM +0900, Masami Hiramatsu wrote:
> On Thu, 9 May 2019 10:14:31 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> > But what I'd love to do is something like the belwo patch, and make all
> > the trampolines (very much including ftrace) use that. Such that we then
> > only have 1 copy of this magic (well, 2 because x86_64 also needs an
> > implementation of this of course).
> 
> OK, but I will make kretprobe integrated with func-graph tracer,
> since it is inefficient that we have 2 different hidden return stack...
> 
> Anyway,
> 
> > Changing ftrace over to this would be a little more work but it can
> > easily chain things a little to get its original context back:
> > 
> > ENTRY(ftrace_regs_caller)
> > GLOBAL(ftrace_regs_func)
> > 	push ftrace_stub
> > 	push ftrace_regs_handler
> > 	jmp call_to_exception_trampoline
> > END(ftrace_regs_caller)
> > 
> > typedef void (*ftrace_func_t)(unsigned long, unsigned long, struct ftrace_op *, struct pt_regs *);
> > 
> > struct ftrace_regs_stack {
> > 	ftrace_func_t func;
> > 	unsigned long parent_ip;
> > };
> > 
> > void ftrace_regs_handler(struct pr_regs *regs)
> > {
> > 	struct ftrace_regs_stack *st = (void *)regs->sp;
> > 	ftrace_func_t func = st->func;
> > 
> > 	regs->sp += sizeof(long); /* pop func */
> 
> Sorry, why pop here? 

Otherwise it stays on the return stack and bad things happen. Note how
the below trampoline thing uses regs->sp.

> > 	func(regs->ip, st->parent_ip, function_trace_op, regs);
> > }
> > 
> > Hmm? I didn't look into the function_graph thing, but I imagine it can
> > be added without too much pain.
> 
> Yes, that should be good for function_graph trampoline too.
> We use very similar technic.

Ideally also the optimized kprobe trampoline, but I've not managed to
fully comprehend that one.

> > 
> > ---
> > --- a/arch/x86/entry/entry_32.S
> > +++ b/arch/x86/entry/entry_32.S
> > @@ -1576,3 +1576,100 @@ ENTRY(rewind_stack_do_exit)
> >  	call	do_exit
> >  1:	jmp 1b
> >  END(rewind_stack_do_exit)
> > +
> > +/*
> > + * Transforms a CALL frame into an exception frame; IOW it pretends the CALL we
> > + * just did was in fact scribbled with an INT3.
> > + *
> > + * Use this trampoline like:
> > + *
> > + *   PUSH $func
> > + *   JMP call_to_exception_trampoline
> > + *
> > + * $func will see regs->ip point at the CALL instruction and must therefore
> > + * modify regs->ip in order to make progress (just like a normal INT3 scribbled
> > + * CALL).
> > + *
> > + * NOTE: we do not restore any of the segment registers.
> > + */
> > +ENTRY(call_to_exception_trampoline)
> > +	/*
> > +	 * On entry the stack looks like:
> > +	 *
> > +	 *   2*4(%esp) <previous context>
> > +	 *   1*4(%esp) RET-IP
> > +	 *   0*4(%esp) func
> > +	 *
> > +	 * transform this into:
> > +	 *
> > +	 *  19*4(%esp) <previous context>
> > +	 *  18*4(%esp) gap / RET-IP
> > +	 *  17*4(%esp) gap / func
> > +	 *  16*4(%esp) ss
> > +	 *  15*415*4(%esp) sp / <previous context>
> 
> isn't this "&<previous context>" ?

Yes.

> > +	 *  14*4(%esp) flags
> > +	 *  13*4(%esp) cs
> > +	 *  12*4(%esp) ip / RET-IP
> > +	 *  11*4(%esp) orig_eax
> > +	 *  10*4(%esp) gs
> > +	 *   9*4(%esp) fs
> > +	 *   8*4(%esp) es
> > +	 *   7*4(%esp) ds
> > +	 *   6*4(%esp) eax
> > +	 *   5*4(%esp) ebp
> > +	 *   4*4(%esp) edi
> > +	 *   3*4(%esp) esi
> > +	 *   2*4(%esp) edx
> > +	 *   1*4(%esp) ecx
> > +	 *   0*4(%esp) ebx
> > +	 */
> > +	pushl	%ss
> > +	pushl	%esp		# points at ss
> > +	addl	$3*4, (%esp)	#   point it at <previous context>
> > +	pushfl
> > +	pushl	%cs
> > +	pushl	5*4(%esp)	# RET-IP
> > +	subl	5, (%esp)	#   point at CALL instruction
> > +	pushl	$-1
> > +	pushl	%gs
> > +	pushl	%fs
> > +	pushl	%es
> > +	pushl	%ds
> > +	pushl	%eax
> > +	pushl	%ebp
> > +	pushl	%edi
> > +	pushl	%esi
> > +	pushl	%edx
> > +	pushl	%ecx
> > +	pushl	%ebx
> > +
> > +	ENCODE_FRAME_POINTER
> > +
> > +	movl	%esp, %eax	# 1st argument: pt_regs
> > +
> > +	movl	17*4(%esp), %ebx	# func
> > +	CALL_NOSPEC %ebx
> > +
> > +	movl	PT_OLDESP(%esp), %eax
> 
> Is PT_OLDESP(%esp) "<previous context>" or "&<previous contex>"?

The latter.

> > +
> > +	movl	PT_EIP(%esp), %ecx
> > +	movl	%ecx, -1*4(%eax)
> 
> Ah, OK, so $func must set the true return address to regs->ip
> instead of returning it.

Just so.

> > +
> > +	movl	PT_EFLAGS(%esp), %ecx
> > +	movl	%ecx, -2*4(%eax)
> > +
> > +	movl	PT_EAX(%esp), %ecx
> > +	movl	%ecx, -3*4(%eax)
> 
> So, at this point, the stack becomes
> 
  3*4(%esp) &regs->sp
  2*4(%esp) RET-IP
  1*4(%esp) eflags
  0*4(%esp) eax

> Correct?

Yes, relative to regs->sp, which is why we need to pop 'func', otherwise
it stays on the stack.

> > +
> > +	popl	%ebx
> > +	popl	%ecx
> > +	popl	%edx
> > +	popl	%esi
> > +	popl	%edi
> > +	popl	%ebp
> > +
> > +	lea	-3*4(%eax), %esp
> > +	popl	%eax
> > +	popfl
> > +	ret
> > +END(call_to_exception_trampoline)
> > --- a/arch/x86/kernel/kprobes/core.c
> > +++ b/arch/x86/kernel/kprobes/core.c
> > @@ -731,29 +731,8 @@ asm(
> >  	".global kretprobe_trampoline\n"
> >  	".type kretprobe_trampoline, @function\n"
> >  	"kretprobe_trampoline:\n"
> > -	/* We don't bother saving the ss register */
> > -#ifdef CONFIG_X86_64
> > -	"	pushq %rsp\n"
> > -	"	pushfq\n"
> > -	SAVE_REGS_STRING
> > -	"	movq %rsp, %rdi\n"
> > -	"	call trampoline_handler\n"
> > -	/* Replace saved sp with true return address. */
> > -	"	movq %rax, 19*8(%rsp)\n"
> > -	RESTORE_REGS_STRING
> > -	"	popfq\n"
> > -#else
> > -	"	pushl %esp\n"
> > -	"	pushfl\n"
> > -	SAVE_REGS_STRING
> > -	"	movl %esp, %eax\n"
> > -	"	call trampoline_handler\n"
> > -	/* Replace saved sp with true return address. */
> > -	"	movl %eax, 15*4(%esp)\n"
> > -	RESTORE_REGS_STRING
> > -	"	popfl\n"
> > -#endif
> > -	"	ret\n"
> 
> Here, we need a gap for storing ret-ip, because kretprobe_trampoline is
> the address which is returned from the target function. We have no 
> "ret-ip" here at this point. So something like
> 
> +	"push $0\n"	/* This is a gap, will be filled with real return address*/

The trampoline already provides a gap, trampoline_handler() will need to
use int3_emulate_push() if it wants to inject something on the return
stack.

> > +	"push trampoline_handler\n"
> > +	"jmp call_to_exception_trampoline\n"
> >  	".size kretprobe_trampoline, .-kretprobe_trampoline\n"
> >  );
> >  NOKPROBE_SYMBOL(kretprobe_trampoline);