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) ®s->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);
