> On May 9, 2019, at 1:14 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
>> On Thu, May 09, 2019 at 10:20:30AM +0900, Masami Hiramatsu wrote:
>> Hi Josh,
>>
>> On Wed, 8 May 2019 13:48:48 -0500
>> Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>>
>>>> On Wed, May 08, 2019 at 05:39:07PM +0200, Peter Zijlstra wrote:
>>>>> On Wed, May 08, 2019 at 07:42:48AM -0500, Josh Poimboeuf wrote:
>>>>> On Wed, May 08, 2019 at 02:04:16PM +0200, Peter Zijlstra wrote:
>>>>
>>>>>> Do the x86_64 variants also want some ORC annotation?
>>>>>
>>>>> Maybe so. Though it looks like regs->ip isn't saved. The saved
>>>>> registers might need to be tweaked. I'll need to look into it.
>>>>
>>>> What all these sites do (and maybe we should look at unifying them
>>>> somehow) is turn a CALL frame (aka RET-IP) into an exception frame (aka
>>>> pt_regs).
>>>>
>>>> So regs->ip will be the return address (which is fixed up to be the CALL
>>>> address in the handler).
>>>
>>> But from what I can tell, trampoline_handler() hard-codes regs->ip to
>>> point to kretprobe_trampoline(), and the original return address is
>>> placed in regs->sp.
>>>
>>> Masami, is there a reason why regs->ip doesn't have the original return
>>> address and regs->sp doesn't have the original SP? I think that would
>>> help the unwinder understand things.
>>
>> Yes, for regs->ip, there is a histrical reason. Since previously, we had
>> an int3 at trampoline, so the user (kretprobe) handler expects that
>> regs->ip is trampoline address and ri->ret_addr is original return address.
>> It is better to check the other archs, but I think it is possible to
>> change the regs->ip to original return address, since no one cares such
>> "fixed address". :)
>>
>> For the regs->sp, there are 2 reasons.
>>
>> For x86-64, it's just for over-optimizing (reduce stack usage).
>> I think we can make a gap for putting return address, something like
>>
>> "kretprobe_trampoline:\n"
>> #ifdef CONFIG_X86_64
>> " pushq %rsp\n" /* Make a gap for return address */
>> " pushq 0(%rsp)\n" /* Copy original stack pointer */
>> " pushfq\n"
>> SAVE_REGS_STRING
>> " movq %rsp, %rdi\n"
>> " call trampoline_handler\n"
>> /* Push the true return address to the bottom */
>> " movq %rax, 20*8(%rsp)\n"
>> RESTORE_REGS_STRING
>> " popfq\n"
>> " addq $8, %rsp\n" /* Skip original stack pointer */
>>
>> For i386 (x86-32), there is no other way to keep ®s->sp as
>> the original stack pointer. It has to be changed with this series,
>> maybe as same as x86-64.
>
> Right; I already fixed that in my patch changing i386's pt_regs.
>
> 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).
>
> 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 */
>
> 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.
>
> ---
> --- 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*4(%esp) sp / <previous context>
> + * 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
> +
> + movl PT_EIP(%esp), %ecx
> + movl %ecx, -1*4(%eax)
> +
> + movl PT_EFLAGS(%esp), %ecx
> + movl %ecx, -2*4(%eax)
> +
> + movl PT_EAX(%esp), %ecx
> + movl %ecx, -3*4(%eax)
> +
> + 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"
> + "push trampoline_handler\n"
> + "jmp call_to_exception_trampoline\n"
> ".size kretprobe_trampoline, .-kretprobe_trampoline\n"
> );
Potentially minor nit: you’re doing popfl, but you’re not doing TRACE_IRQ_whatever. This makes me think that you should either add the tracing (ugh!) or you should maybe just skip the popfl.
