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From: Andrea Corallo <andrea.corallo@arm.com>
To: Richard Earnshaw <Richard.Earnshaw@foss.arm.com>
Cc: Andrea Corallo via Gcc-patches <gcc-patches@gcc.gnu.org>,
	nd <nd@arm.com>, Richard Earnshaw <Richard.Earnshaw@arm.com>
Subject: Re: [PATCH 9/15] arm: Set again stack pointer as CFA reg when popping if necessary
Date: Wed, 11 Jan 2023 10:55:34 +0100	[thread overview]
Message-ID: <gkr4jsxqtt5.fsf@arm.com> (raw)
In-Reply-To: <35e22f98-8c00-e591-112c-64cc6612a7ad@foss.arm.com> (Richard Earnshaw's message of "Mon, 9 Jan 2023 17:22:30 +0000")

Richard Earnshaw <Richard.Earnshaw@foss.arm.com> writes:

> On 09/01/2023 16:48, Richard Earnshaw via Gcc-patches wrote:
>> On 09/01/2023 14:58, Andrea Corallo via Gcc-patches wrote:
>>> Andrea Corallo via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
>>>
>>>> Richard Earnshaw <Richard.Earnshaw@foss.arm.com> writes:
>>>>
>>>>> On 27/09/2022 16:24, Kyrylo Tkachov via Gcc-patches wrote:
>>>>>>
>>>>>>> -----Original Message-----
>>>>>>> From: Andrea Corallo <andrea.corallo@arm.com>
>>>>>>> Sent: Tuesday, September 27, 2022 11:06 AM
>>>>>>> To: Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>
>>>>>>> Cc: Andrea Corallo via Gcc-patches <gcc-patches@gcc.gnu.org>; Richard
>>>>>>> Earnshaw <Richard.Earnshaw@arm.com>; nd <nd@arm.com>
>>>>>>> Subject: Re: [PATCH 9/15] arm: Set again stack pointer as CFA
>>>>>>> reg when
>>>>>>> popping if necessary
>>>>>>>
>>>>>>> Kyrylo Tkachov <Kyrylo.Tkachov@arm.com> writes:
>>>>>>>
>>>>>>>> Hi Andrea,
>>>>>>>>
>>>>>>>>> -----Original Message-----
>>>>>>>>> From: Gcc-patches <gcc-patches-
>>>>>>>>> bounces+kyrylo.tkachov=arm.com@gcc.gnu.org> On Behalf Of Andrea
>>>>>>>>> Corallo via Gcc-patches
>>>>>>>>> Sent: Friday, August 12, 2022 4:34 PM
>>>>>>>>> To: Andrea Corallo via Gcc-patches <gcc-patches@gcc.gnu.org>
>>>>>>>>> Cc: Richard Earnshaw <Richard.Earnshaw@arm.com>; nd <nd@arm.com>
>>>>>>>>> Subject: [PATCH 9/15] arm: Set again stack pointer as CFA reg when
>>>>>>> popping
>>>>>>>>> if necessary
>>>>>>>>>
>>>>>>>>> Hi all,
>>>>>>>>>
>>>>>>>>> this patch enables 'arm_emit_multi_reg_pop' to set again the stack
>>>>>>>>> pointer as CFA reg when popping if this is necessary.
>>>>>>>>>
>>>>>>>>
>>>>>>>>   From what I can tell from similar functions this is correct,
>>>>>>>> but could you
>>>>>>> elaborate on why this change is needed for my understanding please?
>>>>>>>> Thanks,
>>>>>>>> Kyrill
>>>>>>>
>>>>>>> Hi Kyrill,
>>>>>>>
>>>>>>> sure, if the frame pointer was set, than it is the current CFA
>>>>>>> register.
>>>>>>> If we request to adjust the current CFA register offset indicating it
>>>>>>> being SP (while it's actually FP) that is indeed not correct and the
>>>>>>> incoherence we will be detected by an assertion in the dwarf emission
>>>>>>> machinery.
>>>>>> Thanks,  the patch is ok
>>>>>> Kyrill
>>>>>>
>>>>>>>
>>>>>>> Best Regards
>>>>>>>
>>>>>>>     Andrea
>>>>>
>>>>> Hmm, wait.  Why would a multi-reg pop be updating the stack pointer?
>>>>
>>>> Hi Richard,
>>>>
>>>> not sure I understand, isn't any pop updating SP by definition?
>>>
>>>
>>> Back on this,
>>>
>>> compiling:
>>>
>>> =======
>>> int i;
>>>
>>> void foo (int);
>>>
>>> int bar()
>>> {
>>>    foo (i);
>>>    return 0;
>>> }
>>> =======
>>>
>>> With -march=armv8.1-m.main+fp -mbranch-protection=pac-ret+leaf
>>> -mthumb -O0 -g
>>>
>>> Produces the following asm for bar.
>>>
>>> bar:
>>>     @ args = 0, pretend = 0, frame = 0
>>>     @ frame_needed = 1, uses_anonymous_args = 0
>>>     pac    ip, lr, sp
>>>     push    {r3, r7, ip, lr}
>>>     add    r7, sp, #0
>>>     ldr    r3, .L3
>>>     ldr    r3, [r3]
>>>     mov    r0, r3
>>>     bl    foo
>>>     movs    r3, #0
>>>     mov    r0, r3
>>>     pop    {r3, r7, ip, lr}
>>>     aut    ip, lr, sp
>>>     bx    lr
>>>
>>> The offending instruction causing the ICE (without this patch) when
>>> emitting dwarf is "pop {r3, r7, ip, lr}".
>>>
>>> The current CFA reg when emitting the multipop is R7 (the frame
>>> pointer).  If is not the multipop that has the duty to restore SP as
>>> current CFA here which other instruction should do it?
>>>
>> Digging a bit deeper, I'm now even more confused. 
>> arm_expand_epilogue contains (parphrasing the code):
>>   if frame_pointer_needed
>>     {
>>       if arm
>>         {}
>>       else
>>         {
>>           if adjust
>>             r7 += adjust
>>           mov sp, r7    // Reset CFA to SP
>>         }
>>      }
>> so there should always be a move of r7 into SP, even if this is
>> strictly redundant.  I don't understand why this doesn't happen for
>> your testcase.  Can you dig a bit deeper?  I wonder if we've
>> (probably incorrectly) assumed that this function doesn't need an
>> epilogue but can use a simple return?  I don't think we should do
>> that when authentication is needed: a simple return should really be
>> one instruction.
>> 
>
> So I strongly suspect the real problem here is that use_return_insn ()
> in arm.cc needs to be updated to return false when using pointer
> authentication.  The specification for this function says that a
> return can be done in one instruction; and clearly when we need
> authentication more than one is needed.
>
> R.

So yes I agree with your analysis.  I'm respinning 10/15 to include your
suggestion and I believe we can just drop this patch.

Thanks

  Andrea

  reply	other threads:[~2023-01-11  9:55 UTC|newest]

Thread overview: 66+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-08-12 14:26 [PATCH 0/15] arm: Enables return address verification and branch target identification on Cortex-M Andrea Corallo
2022-08-12 15:14 ` [PATCH 1/15] arm: Make mbranch-protection opts parsing common to AArch32/64 Andrea Corallo
2022-12-22 17:04   ` [PATCH 1/15 V2] " Andrea Corallo
2023-01-11 10:48     ` Richard Earnshaw
2022-08-12 15:15 ` [PATCH 2/15] arm: Add Armv8.1-M Mainline target feature +pacbti Andrea Corallo
2022-08-12 15:21 ` [PATCH 3/15] arm: Add option -mbranch-protection Andrea Corallo
2022-08-12 15:22 ` [PATCH 4/15] arm: Add testsuite library support for PACBTI target Andrea Corallo
2022-08-12 15:26 ` [PATCH 5/15] arm: Implement target feature macros for PACBTI Andrea Corallo
2022-08-12 15:29 ` [PATCH 6/15] arm: Add pointer authentication for stack-unwinding runtime Andrea Corallo
2022-08-12 15:30 ` [PATCH 7/15] arm: Emit build attributes for PACBTI target feature Andrea Corallo
2022-09-05 16:53   ` Andrea Corallo
2022-10-20 14:47   ` Kyrylo Tkachov
2022-10-20 15:15     ` Richard Earnshaw
2022-10-21 12:19   ` Richard Earnshaw
2022-08-12 15:33 ` [PATCH 8/15] arm: Introduce multilibs " Andrea Corallo
2022-08-12 15:34 ` [PATCH 9/15] arm: Set again stack pointer as CFA reg when popping if necessary Andrea Corallo
2022-09-05 16:52   ` Andrea Corallo
2022-09-27  9:03   ` Kyrylo Tkachov
2022-09-27 10:05     ` Andrea Corallo
2022-09-27 15:24       ` Kyrylo Tkachov
2022-10-21 12:30         ` Richard Earnshaw
2022-10-26  8:49           ` Andrea Corallo
2022-11-08 14:57             ` Richard Earnshaw
2023-01-09 14:58             ` Andrea Corallo
2023-01-09 15:57               ` Richard Earnshaw
2023-01-09 16:48               ` Richard Earnshaw
2023-01-09 17:22                 ` Richard Earnshaw
2023-01-11  9:55                   ` Andrea Corallo [this message]
2022-08-12 15:36 ` [PATCH 10/15] arm: Implement cortex-M return signing address codegen Andrea Corallo
2022-09-05 16:55   ` Andrea Corallo
2022-09-14 14:20   ` [PATCH 10/15 V2] " Andrea Corallo
2022-10-21 12:58     ` Richard Earnshaw
2022-10-26 15:48       ` Andrea Corallo
2022-10-28 16:34         ` [PATCH 10/15 V3] " Andrea Corallo
2022-11-07  8:57           ` [PATCH 10/15 V4] " Andrea Corallo
2022-12-05 16:38             ` Richard Earnshaw
2022-12-09 14:16               ` [PATCH 10/15 V5] " Andrea Corallo
2022-12-12 10:53                 ` Richard Earnshaw
2022-12-14 16:35                   ` [PATCH 10/15 V6] " Andrea Corallo
2022-12-14 16:45                     ` Richard Earnshaw
2023-01-11  9:58                       ` [PATCH 10/15 V7] " Andrea Corallo
2023-01-11 10:39                         ` Richard Earnshaw
2022-08-12 15:40 ` [PATCH 11/15] aarch64: Make bti pass generic so it can be used by the arm backend Andrea Corallo
2022-09-05 16:56   ` Andrea Corallo
2022-09-27  9:10   ` Kyrylo Tkachov
2022-08-12 15:41 ` [PATCH 12/15] arm: implement bti injection Andrea Corallo
2022-09-05 16:56   ` Andrea Corallo
2022-09-27  9:18   ` Kyrylo Tkachov
2022-09-29 15:45     ` [PATCH 12/15 V2] " Andrea Corallo
2022-10-20 14:56       ` Kyrylo Tkachov
2022-10-28 16:40         ` [PATCH 12/15 V3] " Andrea Corallo
2022-12-05 17:02           ` Richard Earnshaw
2022-12-14 16:40             ` [PATCH 12/15 V4] " Andrea Corallo
2022-12-14 17:00               ` Richard Earnshaw
2022-12-14 17:03                 ` Richard Earnshaw
2022-12-22 17:13                   ` [PATCH 12/15 V5] " Andrea Corallo
2023-01-11 15:08                     ` Richard Earnshaw
2022-08-12 16:44 ` [PATCH 0/15] arm: Enables return address verification and branch target identification on Cortex-M Andrea Corallo
2022-08-12 17:10 ` [PATCH 13/15] arm: Add pacbti related multilib support for armv8.1-m.main Srinath Parvathaneni
2022-10-21 13:00   ` Richard Earnshaw
2022-09-21  8:07 ` [PING][PATCH 0/15] arm: Enables return address verification and branch target identification on Cortex-M Andrea Corallo
2022-10-21 13:01   ` Richard Earnshaw
2022-10-21 13:32     ` Andrea Corallo
2022-12-05 14:10   ` Andrea Corallo
2022-12-05 14:19     ` Kyrylo Tkachov
2023-01-23 10:50   ` [PATCH " Andrea Corallo

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