linux-kernel - Re: [PATCH] loongarch: Only select HAVE_OBJTOOL and allow ORC unwinder if the inline assembler supports R_LARCH_{32,64}

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Message-ID: <82b7e6ea-c2cb-6364-ebe9-bff928028408@loongson.cn>
Date: Wed, 5 Jun 2024 23:47:44 +0800
From: Jinyang He <hejinyang@...ngson.cn>
To: Xi Ruoyao <xry111@...111.site>, Nathan Chancellor <nathan@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>
Cc: Huacai Chen <chenhuacai@...nel.org>, WANG Xuerui <kernel@...0n.name>,
 Tiezhu Yang <yangtiezhu@...ngson.cn>,
 Nick Desaulniers <ndesaulniers@...gle.com>, Bill Wendling
 <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 Youling Tang <tangyouling@...inos.cn>, loongarch@...ts.linux.dev,
 linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
 mengqinggang@...ngson.cn, cailulu@...ngson.cn, wanglei@...ngson.cn,
 luweining@...ngson.cn, Yujie Liu <yujie.liu@...el.com>,
 Heng Qi <hengqi@...ux.alibaba.com>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH] loongarch: Only select HAVE_OBJTOOL and allow ORC
 unwinder if the inline assembler supports R_LARCH_{32,64}_PCREL

On 2024-06-05 23:13, Xi Ruoyao wrote:

> On Wed, 2024-06-05 at 21:18 +0800, Xi Ruoyao wrote:
>> On Wed, 2024-06-05 at 18:57 +0800, Xi Ruoyao wrote:
>>> On Tue, 2024-06-04 at 23:25 -0700, Nathan Chancellor wrote:
>>>> On Wed, Jun 05, 2024 at 01:54:24PM +0800, Xi Ruoyao wrote:
>>>>> On Tue, 2024-06-04 at 22:43 -0700, Nathan Chancellor wrote:
>>>>>> For what it's worth, I have noticed some warnings with clang that I
>>>>>> don't see with GCC but I only filed an issue on our GitHub and never
>>>>>> followed up on the mailing list, so sorry about that.
>>>>>>
>>>>>> https://github.com/ClangBuiltLinux/linux/issues/2024
>>>>>>
>>>>>> Might be tangential to this patch though but I felt it was worth
>>>>>> mentioning.
>>>>> The warnings in GCC build is definitely the issue handled by this patch.
>>>>> But the warnings in Clang build should be a different issue.  Can you
>>>>> attach the kernel/events/core.o file from the Clang build for analysis?
>>>>> I guess we need to disable more optimization...
>>>> Sure thing. Let me know if there are any issues with the attachment.
>>> Thanks!  I've simplified it and now even...
>>>
>>> .global test
>>> .type test,@function
>>> test:
>>>
>>> addi.d	$sp,$sp,-448
>>> st.d	$ra,$sp,440
>>> st.d	$fp,$sp,432
>>> addi.d	$fp,$sp,448
>>>
>>> # do something
>>>
>>> addi.d	$sp,$fp,-448
>>> ld.d	$fp,$sp,432
>>> ld.d	$ra,$sp,440
>>> addi.d	$sp,$sp,448
>>> ret
>>>
>>> .size test,.-test
>>>
>>> is enough to trigger a objtool warning:
>>>
>>> /home/xry111/t1.o: warning: objtool: test+0x20: return with modified stack frame
>>>
>>> And to me this warning is bogus?
>> Minimal C reproducer:
>>
>> struct x { _Alignas(64) char buf[128]; };
>>
>> void f(struct x *p);
>> void g()
>> {
>> 	struct x x = { .buf = "1145141919810" };
>> 	f(&x);
>> }
>>
>> Then objtool is unhappy to the object file produced with "clang -c -O2"
>> from this translation unit:
>>
>> /home/xry111/t2.o: warning: objtool: g+0x50: return with modified stack frame
>>
>> It seems CFI_BP has a very specific semantic in objtool and Clang does
>> not operates $fp in the expected way.  I'm not sure about my conclusion
>> though.  Maybe Peter can explain it better.
> Another example: some simple rust code:
>
> extern { fn f(x: &i64) -> i64; }
>
> #[no_mangle]
> fn g() -> i64 {
>      let x = 114514;
>      unsafe {f(&x)}
> }
>
> It's just lucky GCC doesn't use $fp as the frame pointer unless there's
> some stupid code (VLA etc) thus the issue was not detected.
>
>
I think this because we did not add arch special handle in 
update_cfi_state().
In our 419 repo this func has been renamed arch_update_insn_state (, now it
should be arch_update_cfi_state) and give some actions to deal with the
frame pointer case. If we support it we may deal with some case but for 
clang...

.global test
.type test,@function
test:

addi.d  $sp,$sp,-448
st.d    $ra,$sp,440
st.d    $fp,$sp,432
addi.d  $fp,$sp,448

# do something  <- not change $sp

# addi.d        $sp,$fp,-448  <- not restore sp from cfa(fp)
ld.d    $fp,$sp,432
ld.d    $ra,$sp,440
addi.d  $sp,$sp,448
ret

.size test,.-test

This will cause warning, too. (Gcc should be ok, I don't test.)

source.c: (clang 19, based 0c1c0d53931, -g)
void *foo() { return __builtin_frame_address(0); }

asm.s:
         .cfi_sections .debug_frame
         .cfi_startproc
# %bb.0:
         addi.d  $sp, $sp, -16
         .cfi_def_cfa_offset 16
         st.d    $ra, $sp, 8                     # 8-byte Folded Spill
         st.d    $fp, $sp, 0                     # 8-byte Folded Spill
         .cfi_offset 1, -8
         .cfi_offset 22, -16
         addi.d  $fp, $sp, 16
         .cfi_def_cfa 22, 0                    <- define cfa is $r22
.Ltmp0:
         .loc    0 1 22 prologue_end             # hello.c:1:22
         move    $a0, $fp
         ld.d    $fp, $sp, 0                     # 8-byte Folded Reload  
<- restore regs from non-cfa ?
         ld.d    $ra, $sp, 8                     # 8-byte Folded Reload  
<- restore regs from non-cfa ?
         .loc    0 1 15 epilogue_begin is_stmt 0 # hello.c:1:15
         addi.d  $sp, $sp, 16                                            
<- restore prev-sp from non-cfa ?
         ret

Maybe Clang have anything wrong?


Attach: tmp fix by support arch_update_insn_state.


Thanks,

Jinyang


View attachment "0001-tmp-fix.patch" of type "text/x-patch" (30646 bytes)