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Message-ID: <444ec2031ef6ca016cbfa8dfedc51bddc8529ba7.camel@xry111.site>
Subject: Re: [PATCH] loongarch: Only select HAVE_OBJTOOL and allow ORC
 unwinder if the inline assembler supports R_LARCH_{32,64}_PCREL
From: Xi Ruoyao <xry111@xry111.site>
To: Nathan Chancellor <nathan@kernel.org>, Peter Zijlstra
 <peterz@infradead.org>
Cc: Huacai Chen <chenhuacai@kernel.org>, WANG Xuerui <kernel@xen0n.name>, 
 Tiezhu Yang <yangtiezhu@loongson.cn>, Nick Desaulniers
 <ndesaulniers@google.com>, Bill Wendling <morbo@google.com>, Justin Stitt
 <justinstitt@google.com>, Youling Tang <tangyouling@kylinos.cn>, Jinyang He
 <hejinyang@loongson.cn>,  loongarch@lists.linux.dev,
 linux-kernel@vger.kernel.org, llvm@lists.linux.dev, 
 mengqinggang@loongson.cn, cailulu@loongson.cn, wanglei@loongson.cn, 
 luweining@loongson.cn, Yujie Liu <yujie.liu@intel.com>, Heng Qi
 <hengqi@linux.alibaba.com>, Tejun Heo <tj@kernel.org>
Date: Wed, 05 Jun 2024 21:18:06 +0800
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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 neve=
r
> > > > followed up on the mailing list, so sorry about that.
> > > >=20
> > > > https://github.com/ClangBuiltLinux/linux/issues/2024
> > > >=20
> > > > Might be tangential to this patch though but I felt it was worth
> > > > mentioning.
> > >=20
> > > The warnings in GCC build is definitely the issue handled by this pat=
ch.
> > > But the warnings in Clang build should be a different issue.=C2=A0 Ca=
n you
> > > attach the kernel/events/core.o file from the Clang build for analysi=
s?
> > > I guess we need to disable more optimization...
> >=20
> > Sure thing. Let me know if there are any issues with the attachment.
>=20
> Thanks!=C2=A0 I've simplified it and now even...
>=20
> .global test
> .type test,@function
> test:
>=20
> addi.d	$sp,$sp,-448
> st.d	$ra,$sp,440
> st.d	$fp,$sp,432
> addi.d	$fp,$sp,448
>=20
> # do something
>=20
> addi.d	$sp,$fp,-448
> ld.d	$fp,$sp,432
> ld.d	$ra,$sp,440
> addi.d	$sp,$sp,448
> ret
>=20
> .size test,.-test
>=20
> is enough to trigger a objtool warning:
>=20
> /home/xry111/t1.o: warning: objtool: test+0x20: return with modified stac=
k frame
>=20
> 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 =3D { .buf =3D "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 fra=
me

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.

--=20
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University