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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Date: Mon, 17 Jun 2024 21:47:32 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Xi Ruoyao <xry111@...111.site>
Cc: chenglulu <chenglulu@...ngson.cn>, Jinyang He <hejinyang@...ngson.cn>, 
	Nathan Chancellor <nathan@...nel.org>, Peter Zijlstra <peterz@...radead.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 Mon, Jun 17, 2024 at 9:38 PM Xi Ruoyao <xry111@...111.site> wrote:
>
> On Mon, 2024-06-17 at 21:11 +0800, Huacai Chen wrote:
>
>
>
> > > select HAVE_OBJTOOL if AS_HAS_EXPLICIT_RELOCS && AS_HAS_THIN_ADD_SUB && !CC_IS_CLANG && !RUST
> > Maybe we needn't consider RUST here?
>
> Rustc does use $fp that objtool cannot handle as at now.  It can be
> demonstrated with an over-aligned type, similar to Clang:
>
> $ cat t.rs
> #[repr(C, align(64))]
> struct X(i32);
>
> extern { fn f(x: &X) -> i64; }
>
> #[no_mangle]
> fn g() -> i64 {
>     let x = X(114514);
>     unsafe {f(&x)}
> }
> $ rustc t.rs --emit=asm --crate-type=staticlib -O
> $ grep fp t.s
>         st.d    $fp, $sp, 112
>         addi.d  $fp, $sp, 128
>         addi.d  $sp, $fp, -128
>         ld.d    $fp, $sp, 112
>
> The kernel uses rust-bindgen to generate some .rs file from C headers.
> And __attribute__((aligned(x))) is directly translated to
> repr(align(x)).  As __attribute__((aligned(x))) is very common in the
> kernel I expect objtool will fail to handle some object code from rustc.
My meaning is: the build robot doesn't enable RUST now (maybe I'm
wrong), and this problem will surely be fixed in future, so we can
make the condition as simple as possible.

Huacai

>
> > And can we think AS_HAS_THIN_ADD_SUB always imply
> > AS_HAS_EXPLICIT_RELOCS here?
>
> Maybe, AFAIK there's no assembler using thin add-sub but not explicit
> relocs.
>
> --
> Xi Ruoyao <xry111@...111.site>
> School of Aerospace Science and Technology, Xidian University
>