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: Wed, 05 Jun 2024 23:13:43 +0800
From: Xi Ruoyao <xry111@...111.site>
To: 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>, Jinyang He
 <hejinyang@...ngson.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 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.


-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University