linux-kernel - Re: [PATCH] objtool: ignore .L prefixed local symbols

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Date:   Fri, 14 Feb 2020 19:34:01 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Fangrui Song <maskray@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        jpoimboe@...hat.com, peterz@...radead.org,
        clang-built-linux@...glegroups.com,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] objtool: ignore .L prefixed local symbols

On Fri, Feb 14, 2020 at 07:05:57PM -0500, Arvind Sankar wrote:
> On Fri, Feb 14, 2020 at 02:20:46PM -0800, Fangrui Song wrote:
> > On 2020-02-14, Arvind Sankar wrote:
> > >
> > >I was testing with hidden/protected visibility, I see you want this for
> > >the no-semantic-interposition case. Actually a bit more testing shows
> > >some peculiarities even with hidden visibility. With the below, the call
> > >and lea create relocations in the object file, but the jmp doesn't. ld
> > >does avoid creating a plt for this though.
> > >
> > >	.text
> > >	.globl foo, bar
> > >	.hidden foo
> > >	bar:
> > >		call	foo
> > >		leaq	foo(%rip), %rax
> > >		jmp	foo
> > >
> > >	foo:	ret
> > 
> > Yes, GNU as is inconsistent here.  While fixing
> > https://sourceware.org/ml/binutils/2020-02/msg00243.html , I noticed
> > that the rule is quite complex. There are definitely lots of places to
> > improve.  clang 10 emits relocations consistently.
> > 
> >    call	foo              # R_X86_64_PLT32
> >    leaq	foo(%rip), %rax  # R_X86_64_PC32
> >    jmp	foo              # R_X86_64_PLT32
> > 
> 
> I guess the reason why is that jmp instructions can be optimized to use
> 8-bit signed offset if the destination is close enough, so the assembler
> wants to go through them anyway to check, while such optimization is not
> possible for the call and lea.
> 
> clang 9 emits no relocations for me, unless @PLT/@...PCREL is explicitly
> used. Has that changed? (Just using clang -o test.o test.s on that
> assembler, not too familiar with invokation syntax)

Actually, wait, it does that even with default visibility. The only way
to make it allow for symbol interposition is to explicitly use @PLT etc.
Is the only reason you're adding these local symbols then is to work
around GNU as adding PLT relocations automatically for call foo?