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

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Message-ID: <20200215000556.GA3876732@rani.riverdale.lan>
Date:   Fri, 14 Feb 2020 19:05:57 -0500
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        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 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)