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Re: Patch to not create GOT and dynamic relocation entries for unresolved symbols with --warn-unresolved-symbols.
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Cary Coutant <ccoutant at gmail dot com>
- Cc: Sriraman Tallam <tmsriram at google dot com>, binutils <binutils at sourceware dot org>, Paul Pluzhnikov <ppluzhnikov at google dot com>, Rong Xu <xur at google dot com>, Brooks Moses <bmoses at google dot com>, Ollie Wild <aaw at google dot com>, David Li <davidxl at google dot com>, Teresa Johnson <tejohnson at google dot com>, Ian Lance Taylor <iant at google dot com>
- Date: Sat, 11 Apr 2015 06:28:22 -0700
- Subject: Re: Patch to not create GOT and dynamic relocation entries for unresolved symbols with --warn-unresolved-symbols.
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On Fri, Apr 10, 2015 at 7:18 PM, Cary Coutant <firstname.lastname@example.org> wrote:
>> However, with -fPIE
>> $ g++ -O2 -fPIE foo.cc -pie
>> foo.cc:function main: warning: undefined reference to 'foo()'
>> ./a.out: symbol lookup error: ./a.out: undefined symbol: _Z3foov
>> because with fPIE, a function pointer access is using a GOTPCREL
>> relocation which creates a GOT entry and a dynamic relocation for it.
>> The dynamic linker does not like the unresolved symbol any more.
>> I have attached a patch to prevent creation of GOT and dynamic
>> relocation entries with --warn-unresolved-symbols in general. Is this
> No, I don't think so. For static links, we statically resolve
> undefined symbols to zero, but for non-static links, we leave them for
> dynamic resolution, presuming that they might be resolvable at
> runtime. This patch would change linker behavior that is fairly well
> established. (This patch also changes only x86_64 behavior; you'd need
> a similar patch for all targets. For a problem that isn't really
> target-specific, I'd prefer a target-independent solution.)
> The difference between non-PIE and PIE here is not due to the linker's
> -pie option, but because the compilers -fPIE option caused the
> compiler to use the GOTPCREL relocation, which requires the linker to
> create a GOT entry.
> If the reference to foo is weak, the dynamic loader won't complain --
> I think that's really what you want. I'm not sure whether it's
> reasonable for you to have the compiler make these symbols weak unsats
> or not, but if you can, you wouldn't need --warn-unresolved-symbols
> any longer. If not, I wouldn't object to a new
> --weak-unresolved-symbols option that would have the linker convert
> any unresolved symbols (that it warned about) into weak references. I
> don't think we want to do that by default, since it changes existing
> expectations, but I think it's fine as an option.
> HJ, does that sound like a reasonable option for Gnu ld as well?
This sounds reasonable to me.