Re: [PATCH] remove remaining traces of __tls_get_new

[Rich Felker <dalias@libc.org>]

On Mon, Oct 21, 2019 at 05:50:30PM -0700, Fangrui Song wrote:
> On 2019-09-29, Rich Felker wrote:
> >On Sun, Sep 29, 2019 at 03:05:27PM +0200, Szabolcs Nagy wrote:
> >>reported on irc by malc_
> >
> >>>From 4a7090ab76d81b59f57a83bce9d22582e35a8b2b Mon Sep 17 00:00:00 2001
> >>From: Szabolcs Nagy <nsz@port70.net>
> >>Date: Sun, 29 Sep 2019 12:25:39 +0000
> >>Subject: [PATCH] remove remaining traces of __tls_get_new
> >>
> >>Some declarations of __tls_get_new were left in the code, even
> >>though the definition got removed in
> >>
> >>  commit 9d44b6460ab603487dab4d916342d9ba4467e6b9
> >>  install dynamic tls synchronously at dlopen, streamline access
> >>
> >>this can make the build fail with
> >>
> >>  ld: lib/libc.so: hidden symbol `__tls_get_new' isn't defined
> >>
> >>when libc.so is linked without --gc-sections, because a .hidden
> >>declaration in asm code creates a reference even if the symbol
> >>is not actually used.
> >
> >This is definitely a tooling bug. There is no reference to the symbol,
> >only declarations of it. I think it's a good idea to cleanup the
> >spurious mentions of it anyway, though.
> 
> I think it is hard to simply state that this is a tooling bug.
> For the visibility attribute of a symbol, it may still be emitted into
> the symbol table even if it is unused. For example:
> 
> ..protected foo => STV_PROTECTED foo
> ..hidden foo => STV_HIDDEN foo
> 
> A relocation referencing a symbol may be dropped due to --gc-sections.
> Shall we consider the symbol unused if all relocations to it are
> dropped?

I don't think --gc-sections is relevant except that it happened to
make the problem go away. This isn't about ld spuriously thinking
there's a reference to a symbol after --gc-sections dropped the
sections containing relocations referencing it.

Rather, it looks like a bug in --no-undefined, which as documented
should only produce an error when there's a *reference* (i.e. an
outstanding relocation using) the undefined symbol, instead erroring
out just because an undefined symbol appears in the linker's working
symbol table for the purpose of tracking its declared visibility.

Rich