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Date: Thu, 28 May 2015 21:47:34 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: Progress since 1.1.9

* Rich Felker <dalias@...c.org> [2015-05-28 13:12:41 -0400]:

> On Thu, May 28, 2015 at 01:45:15PM +0200, Szabolcs Nagy wrote:
> > * Rich Felker <dalias@...c.org> [2015-05-27 19:13:08 -0400]:
> > > The one roadmap item I don't have any progress on is what to do with
> > > the libgcc_s symbol version mess, and I think we should probably just
> > > hold off until the next release cycle for that now.
> > 
> > in case anyone wonders about the issue:
> > 
> > libgcc_s has a symbolic reference to an internal deprecated
> > symbol with no default version on x86 (in gcc trunk).
> > (so no @version only @@version which makes it invisible to
> > musl and ld).
> > 
> > the deprecated function is a constructor in libgcc_s that
> > initializes an unused struct in libgcc_s.
> > 
> > the only reason the symbol got versioned this way is because
> > they want old binaries to work and remove the symbol from
> > libgcc_s for new binaries, while still using the same symbol
> > name in new binaries but with different abi behaviour: now
> > it's only available in libgcc.a to fix their ifunc hack for
> > multi-versioning.
> > 
> > since it's a symbol internal to libgcc and the semantics
> > of the symbol changed they could have just used a new name
> > and left the old one alone (so old stuff is guaranteed to not
> > break including musl and new binaries use the new symbol with
> > new semantics).
> 
> Yes but if the new symbol is not linkable like they made it, then old
> musl-linked binaries depending on getting it from libgcc_s would fail
> to find it at dynamic-link time and error out. I don't know whether
> this can happen without use of the multiversioning feature, though,
> which would not have worked with musl anyway since we don't have
> ifunc.

old binaries don't reference the new name unless new name == old name.

there is no reason to make the old name unlinkable in libgcc_s.so if
the new symbol has new name.  (old binaries only reference the old
name which is available in libgcc_s.so and libgcc.a like before, new
binaries only reference the new name that is only in libgcc.a).
(this is what my patch does).

(it was possible to reference __cpu_indicator_init in libgcc_s.so without
using ifunc based multi-versioning: the x86 specific __builtin_cpu_init
can be called directly to make __builtin_cpu_is and __builtin_cpu_supports
work, but i think only c++ code used -lgcc_s).

> > so i still think my proposed libgcc patch makes more sense
> > than versioning:
> > https://gcc.gnu.org/ml/gcc-patches/2015-05/msg00899.html
> > 
> > gcc is unlikely to fix this, but this is a nonsense usecase
> > (and there is no other known use of versioning that is broken
> > with musl.. in theory other libs may deprecate symbols in a
> > similar way while still keeping symbolic references to them,
> > but such use was not yet observed).
> 
> Are you sure? have you tried building C++ programs with gcc 3.x or 4.2
> then using a libstdc++ from recent gcc? Unless the soname is
> different, I suspect they're using symbol versions to make it "work"
> and it will probably break catastrophically.

ok that may cause problems (but won't make the dynamic linking fail:
all versioned symbols in libstdc++ has default version).

but mixing libs linked with different versions of libstdc++ is
broken anyway if any two libs happen to use the same symbol with
different versions.

> > in musl-gcc it can be worked around by preloading a noop
> > __cpu_indicator_init and in a musl based gcc it can be
> > patched out.
> 
> But it sounds like that patch will be unacceptable for upstream. There
> are alternatives we could do, like providing in musl a list of useless
> symbols to ignore (resolve to a nop func or a dummy data symbol) if
> they're not found, but these are all a bit hackish and I worry more
> stuff with symbol versioning will come back to bite us in the future.
> The other option would be to teach gcc and binutils that musl does not
> support symbol versions, but I worry that might break even more things
> that expect versions to be available on Linux but that work fine with
> musl's current dummying-out of version matching.

if things use symbol versioning then musl will have problems.

but is that really widespread?

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