Date: Thu, 28 May 2015 16:43:18 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: Progress since 1.1.9 On Thu, May 28, 2015 at 09:47:34PM +0200, Szabolcs Nagy wrote: > * 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). Yes, that would have been preferable, but it looks hard to get gcc to change this... :( > (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). Bleh. > > > 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). If so, it's worse than making dynamic linking fail -- it presumably causes silent runtime memory corruption due to using wrong data structures. > 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. I agree completely that symbol versioning is a horrible idea and does not properly solve the problem it was invented to solve. (And this problem can be addressed much better, though still not perfectly, with macro-based, inline-function-based, or asm-label-based redirects in header files.) But I believe the way gcc is doing things _does_ work reliably when you do not have C++ types crossing DSO interface boundaries, and that's a fairly large portion of real-world usage. (And when C++ types do cross DSO boundaries, it's usually intra-project DSOs, not system-wide ones in the public lib dirs, because people generally know C++ ABI is continually broken for public library interfaces.) > > > 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? I recall ALSA libs were using it; this is why we added the hack that we have now, always using the version ld would choose instead of the first-found version. Rich
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