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Gold readiness obstacle #6: more versioning trouble

And let’s keep talking about gold and the issues I’m encountering trying to build Gentoo with it. Waiting to see if glibc will ever get to implement the base versioning or if Ian would like to implement default versions I’ve found a third problem with the versioning support.

The problem, which I’ve reported as Sourceware bug #12893 as of today, is displayed by the libdebian-installer package: somehow the link editor is reporting two duplicated symbols… in the same source file. At first I thought it was some nasty bug with the link editor’s internals; what I found instead was a curious setting fort the source code itself.

What happens is this: one source file defines a function (with an additional alias, let’s ignore that for now); then it also uses the already described .symver directive to provide an alias which has a version:

int di_system_prebaseconfig_append (const char *udeb, const char *format, ...)
{ /* doesn't care about its body */ }

__asm__ (".symver di_system_prebaseconfig_append,di_system_prebaseconfig_append@LIBDI_4.0");

Do note that the original symbol is not marked static, which means it is also exposed as a base version. This by itself would be just fine, if it wasn’t that said source file is compiled into a translation unit, which is (indirectly, but I’ll simplify) used to produce a shared library. Said shared library uses a link editor version script to set the symbol/version pair of various symbols, like it is often done by properly-designed shared objects.

What becomes a problem is that the symbol’s name is also listed in the version script; which means the link editor will take the unversioned (base) symbol, and label it with the designed version (LIBDI_4.0)… causing two symbols with same name and version to be created. It should appear obvious that something’s wrong with the logic of this whole situation.

Why does this not cause a problem with the old bfd link editor? I’ve got no idea, although I can possibly speculate on the fact that the two symbols not only have the same name and version, but also the same address, which is likely to give the link editor enough clue to simply disregard the duplicate. On the other hand, the solution of this should probably be applied to the libdebian-installer package, the design of which I know nothing about; it might have been intended to support both shared and static linking, but it would look quite strange even in such a case.

At any rate, I’ll have to wait for Ian to express his opinion, and in the mean time I’ll be catching up with a few more buggy packages. I guess I don’t have any hurry, given that libtirpc is not fixed after all as it still reports missing symbols, but it does so only on glibc 2.14, which means that the main tinderbox won’t be able to be of much help for a little while.

Comments 2
  1. This series of comments on gold belongs to the most interesting things I’ve read in the past year. Thanks a lot, Diego, for sharing your insights!

  2. Thanks Nico, glad to hear that! Hopefully at the end of the series, at least the most important points would have been addressed, so that the link editor could be used without too many curses.

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