Re: [PATCH] dlsym: Make RTLD_NEXT prefer default version definition [#BZ #14932]

[Florian Weimer <fweimer@redhat.com>]

* Fangrui Song:

> I suspect the code (from 199x) has a hidden assumption that for a stem
> symbol (say foo), the .dynsym entries (say foo@v1, foo@v2, foo@@v3) are
> ordered by the version node indices (in .gnu.version_d).

Right, the code clearly has this assumption.  There has to be some
ordering because we don't want to do a topological sort to find the
lowest and highest versions.

> If so:
>
> * (flags & DL_LOOKUP_RETURN_NEWEST) == 0 => the symbol with
>     verstab[symidx]==2 will skip the check and be returned
> * (flags & DL_LOOKUP_RETURN_NEWEST) != 0 => only the default version
>   symbol gets assigned in `*versioned_sym = sym`
>
> Unfortunately, this assumption hold for none of GNU ld, gold, and
> ld.lld (seems unrelated to --hash-style=):
>
> ```
> % ld.bfd @response.txt && readelf -W --dyn-syms nextmod3.so | grep foo@
>      5: 0000000000001100     6 FUNC    GLOBAL DEFAULT   13 foo@v1
>      8: 0000000000001120     6 FUNC    GLOBAL DEFAULT   13 foo@@v3
>      9: 0000000000001110     6 FUNC    GLOBAL DEFAULT   13 foo@v2
> % gold @response.txt && readelf -W --dyn-syms nextmod3.so | grep foo@
>      7: 00000000000007c0     6 FUNC    GLOBAL DEFAULT   14 foo@@v3
>      9: 00000000000007a0     6 FUNC    GLOBAL DEFAULT   14 foo@v1
>     10: 00000000000007b0     6 FUNC    GLOBAL DEFAULT   14 foo@v2
> % ld.lld @response.txt && readelf -W --dyn-syms nextmod3.so | grep foo@
>      7: 0000000000001740     6 FUNC    GLOBAL DEFAULT   13 foo@@v3
>      8: 0000000000001720     6 FUNC    GLOBAL DEFAULT   13 foo@v1
>      9: 0000000000001730     6 FUNC    GLOBAL DEFAULT   13 foo@v2

The order of symbols in the symbol table is not what counts here, it's
the order in the list for the hash bucket.

I'm not sure if it's possible to dump this with readelf.  I'm writing a
custom dumper for this.

Thanks,
Florian