Re: [Qemu-devel] RFC: DSO (dynamic shared objects) support

[Richard Henderson <rth@twiddle.net>]

On 06/18/2013 01:15 PM, Daniel P. Berrange wrote:
> By having all the private symbols change their version tag on every
> release, we make it a total PITA for anyone to provide closed source,
> out of tree, third party modules. Which is nice a feature :-)

Using a symbol version file also means that symbols that aren't
explicitly exported are not present in the dynamic symbol table.

Which isn't *quite* as good as using compiler-level symbol visibility
markup, but the linker can sometimes optimize code sequences in place
or at minimum reduce a full symbol relocation to a relative relocation.

> Symbol versioning like this isn't portable to Windows, but it works
> on Solaris and Linux at least. I'm not sure about BSD support for
> symbol versioning.

Given that symbol versioning is mostly a forward compatible extension
to ELF, assuming the linker doesn't date from the dark ages one should
at least be able to take advantage of visibility improvements, even if
actual symbol versions aren't enforced.


r~