Re: Symbol visibility revised

["H . J . Lu" <hjl at valinux dot com>]

On Mon, May 01, 2000 at 07:38:18PM +0200, Martin v. Loewis wrote:
> This patch implements some of the symbol visibility aspects in the
> static linker, which my previous patch assumed as the dynamic linker's
> responsibility. It includes the patch Chip sent last month. The

Last time when I tried Chip's patch, the resulting binary in one
of my testcases dumps core. I will take another look at your patch.

> immediate need for this patch is GCC's usage of .hidden as part of the
> new C++ ABI, and the lack for .hidden support in dynamic linkers.
> 
> In particular, hidden and internal visibility are implemented using
> the local symbols extension of the versioning feature. The linker
> avoids assigning dynamic indices if possible, or clears them if the
> already have been assigned, setting ELF_LINK_FORCED_LOCAL at the same
> time. Provided that symbol versioning is already available for a
> target, the hidden symbols mechanism should also work on that target.
> 
> It would be possible to implement protected visibility in the static
> linker as well, however, that would require changes to the backends
> (specifically, many places that check for ->symbolic || dynindx == -1
> should also check for visibility). Since there is no immediate need
> for supporting .protected, and since that will be most likely added to
> dynamic linkers, anyway, this patch does not attempt to completely
> perform .protected visibility.
> 
> As a result, .protected symbols can be overridden from another DSO
> (i.e. they are visible, but not protected).  As a special case, PLT
> entries for protected functions are eliminated, so protection works
> for function calls already (based on the -Bsymbolic support in that
> case).

That is incorrect. At our last ABI meeting, we have cleared that
confusion. We decided that the normal symbol resolution shouldn't
be changed due to STV_PROTECTED. That means we cannot do it in the
way of DT_SYMBOLIC. The PLT entry has to be there and the dynamic
linker has to support it.


H.J.