removing USDT's libelf dependency

[Mark Johnston]

On Fri, Jan 30, 2015 at 10:15:09AM -0500, Ed Maste wrote:
> On 30 January 2015 at 01:13, Mark Johnston <markj at freebsd.org> wrote:
> >
> > It turns out that GNU ld implements a similar feature, albeit in a more
> > restrictive manner: a symbol named __start_<id> will have its value
> > filled in with the address of the section named <id>. The catch is that
> > <id> has to be a valid C identifier, and the DOF section is
> > conventionally named ".SUNW_dof", which of course doesn't quite work. I
> > propose using "SUNW_dof" instead so that we can make use of this linker
> > feature and eliminate the libelf dependency.
> >
> > DOF sections have their own ELF section type (SHT_SUNW_dof), so
> > anything that might want to find a DOF section should already be
> > searching for that rather than the conventional name. Depending on this
> > linker feature is also arguably a portability concern given that lld is
> > on the horizon, but we depend on it already anyway - this feature is
> > used in FreeBSD's linker set implementation (which in turn is used by
> > our SDT code :). So I don't think this would introduce a new problem.
> 
> I'm not worried about issues with lld here - we can make it meet our
> requirements. I am a little worried that there will be code floating
> around that does strcmp(foo, ".SUNW_dof") though. A quick search
> turned up some examples.  Most are copies of drti.c but I did find one
> case in some markjdb Github repo :-) 

Heh, oops.

> Even if we don't end up changing
> .SUNW_dof I think we should convert these cases to use the section
> type instead of name.

I agree. In the case of drti.c, we look for both the section type and
name. There isn't much else out there that looks for a DOF section.

> 
> We could give GNU ld 2.17.50 and lld knowledge of .SUNW_dof and
> __SUNW_dof. This looks like it would be straightforward, but at the
> cost of carrying around different FreeBSD-specific changes.
> 
> I think our best approach will be to find an approach that upstream
> linker projects (GNU ld, gold, lld) are willing to support. But, until
> or unless that happens I'd much rather we switch to SUNW_dof than
> require linking against libelf.

Ok. It looks like gold also supports the __start_<secname> feature from
GNU ld; there's a thread about handling the case where <secname> starts
with a period here: http://comments.gmane.org/gmane.comp.gnu.binutils/61852,
though it didn't really go anywhere. lld doesn't support the feature
AFAICS. For now I'd rather just change the section name.

-Mark