[CFR] Adding a function to rtld-elf.so, how to handle Symbol.map?

[Ian Lepore]

On Thu, 2014-07-17 at 20:29 +0300, Konstantin Belousov wrote:
> On Thu, Jul 17, 2014 at 11:09:50AM -0600, Ian Lepore wrote:
> > On Thu, 2014-07-17 at 03:45 +0300, Konstantin Belousov wrote:
> > > On Wed, Jul 16, 2014 at 03:23:53PM -0600, Ian Lepore wrote:
> > [snip]

> > I did some looking around and Netbsd, Android, and Risc Os all
> > implemented this in their dynamic loaders, so that seemed like the way
> > to go.  Android actually puts a function with this __gnu name in its
> > libc, but all that function does is calls dl_unwind_find_exidx() which
> > is implemented in their loader.
> > 
> > I've just discovered that the arm unwind support code that will arrive
> > as part of clang 3.5 appears to assume the Android way of things unless
> > __LINUX__ is defined, so maybe it would be good to follow that model
> > ourselves and add a dl_unwind_find_exidx() stub to libc/gen/dlfcn.c and
> > name the new implementation in ld-elf to match.
> I think that Android/__LINUX__ combination does the right thing, by
> providing the symbol in libc. A libc implementation does not need any
> additional service from rtld, except already existing _rtld_addr_phdr().
> 

Android provides a stub of dl_unwind_find_exidx() in libdl and the
shared-object implementation in the dynamic linker.  What it puts in
libc is the __gnu_Unwind_Find_exidx() symbol, which just calls through
to the dl_unwind_find_exidx() implementation in the dynamic linker.

That aside, I've reworked my code so it all lives in libc instead of
rtld, as you suggested.  It seems to work fine, and I guess I'm agnostic
about whether we're exporting a new function from libc versus rtld.  It
seems a bit strange to me to have just one dl_something() function with
its shared/dynamic implementation in libc, while all the other functions
with dl-prefix names are implemented in rtld.  But not so weird that
it's a big deal.

-- Ian

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