Re: Removing glibc from the musl .2 ABI

[Szabolcs Nagy <nsz@port70.net>]

* Rich Felker <dalias@libc.org> [2019-07-22 11:52:59 -0400]:
> So, what I'd (tentatively; for discussion) like to do:
> 
> When ldso loads an application or shared library and detects that it's
> glibc-linked (DT_NEEDED for libc.so.6), it both loads a gcompat
> library instead *and* flags the dso as needing ABI-compat. The gcompat
> library would be permanently RTLD_LOCAL, unable to be used for
> resolving global symbols, since it would have to define symbols
> conflicting with libc symbols names and with future directions of the
> musl ABI.
> 
> Symbol lookups when relocating such a flagged dso would take place by
> first processing gcompat (logically, adding it to the head of the dso
> search list), then the normal symbol search order. The gcompat library
> could also provide a replacement dlsym function, so that dlsym calls
> from the glibc-linked DSO also follow this order, and a replacement
> dlopen, so that dlopen of libc from the glibc-linked DSO would get the
> gcompat module.
> 
> I'm not sure what mechanism gcompat would then use to make its own
> references to the underlying real libc functions. This is something
> we'd need to think about.

i'm not sure how gcompat would implement dlsym, if it's
on top of the musl dlsym, then that needs to be accessible
already (e.g. by exposing a __musl_dlsym alias) and can be
used to do lookups in libc.so.

> 
> Before we decide to do it, please be aware that this would be a bit of
> a burden on gcompat to do more than it's doing now. But it would also
> make lots of cases work that fundamentally *can't* work now -- compat
> with 32-bit code using the legacy 32-bit off_t functions, compat with
> 64-bit code using regexec, etc. -- anywhere the musl ABI currently
> conflicts with the glibc ABI. Of course much of this is optional. The
> new things that would be mandatory would mainly be moving over
> existing glibc compat shims (like the __ctype and __xstat stuff) and
> implementing converting wrappers where musl's use of reserved space
> creates unsafety/incompatibility with the existing glibc code.
> 
> Rich