[Bug dynamic-link/21050] Prevent tailcall optimizations of libdl functions

["bugdal at aerifal dot cx" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=21050

--- Comment #3 from Rich Felker <bugdal at aerifal dot cx> ---
The suggestion in comment 2 is a spec violation; it has a static identifier
dlsym in place of the required extern one, and also breaks the identity of
function pointers (different pointer per translation unit). You have to use
function-like macros to do this kind of thing in a conforming way, and the
macro still does not cover the case where the caller undefines the macro or
directly calls the function via its address.

I believe the right solution is to deprecate the old constant value of
RTLD_NEXT and instead define RTLD_NEXT to expand to the address of a static
anchor object inside the translation unit. The dynamic linker can then use that
anchor object, rather than the return address, to identify the DSO from which
it was called. This also fixes a lot of other usage patterns, e.g. where
library A calls dlsym_wrapper(RTLD_NEXT, "foo") and dlsym_wrapper is defined in
library B.

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