c18n: Miscellaneous improvements #2266
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Turn all use sites of __builtin_return_address(0) to use the newly defined rtld_get_return_address macro, which is overridden to take into account the existence of trampolines when c18n is enabled.
In the following example, bar makes a tail-call to foo, which returns a
value that is observable to the caller of bar, even though bar returns
nothing.
void *foo();
void bar() { foo(); }
When bar is called, previous versions of the trampoline clears return
value registers as if foo is being called directly, leaking a
capability. Instead, clear the maximum number of return value registers
as required for both foo and bar.
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Improve trampolines to clear the correct number of return values during tail-call.
Improve previous uses of c18n_return_address.