When do functions get .plt entries in GLIBC when there are multiple definitions?

Question

Some functions are defined twice across GLIBC libraries. For example, consider _dl_signal_exception :

$ readelf -s libc-2.31.so | grep _dl_signal_exception
   103: 0000000000138130    77 FUNC    GLOBAL DEFAULT   14 _dl_signal_exception@@GLIBC_PRIVATE
$ readelf -s ld-2.31.so | grep _dl_signal_exception
     2: 000000000001a8b0    73 FUNC    GLOBAL DEFAULT   12 _dl_signal_exception@@GLIBC_PRIVATE

Other functions in ld.so call _dl_signal_exception and thus _dl_signal_exception gets a .plt entry which at runtime will actually resolve to libc.so 's _dl_signal_exception . As I understand it, shared libraries (on Linux) will make function calls indirectly (unless static) to support symbol interposition.

However, other functions in libc.so call _dl_signal_exception , but there is no .plt entry in libc.so for this function.

So I have two questions:

1. Why does ld.so define _dl_signal_exception when it can just get it from libc.so ?
2. Why don't we see a .plt entry for _dl_signal_exception in libc.so ?

Answer 1

Why does ld.so define _dl_signal_exception when it can just get it from libc.so?

The ld.so is special -- it is what the kernel transfers control to after mapping the main executable and ld.so (the path to which is hard-coded into the main executable's PT_INTERP segment).

No other library is present yet -- it is the job of ld.so to load all other libraries.

So ld.so must be completely stand-alone, and can't depend on any other library (including libc.so ). Because of this, ld.so has its own (smallish) malloc , and other parts of libc which it needs to open and mmap files, etc.

Why don't we see a.plt entry for _dl_signal_exception in libc.so?

A lot of libc.so calls are considered internal implementation detail, and bypass the .plt . This is achieved using internal_call macro in GLIBC sources.

For example, the fact that fopen (potentially) uses mmap is internal implementation detail, and the call from fopen to mmap is hidden from interposition.