[7.5/10] arch/arm: Build a loadable module and a shared library as FDPIC too.

[casaroli]

Summary

Part of the FDPIC series. [5.5/10] #19940 taught the ELF module path what an FDPIC object is, so an application built as a module gets -mfdpic -fPIC and the arm-uclinuxfdpiceabi linker, and comes out a shared object the loader can bind.

The loadable module path was left behind. apps/Application.mk reaches it with DYNLIB = y, apps/Library.mk builds every shared library with it, and it is still a -r partial link with the stock linker. Such an object has no dynamic section, so nothing can bind an import to it, and there is no way to build a library an FDPIC module can call.

So CMODULEFLAGS and CXXMODULEFLAGS gain the same compiler flags, and LDMODULEFLAGS links a shared object rather than a partial one. The entry point stays with the caller, because a module is entered at _start while a library is only ever called into.

CXXMODULEFLAGS is defined here for the first time. apps/Library.mk compiles every C++ source of a shared library with it and no architecture ever defined it, so those sources were compiled with no architecture flags at all.

apache/nuttx-apps#3762 is the first user: its module fixtures stop hand-rolling the link and use these variables.

Impact

Everything else is under CONFIG_FDPIC, which defaults off and is selectable only where ARCH_HAVE_ELF_FDPIC is set.

CXXMODULEFLAGS replaces nothing, since it was empty.

Testing

Compiled and linked a library with the variables this patch produces, taken from a pimoroni-pico-2-plus tree configured with CONFIG_FDPIC:

$ arm-uclinuxfdpiceabi-readelf -dh libcounter.so
  OS/ABI:      ARM FDPIC
  Type:        DYN (Shared object file)
  Entry point: 0x0
  0x0e (SONAME)  Library soname: [libcounter.so]

Two PT_LOAD segments, R E and RW, as an FDPIC object needs.

tools/checkpatch.sh -c -u -m -g passes.

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb