Support simply executing the built shared libraries.

Some system libraries support being run as regular executables.
Now that I have figured out how to do it, add support for libcap.so
and libpsx.so to print some information and exit.

Note, I've explained how most of this stuff works in this answer:

  https://stackoverflow.com/a/68339111/14760867

Signed-off-by: Andrew G. Morgan <morgan@kernel.org>

1 files changed, 49 insertions, 16 deletions

diff --git a/libcap/Makefile b/libcap/Makefile
index bdd32a9..47cf8f4 100644
--- a/libcap/Makefile
+++ b/libcap/Makefile
@@ -9,11 +9,18 @@ include ../Make.Rules
 CAPLIBNAME=$(LIBTITLE).so
 STACAPLIBNAME=$(LIBTITLE).a
 #
-PSXLIBNAME=libpsx.so
-STAPSXLIBNAME=libpsx.a
+PSXTITLE=libpsx
+PSXLIBNAME=$(PSXTITLE).so
+STAPSXLIBNAME=$(PSXTITLE).a
 
 CAPFILES=cap_alloc cap_proc cap_extint cap_flag cap_text cap_file
+CAPMAGICOBJ=cap_magic.o
 PSXFILES=../psx/psx
+PSXMAGICOBJ=psx_magic.o
+
+# The linker magic needed to build a dynamic library as independently
+# executable
+MAGIC=--entry=__so_start
 
 INCLS=libcap.h cap_names.h $(INCS)
 GPERF_OUTPUT = _caps_output.gperf
@@ -37,9 +44,9 @@ ifeq ($(SHARED),yes)
 endif
 endif
 
-pcs: libcap.pc
+pcs: $(LIBTITLE).pc
 ifeq ($(PTHREADS),yes)
-	$(MAKE) libpsx.pc
+	$(MAKE) $(PSXTITLE).pc
 endif
 
 ifeq ($(BUILD_GPERF),yes)
@@ -47,7 +54,7 @@ USE_GPERF_OUTPUT = $(GPERF_OUTPUT)
 INCLUDE_GPERF_OUTPUT = -DINCLUDE_GPERF_OUTPUT='"$(GPERF_OUTPUT)"'
 endif
 
-libcap.pc: libcap.pc.in
+$(LIBTITLE).pc: $(LIBTITLE).pc.in
 	sed -e 's,@prefix@,$(prefix),' \
 		-e 's,@exec_prefix@,$(exec_prefix),' \
 		-e 's,@libdir@,$(LIBDIR),' \
@@ -56,7 +63,7 @@ libcap.pc: libcap.pc.in
 		-e 's,@deps@,$(DEPS),' \
 		$< >$@
 
-libpsx.pc: libpsx.pc.in
+$(PSXTITLE).pc: $(PSXTITLE).pc.in
 	sed -e 's,@prefix@,$(prefix),' \
 		-e 's,@exec_prefix@,$(exec_prefix),' \
 		-e 's,@libdir@,$(LIBDIR),' \
@@ -93,13 +100,26 @@ $(STAPSXLIBNAME): $(PSXOBJS) include/sys/psx_syscall.h
 	$(RANLIB) $@
 
 ifeq ($(SHARED),yes)
-$(CAPLIBNAME) $(MAJCAPLIBNAME) $(MINCAPLIBNAME): $(CAPOBJS)
-	$(LD) $(CFLAGS) $(LDFLAGS) -Wl,-soname,$(MAJCAPLIBNAME) -o $(MINCAPLIBNAME) $^
+
+empty: empty.c
+	$(CC) -o $@ $<
+
+loader.txt: empty
+	$(OBJCOPY) --dump-section .interp=/dev/stdout $< > $@
+
+cap_magic.o: execable.h execable.c loader.txt
+	$(CC) $(CFLAGS) $(IPATH) -DLIBRARY_VERSION=\"$(LIBTITLE)-$(VERSION).$(MINOR)\" -DSHARED_LOADER=\"$(shell cat loader.txt)\" -c execable.c -o $@
+
+$(CAPLIBNAME) $(MAJCAPLIBNAME) $(MINCAPLIBNAME): $(CAPOBJS) $(CAPMAGICOBJ)
+	$(LD) $(CFLAGS) $(LDFLAGS) -Wl,-soname,$(MAJCAPLIBNAME) -o $(MINCAPLIBNAME) $^ $(MAGIC)
 	ln -sf $(MINCAPLIBNAME) $(MAJCAPLIBNAME)
 	ln -sf $(MAJCAPLIBNAME) $(CAPLIBNAME)
 
-$(PSXLIBNAME) $(MAJPSXLIBNAME) $(MINPSXLIBNAME): $(PSXOBJS) include/sys/psx_syscall.h
-	$(LD) $(CFLAGS) $(LDFLAGS) -Wl,-soname,$(MAJPSXLIBNAME) -o $(MINPSXLIBNAME) $(PSXOBJS) $(PSXLINKFLAGS)
+psx_magic.o: execable.h execable.c loader.txt
+	$(CC) $(CFLAGS) $(IPATH) -DLIBRARY_VERSION=\"$(PSXTITLE)-$(VERSION).$(MINOR)\" -DSHARED_LOADER=\"$(shell cat loader.txt)\" -c execable.c -o $@
+
+$(PSXLIBNAME) $(MAJPSXLIBNAME) $(MINPSXLIBNAME): $(PSXOBJS) include/sys/psx_syscall.h $(PSXMAGICOBJ)
+	$(LD) $(CFLAGS) $(LDFLAGS) -Wl,-soname,$(MAJPSXLIBNAME) -o $(MINPSXLIBNAME) $(PSXOBJS) $(PSXMAGICOBJ) $(MAGIC) $(PSXLINKFLAGS)
 	ln -sf $(MINPSXLIBNAME) $(MAJPSXLIBNAME)
 	ln -sf $(MAJPSXLIBNAME) $(PSXLIBNAME)
 endif
@@ -113,8 +133,20 @@ cap_text.o: cap_text.c $(USE_GPERF_OUTPUT) $(INCLS)
 cap_test: cap_test.c libcap.h $(CAPOBJS)
 	$(CC) $(CFLAGS) $(IPATH) $< $(CAPOBJS) -o $@
 
+libcapsotest: $(CAPLIBNAME)
+	./$(CAPLIBNAME)
+
+libpsxsotest: $(PSXLIBNAME)
+	./$(PSXLIBNAME)
+
 test: cap_test
 	./cap_test
+ifeq ($(SHARED),yes)
+	$(MAKE) libcapsotest
+ifeq ($(PTHREADS),yes)
+	$(MAKE) libpsxsotest
+endif
+endif
 
 install: install-static
 ifeq ($(SHARED),yes)
@@ -163,17 +195,17 @@ ifeq ($(FAKEROOT),)
 	-/sbin/ldconfig
 endif
 
-install-common-cap: install-common libcap.pc
+install-common-cap: install-common $(LIBTITLE).pc
 	install -m 0644 include/sys/capability.h $(FAKEROOT)$(INCDIR)/sys
-	install -m 0644 libcap.pc $(FAKEROOT)$(PKGCONFIGDIR)/libcap.pc
+	install -m 0644 $(LIBTITLE).pc $(FAKEROOT)$(PKGCONFIGDIR)/$(LIBTITLE).pc
 
 include/sys/psx_syscall.h: ../psx/psx_syscall.h
 	rm -f $@
 	ln -s ../../../psx/psx_syscall.h $@
 
-install-common-psx: install-common libpsx.pc include/sys/psx_syscall.h
+install-common-psx: install-common $(PSXTITLE).pc include/sys/psx_syscall.h
 	install -m 0644 include/sys/psx_syscall.h $(FAKEROOT)$(INCDIR)/sys
-	install -m 0644 libpsx.pc $(FAKEROOT)$(PKGCONFIGDIR)/libpsx.pc
+	install -m 0644 $(PSXTITLE).pc $(FAKEROOT)$(PKGCONFIGDIR)/$(PSXTITLE).pc
 
 install-common:
 	mkdir -p -m 0755 $(FAKEROOT)$(INCDIR)/sys
@@ -182,8 +214,9 @@ install-common:
 
 clean:
 	$(LOCALCLEAN)
-	rm -f $(CAPOBJS) $(CAPLIBNAME)* $(STACAPLIBNAME) libcap.pc
-	rm -f $(PSXOBJS) $(PSXLIBNAME)* $(STAPSXLIBNAME) libpsx.pc
+	rm -f $(CAPOBJS) $(CAPLIBNAME)* $(STACAPLIBNAME) $(LIBTITLE).pc
+	rm -f $(PSXOBJS) $(PSXLIBNAME)* $(STAPSXLIBNAME) $(PSXTITLE).pc
 	rm -f cap_names.h cap_names.list.h _makenames $(GPERF_OUTPUT) cap_test
 	rm -f include/sys/psx_syscall.h
+	rm -f $(CAPMAGICOBJ) $(PSXMAGICOBJ) empty loader.txt
 	cd include/sys && $(LOCALCLEAN)