Stop pretending to be an executable.

Fixes support on glibc >=2.30 , which otherwise emits a message like:
ERROR: ld.so: object '.../parts/userhosts/userhosts' from LD_PRELOAD cannot be preloaded (cannot dynamically load position-independent executable): ignored.

Makefile

 CFLAGS ?= -O2 -s
-CFLAGS += -Wall -fPIC -pie
+CFLAGS += -Wall -fPIC -shared
 LDLIBS = -ldl
 PREFIX = /usr/local
 
 .PHONY: all install uninstall clean
 
-all: userhosts
+all: userhosts.so
 
-userhosts: userhosts.c
+userhosts.so: userhosts.c
 	$(CC) $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) -o $@ $< $(LDLIBS)
 
 install: all
-	install -Dp userhosts $(DESTDIR)$(PREFIX)/bin/userhosts
+	install -Dp userhosts $(DESTDIR)$(PREFIX)/lib/userhosts.so
 
 uninstall:
-	-cd $(DESTDIR)$(PREFIX) && rm -f bin/userhosts
+	-cd $(DESTDIR)$(PREFIX) && rm -f lib/userhosts.so
 
 clean:
-	-rm -f userhosts
+	-rm -f userhosts.so
 
-check: userhosts
+check: userhosts.so
 	@export HOSTS=`mktemp` && trap "rm $$HOSTS" 0 && \
 	echo 2001:db8::1 $<.example.com >$$HOSTS && \
 	set getent hosts $<.example.com && \
-	a=`./$< $$@` && b=`LD_PRELOAD=./$< $$@` && [ "$$a" = "$$b" ] && \
+	a=`LD_PRELOAD=./$< $$@` && \
 	echo "$$a" |grep -q ^2001:db8::1 && ! $$@

userhosts.c

@@ -90,36 +90,3 @@ FILE *fopen(const char *path, const char *mode) {
     path = replacement_hosts;
   return (*original_fopen)(path, mode);
 }
-
-int main(int argc, char *argv[]) {
-  char *ld_preload, *p;
-  int ret = -1;
-
-  if (argc <= 1)
-    dprintf(STDERR_FILENO, "usage: userhosts <command> [<args>]\n");
-  else {
-    /* In order not to depend on /proc, getauxval(AT_EXECFN) from <sys/auxv.h>
-     * should be used. However, this requires glibc >= 2.16. */
-    ld_preload = realpath("/proc/self/exe", NULL);
-    if (!ld_preload)
-      p = "realpath";
-    else {
-      if ((p = getenv("LD_PRELOAD"))) {
-        size_t n = strlen(ld_preload);
-        ld_preload = realloc(ld_preload, n + strlen(p) + 2);
-        ld_preload[n++] = ':';
-        strcpy(ld_preload + n, p);
-      }
-      ret = setenv("LD_PRELOAD", ld_preload, 1);
-      free(ld_preload);
-      if (ret)
-        p = "setenv";
-      else {
-        ret = execvp(argv[1], &argv[1]);
-        p = "execvp";
-      }
-    }
-    perror(p);
-  }
-  return ret;
-}