Make userhosts an executable library which can be used as a wrapper

.gitignore

-/libuserhosts.so
+/userhosts

Makefile

 CFLAGS ?= -O2 -s
-CFLAGS += -Wall -fPIC
+CFLAGS += -Wall -fPIE -pie
 LDLIBS = -ldl
 PREFIX = /usr/local
 
 .PHONY: all install uninstall clean
 
-all: userhosts.so
+all: userhosts
 
-userhosts.so: userhosts.c
-	$(CC) $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) -shared -Wl,-soname,$@ $(LDLIBS) -o $@ $<
+userhosts: userhosts.c
+	$(CC) $(CPPFLAGS) $(CFLAGS) $(LDFLAGS) $(LDLIBS) -o $@ $<
 
 install: all
-	install -Dpm 0644 userhosts.so $(DESTDIR)$(PREFIX)/lib/userhosts/userhosts.so
+	install -Dp userhosts $(DESTDIR)$(PREFIX)/bin/userhosts
 
 uninstall:
-	-cd $(DESTDIR)$(PREFIX) && rm -f lib/userhosts/userhosts.so
+	-cd $(DESTDIR)$(PREFIX) && rm -f bin/userhosts
 
 clean:
-	-rm -f libuserhosts.so
+	-rm -f userhosts
+
+check: userhosts
+	@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" ] && \
+	echo "$$a" |grep -q ^2001:db8::1 && ! $$@

README

@@ -31,4 +31,8 @@ Just run "make".
 Usage
 -----
 
-$ env HOSTS=/path/to/custom/hosts LD_PRELOAD=/path/to/libuserhosts.so <command>
+$ HOSTS=/path/to/custom/hosts LD_PRELOAD=/path/to/userhosts <command>
+
+`userhosts` is also an executable wrapper that appends itself to LD_PRELOAD:
+
+$ HOSTS=/path/to/custom/hosts userhosts <command>

userhosts.c

@@ -26,6 +26,7 @@
 #include <stdlib.h>
 #include <dlfcn.h>
 #include <fcntl.h>
+#include <unistd.h>
 
 #define ORIGINAL_HOSTS_PATH "/etc/hosts"
 
@@ -85,3 +86,36 @@ 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)
+    fprintf(stderr, "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;
+}