Add the setruid utility

.gitignore

@@ -1,3 +1,4 @@
 *.swp
 *.o
 pidfilewrapper
+setruid

Makefile

@@ -9,21 +9,24 @@ CFLAGS += -std=c11 -Os -g -Wall -Wextra -pedantic
 CPPFLAGS +=
 LDFLAGS +=
 
-all: pidfilewrapper
+all: pidfilewrapper setruid
 
 pidfilewrapper: pidfilewrapper.c
 	$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) -o $@ $<
 
+setruid: setruid.c
+	$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) -o $@ $<
+
 .PHONY: all install uninstall clean distclean
 
-install: all
+install: all pidfilewrapper.1 setruid.1
 	mkdir -p $(BINDIR)
-	install pidfilewrapper $(BINDIR)
+	install pidfilewrapper setruid $(BINDIR)
 	mkdir -p $(DESTDIR)$(MANDIR)/man1
-	cp pidfilewrapper.1 $(DESTDIR)$(MANDIR)/man1/pidfilewrapper.1
+	cp pidfilewrapper.1 setruid.1 $(DESTDIR)$(MANDIR)
 
 uninstall:
-	rm -f $(BINDIR)/pidfilewrapper $(DESTDIR)$(MANDIR)/man1/pidfilewrapper.1
+	rm -f $(BINDIR)/pidfilewrapper $(BINDIR)/setruid $(DESTDIR)$(MANDIR)/man1/pidfilewrapper.1 $(DESTDIR)$(MANDIR)/setruid.1
 
 clean:
 	rm -f pidfilewrapper

pidfilewrapper.c

@@ -1,8 +1,8 @@
-#include <unistd.h>
+#include <stdint.h>
 #include <stdio.h>
 #include <sys/types.h>
 #include <unistd.h>
-#include <stdint.h>
+#include <unistd.h>
 
 int main(int argc, char **argv) {
 	if(argc < 3) {

setruid.1

@@ -0,0 +1,25 @@
+.Dd Jul 11, 2018
+.Dt setruid 1
+.Os
+.Sh NAME
+.Nm setruid
+.Nd set the real UID for a command
+.Sh SYNOPSIS
+.Nm
+.Ar command
+.Op Ar arguments
+.Sh DESCRIPTION
+.Nm
+sets the real UID while keeping the effective UID the same. It is indended for
+simple servers that need to bind on a low port and drop privileges by setting
+effective UID to real UID.
+.Pp
+.Nm
+executes the given command as the same process.
+.Sh EXIT STATUS
+.Nm
+will exit with status 1 if it fails to set the UID or execute the given command.
+If it is succesful, exit status will be that of the given command.
+.Sh AUTHORS
+.Nm
+has been written by nortti.

setruid.c

@@ -0,0 +1,36 @@
+#define _BSD_SOURCE
+#include <errno.h>
+#include <pwd.h>
+#include <stdio.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+int main(int argc, char **argv) {
+	if(argc < 3) {
+		fprintf(stderr, "Usage: %s username command [arguments]\n", argv[0]);
+		return 1;
+	}
+
+	const char *username = argv[1];
+	errno = 0;
+	struct passwd *passwd_entry = getpwnam(username);
+	if(passwd_entry == NULL) {
+		perror("getpwnam");
+		return 1;
+	}
+
+	uid_t ruid = passwd_entry->pw_uid;
+
+	if(setreuid(ruid, -1) != 0) {
+		perror("getpwnam");
+		return 1;
+	}
+
+	char **daemon_argv = &argv[2];
+	const char *daemon_command = daemon_argv[0];
+
+	execvp(daemon_command, daemon_argv);
+
+	perror("execvp");
+	return 1;
+}