initial commie

README.md

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+# US - User Switcher

makefile

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+CC ?= gcc
+CFLAGS = -Wall -pedantic --std=c99 -O2
+PREFIX = /usr/local
+MANPREFIX = ${PREFIX}/share/man
+
+us: us.c
+
+dbg:
+	gcc -O0 -g us.c -o us
+
+install: us
+	mkdir -p ${DESTDIR}${PREFIX}/bin
+	cp -f us ${DESTDIR}${PREFIX}/bin/us
+	chown root:root ${DESTDIR}${PREFIX}/bin/us
+	chmod 4755 ${DESTDIR}${PREFIX}/bin/us
+#	mkdir -p ${DESTDIR}${MANPREFIX}/man1
+#	cp -f us.1 ${DESTDIR}${MANPREFIX}/man1/us.1
+#	chmod 644 ${DESTDIR}${MANPREFIX}/man1/us.1
+
+uninstall:
+	rm -f ${DESTDIR}${PREFIX}/bin/us
+#		${DESTDIR}${MANPREFIX}/man1/us.1
+
+clean:
+	rm -f us us-dbg

us.c

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+#define _POSIX_C_SOURCE 200809L
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+#include <sys/types.h>
+
+static void usage (void);
+static int perm_set (uid_t);
+
+int main (int argc, char *argv[])
+{
+	if (argc < 2) {
+		usage();
+		exit(1);
+	}
+
+	uid_t ruid = getuid();
+
+	// Copy argv and argc
+	int c_argc = argc - 1;
+	char **c_argv = malloc((c_argc + 1) * sizeof(char *));
+	if (!c_argv) {
+		fprintf(stderr, "malloc: %s\n", strerror(errno));
+		goto fail_end;
+	}
+	for (int i = 0; i < c_argc; i++)
+		c_argv[i] = strdup(argv[i+1]);
+	c_argc[c_argv] = NULL;
+
+	if (perm_set(0) == -1) { // 0 = root
+		fprintf(stderr, "perm_set: %s\n", strerror(errno));
+		goto fail_end;
+	}
+	if (execvp(*c_argv, c_argv) == -1) // execvp searches in path
+		fprintf(stderr, "execv: %s\n", strerror(errno));
+	// if exec fails reset the permissions
+	if (perm_set(ruid) == -1) { // 0 = root
+		fprintf(stderr, "perm_set: %s\n", strerror(errno));
+		goto fail_end;
+	}
+
+	fail_end:
+	return 0;
+}
+
+static inline void usage (void)
+{
+	printf("usage: us [command]\n");
+}
+
+static inline int perm_set (uid_t id)
+{
+	return seteuid(id);	
+}