libauth: add procsetuser() function to change user id of the calling process

Provide a central function to change the user id
of the calling process.

This is mostly used by programs to become the none
user, followed by a call to newns().

2 files changed, 21 insertions, 0 deletions

diff --git a/sys/src/libauth/mkfile b/sys/src/libauth/mkfile
index 2030db8f1..65505703e 100644
--- a/sys/src/libauth/mkfile
+++ b/sys/src/libauth/mkfile
@@ -17,6 +17,7 @@ OFILES=\
 	login.$O\
 	newns.$O\
 	noworld.$O\
+	procsetuser.$O\
 
 HFILES=\
 	/sys/include/auth.h\
diff --git a/sys/src/libauth/procsetuser.c b/sys/src/libauth/procsetuser.c
new file mode 100644
index 000000000..f9cac306b
--- /dev/null
+++ b/sys/src/libauth/procsetuser.c
@@ -0,0 +1,20 @@
+#include <u.h>
+#include <libc.h>
+#include <auth.h>
+
+int
+procsetuser(char *user)
+{
+	int fd, n;
+
+	fd = open("#c/user", OWRITE|OCEXEC);
+	if(fd < 0)
+		return -1;
+	n = strlen(user);
+	if(write(fd, user, n) != n){
+		close(fd);
+		return -1;
+	}
+	close(fd);
+	return 0;
+}