bindmount-regfile.c

#include <errno.h>                                                                                                                                                                                                                                                                                   
#include <stdio.h>                                                                                                                                                                                                                                                                                   
#include <string.h>                                                                                                                                                                                                                                                                                  
#include <unistd.h>                                                                                                                                                                                                                                                                                  
#include <fcntl.h>                                                                                                                                                                                                                                                                                   
#include <sys/stat.h>                                                                                                                                                                                                                                                                                
#include <sys/types.h>                                                                                                                                                                                                                                                                               
#include <sys/mount.h>                                                                                                                                                                                                                                                                               
                                                                                                                                                                                                                                                                                                     
int main(){
       char *from = "/tmp/bm-source";
       char *to = "/tmp/bm-target";
       char *testfile = "/tmp/bm-source/testfile";
       int rs1 = remove(from);
       int rs2 = remove(to);
       int rs3 = mkdir(from, 0777);
       int rs5 = close(open(to, O_RDWR|O_CREAT, 0777));
       int rs6 = close(open(testfile, O_RDWR|O_CREAT, 0777));
       int rs7 = mount(from, to, NULL, MS_BIND, NULL);

       printf("---\nmount: %s\n---\n", strerror(errno));
       return rs7;
       
}

$ sudo rm -rf /tmp/bm-* ; cc bindmountf.c ; sudo ./a.out ; uname -r ; ls -la /tmp/bm-*

---
mount: Not a directory

---
4.5.0-2-amd64
-rwxr-xr-x 1 root root    0 May 20 16:55 /tmp/bm-target

/tmp/bm-source:
total 8
drwxr-xr-x  2 root root 4096 May 20 16:55 .
drwxrwxrwt 21 root root 4096 May 20 16:55 ..
-rwxr-xr-x  1 root root    0 May 20 16:55 testfile