Posts in linux

Why df fails to show one or more file systems when run as an unprivileged user

One of my friends recently reached out with a fun problem. His monitoring system was periodically not firing when file systems grew past the thresholds he defined. When we hopped on one of his EC2 instances to debug the issue, I noticed that we were getting a permission denied (EACCES) errno when running df as their monitoring user: When we ran the same command as trusty UID 0, everything worked as expected: A quick check with strace verified this as well: If you aren't familiar with statfs(2), it returns information about a mounted file system in a statfs structure. Here is a blurb from the manual page describing which information is returned: I thought that df was setuid root like the mount uility, so when I initially saw the permission denied error I thought it was something unrelated to permissions…