How to find which mountpoint current working directory of a process correspond to?

Question

I have a program a.out running from /tmp/test. If I list /proc/$(pgrep a.out)/cwd, it is a symbolic link to /tmp/test.

Now, I bind /mnt/othertest onto /tmp/test with mount -o bind /mnt/othertest /tmp/test. Thus, this new mount is stacked and old /tmp/test is hidden.

Still, since a.out was launched before mount command, it has access to the old path.

The problem I want to tackle is how to know if a process with cwd set to /tmp/test refers to the directory from /mnt/othertest or not.

For opened files, it is easy, I can use fdinfo from proc pseudo-filesyste and read the mount id, but I did not find such information for cwd. I could check the start time of the process and compare it with the time when mount was launched, but because of possible forks it could be wrong.

Answer 1

Call stat(2) on /proc/PID/cwd and on the other directory and compare them by the device:inode tuple.

Example (notice that unless given the -L switch, the stat(1) utility will perform a lstat(2) system call instead of a stat(2)):

$ mkdir -p /tmp/foo /tmp/bar /tmp/baz
$ cd /tmp/foo
$ echo $$
20890

# mount -B /tmp/bar /tmp/foo
# stat -Lc %d:%i /proc/20890/cwd
2055:1835959

# stat -Lc %d:%i "$(readlink /proc/20890/cwd)" /tmp/foo /tmp/bar
2055:1843068
2055:1843068
2055:1843068

# umount /tmp/foo
# stat -Lc %d:%i /tmp/foo
2055:1835959

Just like the files under /proc/PID/fd, /proc/PID/cwd is not a real symlink -- an open(2), stat(2), etc. will not follow its target, but act directly on the actual object it refers to.