libbpf-tools/filelife: Fix wrong full-path

[Rtoax]

The process that creates the file and the process that deletes the file may be two completely different processes, and their cwds are different. We get the file name from the file dentry and concatenate the cwd to create the full-path. If we use the dentry name of the deleted file + the cwd-dentry of the process that created the file, we will get an incorrect full-path. For example:

$ realpath .
/home/sda/git-repos/iovisor/bcc/libbpf-tools

(1) correct full-path

$ touch a.out               cwd_dentry = /home/sda/git-repos/iovisor/bcc/libbpf-tools
$ cd ..
$ rm libbpf-tools/a.out     dentry = a.out

(2) wrong full-path

$ touch libbpf-tools/a.out  cwd_dentry = /home/sda/git-repos/iovisor/bcc/
$ cd libbpf-tools/
$ rm a.out                  dentry = a.out

$ sudo ./filelife -F
Tracing the lifespan of short-lived files ... Hit Ctrl-C to end.
TIME     PID    COMM  AGE(s)  FILE
08:56:30 13015  rm    5.17    /home/sda/git-repos/iovisor/bcc/libbpf-tools/a.out (1)
08:56:45 13075  rm    5.17    /home/sda/git-repos/iovisor/bcc/a.out              (2)

In fact, we do not need to record the cwd-dentry of the process that created the file. We only need to pass the dentry of the deleted file as the dentry parameter of bpf_dentry_full_path(). Of course, the vfsmount of cwd still needs to be obtained when creating it.

Fix: commit 74bddcb ("libbpf-tools/filelife: support full-path")