Add fs mount point identification in 'tools/opensnoop.py'

[dennisorlando]

Hi :)
Not sure if this addresses existing issues, a quick look didn't give me any result..
Anyways: there are a ton of filesystem roots in the linux filetree which mess up the output of opensnoop.py.
E.g. when working with files in /tmp, let's say /tmp/averynicefile, only /averynicefile gets displayed since that file is in the "root" / of a tmpfs file system.
This simple commit reconstructs the mountpoint information by passing the device identifier (dev_t) field of the BPF's PWD directory to the userspace python program, which retrieves the mountpoint information by grepping /proc/self/mountinfo

[dennisorlando]

Update: I just stumbled upon #4285 but that's literally 2 years old... + it's outdated: struct vfsmount doesn't appear to include mnt_parent anymore

[xingfeng2510]

Update: I just stumbled upon #4285 but that's literally 2 years old... + it's outdated: struct vfsmount doesn't appear to include mnt_parent anymore

Not outdated actually, struct vfsmount never includes mnt_parent, the fixup still works now, you should try it.

And, unwind dentry by reading /proc/self/mountinfo frequently is inefficient. #4285 refers to kernel function d_path, which returns the path of a dentry and unwinds in kernel.

for example, when reading such a link file /proc/793073/fd/4, it converts dentry to full path /usr/local/agent/log/agent.log, which contains at least two mountpoints, by invoking kernel functions do_proc_readlink -> d_path -> __prepend_path source link: d_path

# ls -l /proc/793073/fd/4
lrwx------ 1 root root 64 Nov 17 22:41 /proc/793073/fd/4 -> /usr/local/agent/log/agent.log