Some fixes to mounting path validation (fix #3733)

[ntorresalberto]

Fixes for #3733 and Additional Issues Found During Inspection

This PR addresses the problems outlined in #3733, along with some related issues discovered during review. Details below.

Issues Fixed

Relative Paths Incorrectly Resolving to /home/ubuntu

The issue occurs when relative paths unintentionally resolve to /home/ubuntu, allowing unexpected mounts. Here’s how to reproduce it (same steps as in the issue, repeated for clarity):

$ mkdir foodir
$ multipass delete -p foo
$ multipass launch --name foo

# Expected to fail, as it does:
$ multipass mount foodir foo:/home/ubuntu  
mount failed: The following errors occurred:
unable to mount to "/home/ubuntu"

# Expected not to work, but it does (matches /home/ubuntu):
$ multipass mount foodir foo:.              
$ multipass mount foodir foo:../ubuntu      

Relative Paths Allowing Mounts to Restricted Directories

Similar path resolution issues allowed mounting to restricted locations such as /dev/, /:

# Expected not to work, but it does (matches /dev):
$ multipass mount foodir foo:../../dev  

# Expected not to work, but it does (matches /):
$ multipass mount foodir foo:../..          

Issues with Non-Normalized Paths Causing Duplicate Entries

Paths should be normalized before being stored in vm_mounts to prevent different representations of the same path from being treated as separate entries. Without normalization, paths like /home/foo and ../foo are considered distinct, leading to inconsistencies.

# Expected to work, and it does:
$ multipass mount foodir foo:/home/foo                     

# Expected NOT to work, and correctly fails (absolute paths are handled properly):
$ multipass mount foodir foo:/home/../home/foo  
mount failed: The following errors occurred:
"/home/foo" is already mounted in 'foo'

# Expected not to work, but it does:
$ multipass mount foodir foo:../foo                                

---

Fix Implementation

A new function, make_abspath, has been added to src/utils/utils.cpp.

• Converts relative paths to absolute paths, defaulting from /home/ubuntu.
• Guarantees unique, normalized paths for each mount, preventing duplicate entries.

Validation & Tests After Fixes

$ multipass delete -p foo
$ multipass launch --name foo
$ multipass mount foodir foo:/home/ubuntu
mount failed: The following errors occurred:
unable to mount to "/home/ubuntu"
$ multipass mount foodir foo:.
mount failed: The following errors occurred:
unable to mount to "/home/ubuntu"
$ multipass mount foodir foo:../ubuntu
mount failed: The following errors occurred:
unable to mount to "/home/ubuntu"
$ multipass mount foodir foo:../../dev
mount failed: The following errors occurred:
unable to mount to "/dev"
$ multipass mount foodir foo:../..
mount failed: The following errors occurred:
unable to mount to "/"
$ multipass mount foodir foo:/home/foo
$ multipass mount foodir foo:/home/../home/foo
mount failed: The following errors occurred:
"/home/foo" is already mounted in 'foo'
$ multipass mount foodir foo:../foo
mount failed: The following errors occurred:
"/home/foo" is already mounted in 'foo'

[georgeliao]

@ntorresalberto
Thank you for your contribution.
We’ll review the code once we have some time to dedicate to it.