Yet another --private-etc symlink issue, with /etc/alternatives/

[JulienPalard]

Description

Depending on how I set --private-etc, /etc/alternatives/cc is sometimes a symlink, sometimes a copy:

Steps to Reproduce

If I whitelist /etc/alternatives/cc alone, I get a copy of cc instead of its symlink:

$ ls -lah /etc/alternatives/cc
lrwxrwxrwx 1 root root 12 Oct 14  2019 /etc/alternatives/cc -> /usr/bin/gcc
$ firejail --quiet --private-etc=alternatives/cc ls -lah /etc/alternatives/cc
-rwxr-xr-x 1 65534 65534 1.3M Sep 23 12:22 /etc/alternatives/cc

Which is bad (cc is not happy to be here). On the other hand, if I just whielist the whole alternatives directory:

$ firejail --quiet --private-etc=alternatives ls -lah /etc/alternatives/cc
Error fcopy: size limit of 500 MB reached
lrwxrwxrwx 1 65534 65534 32 Sep 23 12:22 /etc/alternatives/cc -> /usr/bin/x86_64-linux-gnu-gcc-12

this time /etc/alternatives/cc is a symlink (and cc is happy). But fcopy whines about a 500MB limit being reached, which is probably wrong as /etc/alternatives only contains symlinks:

$ du -sh /etc/
4.0K	/etc/

Expected behavior

I know that symlink is a security issue magnet, so I may miss something obvious, but as there's already a way to have the symlink I quickly though "what about the other way also giving a symlink ?".

Environment

• Debian testing (bookworm)
• Firejail 0.9.70

Checklist

• The issues is caused by firejail (i.e. running the program by path (e.g. /usr/bin/vlc) "fixes" it).
• I can reproduce the issue without custom modifications (e.g. globals.local).
• The program has a profile. (If not, request one in https://github.com/netblue30/firejail/issues/1139)
• The profile (and redirect profile if exists) hasn't already been fixed upstream.
• I have performed a short search for similar issues (to avoid opening a duplicate).
  • I'm aware of browser-allow-drm yes/browser-disable-u2f no in firejail.config to allow DRM/U2F in browsers.
• I used --profile=PROFILENAME to set the right profile. (Only relevant for AppImages)

[gerasiov]

The problem is in the logic used for size calculation: for symlinks fcopy looks at target file, not at the symlink itself.

Yet another... FYI, there's been quite a few PR's marked as drafts due to the private-etc refactoring. Most importantly IMO is #5645. Somewhere along the line this should be decided upon. IMO best before the next release.

Just my $ 0.02