See also: #4128, #4419

Feedback from Ubuntu user:

https://bugs.launchpad.net/ubuntu/+source/mc/+bug/2097560

I have a zip file where the first enclosed file is PDF. For a reason (I strictly prefer okular over evince and was lazy to find another place to set the priority), I changed the mc extension file (mc.ext) entry to the following:

type/^PDF
        Open=okular %f & disown

Unexpectedly, this caused misopening of some zip files. Checking what is happening by strace showed that mc calls file with arguments to detect the enclosed contents:

1368162 19:40:43.017932 execve("/usr/bin/file", ["file", "-z", "-S", "-L", "/home/netch/Downloads/sql_12.zip"], 0x6017f1d3d398 /* 68 vars */ <unfinished ...>

and this file helper returns:

1368162 19:40:43.040724 write(1, "PDF document, version 1.3, 90 pages (Zip archive data, at least v2.0 to extract, compression method=deflate)\n", 109) = 109

The rule in the extension file detects, according to "^PDF", this is PDF and runs okular instead of ark for this file. Okular deliberately rejects opening zip.

With the default extension file contents:

    Open=/usr/lib/mc/ext.d/doc.sh open pdf

the bug still has place but is masked because passing through doc.sh and then xdg-open rechecks the file type using file (technically, with kioslave5) without -z that allows proper detection.

So using simple file/ rules in mc extension file is incompatible with using file -z for deeper analysis (if this deeper analysis has sense in general).

Iʼm uncertain for security impact but it could have taken place in a more complicated scenario.