bug#45258: mkdir man page unclear in describing -m flag

[Chris Colohan]

I like to present a challenge to my software engineer friends:  can you
tell me what this command does on Linux, if run in an empty directory?

mkdir -m 0755 -p ./usr/bin/foo

If they read the mkdir man page (
https://man7.org/linux/man-pages/man1/mkdir.1.html), they almost always say
the answer is:

- create the directory ./usr, with the mode 0755
- create the directory ./usr/bin, with the mode 0755
- create the directory ./usr/bin/foo, with the mode 0755

They are wrong.  (Side note -- this misunderstanding contributed to one of
the scariest outages Google has ever seen,
https://www.pdl.cmu.edu/SDI/2012/083012b.html).

What it actually does:

- create the directory ./usr, with the mode based on the umask
- create the directory ./usr/bin, with the mode based on the umask
- create the directory ./usr/bin/foo, with the mode 0755

I tried at the time to get the man page corrected, but I was told at the
time that nobody reads man pages, and the info page is correct, so it won't
be fixed.

I figured after almost 10 years, perhaps thinking has evolved.  Can we fix
the man page?

I have a suggested fix:  the current man page reads:

       -p, --parents
              no error if existing, make parent directories as needed

I can be updated to read:

       -p, --parents
              no error if existing, make parent directories as needed,
setting
              their file permission bits to the umask modified by ‘u+wx’.

I copied the new text from the info page.

Thanks!

Chris