Re: chmod octal form of sgid/suid removal fails

[Eric Blake]

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According to Jan Engelhardt on 5/12/2007 8:50 AM:
> Cc'ed bug-coreutils.
> The following bug affects at least:
>  coreutils 6.4 (used in opensuse 10.2 - open a bug report here)
>  coreutils 6.9

> 
> $ mkdir /dev/shm/me
> $ strace -e chmod chmod 0755 me
> chmod("me", 02755)                      = 0

Not a bug, and this is becoming a FAQ.  POSIX explicitly leaves the
special mode bits undefined when using octal permissions.  And the NEWS
for coreutils 6.0 is quite explicit:

  chmod, install, and mkdir now preserve a directory's set-user-ID and
  set-group-ID bits unless you explicitly request otherwise.  E.g.,
  `chmod 755 DIR' and `chmod u=rwx,go=rx DIR' now preserve DIR's
  set-user-ID and set-group-ID bits instead of clearing them, and
  similarly for `mkdir -m 755 DIR' and `mkdir -m u=rwx,go=rx DIR'.  To
  clear the bits, mention them explicitly in a symbolic mode, e.g.,
  `mkdir -m u=rwx,go=rx,-s DIR'.  To set them, mention them explicitly
  in either a symbolic or a numeric mode, e.g., `mkdir -m 2755 DIR',
  `mkdir -m u=rwx,go=rx,g+s' DIR.  This change is for convenience on
  systems where these bits inherit from parents.  Unfortunately other
  operating systems are not consistent here, and portable scripts
  cannot assume the bits are set, cleared, or preserved, even when the
  bits are explicitly mentioned.  For example, OpenBSD 3.9 `mkdir -m
  777 D' preserves D's setgid bit but `chmod 777 D' clears it.
  Conversely, Solaris 10 `mkdir -m 777 D', `mkdir -m g-s D', and
  `chmod 0777 D' all preserve D's setgid bit, and you must use
  something like `chmod g-s D' to clear it.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             address@hidden
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