Re: set-file-extended-attributes and backups

[Paul Eggert]

On 12/21/12 08:44, Eli Zaretskii wrote:
> How about if it tried to copy ACLs, and if that failed, attempted to
> copy the file modes?  That would DTRT if possible, and fall back on
> the pre-ACL method if not.

That could lead to security issues if the file modes are more
permissive than the ACLs.

Is there an easy way to test whether a file's ACLs could deny
access when the file's modes would allow it?  If so, Emacs could
follow your suggestion when that test says "no".  If not, Emacs
could fall back on a conservative approximation to that test.
The simplest conservative approximation that I can think of offhand
is to test whether a file has any nontrivial ACLs.

Whatever test Emacs uses, if the test says "yes" Emacs should
be more cautious: create a destination file with a restrictive
mode (e.g., -rw-------), copy the data, then attempt to copy the ACLs,
and if the ACL copy fails then Emacs should not attempt to change
the mode.