rsync - Feb 2016 - [Bug 11726] New: --copy-links cause rsync to fail with ancestor symlink (..)

https://bugzilla.samba.org/show_bug.cgi?id=11726

            Bug ID: 11726
           Summary: --copy-links cause rsync to fail with ancestor symlink
                    (..)
           Product: rsync
           Version: 3.1.2
          Hardware: All
                OS: All
            Status: NEW
          Severity: major
          Priority: P5
         Component: core
          Assignee: wayned at samba.org
          Reporter: cyphar at cyphar.com
        QA Contact: rsync-qa at samba.org

If you have a directory created like so:

% mkdir -p from/dir/
% ln -s .. from/dir/loop
% rsync -a --copy-links from to
rsync:
readlink_stat("/tmp/from/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop/dir/loop")
failed: Too many levels of symbolic links (40)
rsync error: some files/attrs were not transferred (see previous errors) (code
23) at main.c(1178) [sender=3.1.2]

For a trivial case of infinitely recursive symlinks (in other words when you go
through a symlink that takes you to an ancestor), rsync should just declare
"that's as far as we need to go here".

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--- Comment #1 from cyphar at cyphar.com ---
I'm working on writing a patch for this (although this is my first time
looking
at the rsync codebase, so it might take a while). I'll ping this thread if
I'm
unable to write one.

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--- Comment #2 from cyphar at cyphar.com ---
It turns out that this issue also happens even when you don't use
--copy-link
(albeit a different error):

% rsync -a from to
cannot delete non-empty directory: from/dir/loop
could not make way for new symlink: from/dir/loop
rsync error: some files/attrs were not transferred (see previous errors) (code
23) at main.c(1178) [sender=3.1.3dev]

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Wayne Davison <wayned at samba.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |WONTFIX
             Status|NEW                         |RESOLVED

--- Comment #3 from Wayne Davison <wayned at samba.org> ---
Yes, if you tell rsync to copy the symlinks it does it. If you have one or more
looping symlinks then you need to exclude them from the copy or not use
--copy-links. As for the other error, you need to use --delete or --force to
copy over a non-empty directory, as mentioned in the man page.

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--- Comment #4 from cyphar at cyphar.com ---
(In reply to Wayne Davison from comment #3)
> Yes, if you tell rsync to copy the symlinks it does it. If you have one or
more looping symlinks then you need to exclude them from the copy or not use
--copy-links.
That's just patently ridiculous. cp -r doesn't fail when you ask it to
copy a
directory that has symlink loops because "that's what you asked me to
do". It
literally just takes doing some fairly trivial checks before adding extra files
to file_list (i.e. does the path we're adding resolve to an ancestor of a
path
we've already seen?).

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cyphar at cyphar.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|WONTFIX                     |---
             Status|RESOLVED                    |REOPENED

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Wayne Davison <wayned at samba.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|REOPENED                    |RESOLVED
         Resolution|---                         |WONTFIX

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Wayne Davison <wayned at samba.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|rsync fails to copy         |rsync fails to copy
                   |directory containing        |directory containing
                   |ancestor symlink (..)       |ancestor symlink with
                   |                            |--copy-links

--- Comment #5 from Wayne Davison <wayned at samba.org> ---
Keep in mind that "cp -r" doesn't dereference symlinks -- that
takes their -L
option (similar to rsync's --copy-links). If someone is copying a file tree
and
turning symlinks into real files/dirs, they should really know in advance what
the symlinks are in order for the copy to be safe. Yes, we could stop looping
with some additional checks. It might be worth it to at least not follow
'.'
and "..".

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