06xattr.txt

# Copyright (C) 2014-2020 Junjiro R. Okajima
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Listing XATTR/EA and getting the value
----------------------------------------------------------------------
For the inode standard attributes (owner, group, timestamps, etc.), aufs
shows the values from the topmost existing file. This behaviour is good
for the non-dir entries since the bahaviour exactly matches the shown
information. But for the directories, aufs considers all the same named
entries on the lower branches. Which means, if one of the lower entry
rejects readdir call, then aufs returns an error even if the topmost
entry allows it. This behaviour is necessary to respect the branch fs's
security, but can make users confused since the user-visible standard
attributes don't match the behaviour.
To address this issue, aufs has a mount option called dirperm1 which
checks the permission for the topmost entry only, and ignores the lower
entry's permission.

A similar issue can happen around XATTR.
getxattr(2) and listxattr(2) families behave as if dirperm1 option is
always set. Otherwise these very unpleasant situation would happen.
- listxattr(2) may return the duplicated entries.
- users may not be able to remove or reset the XATTR forever,


XATTR/EA support in the internal (copy,move)-(up,down)
----------------------------------------------------------------------
Generally the extended attributes of inode are categorized as these.
- "security" for LSM and capability.
- "system" for posix ACL, 'acl' mount option is required for the branch
  fs generally.
- "trusted" for userspace, CAP_SYS_ADMIN is required.
- "user" for userspace, 'user_xattr' mount option is required for the
  branch fs generally.

Moreover there are some other categories. Aufs handles these rather
unpopular categories as the ordinary ones, ie. there is no special
condition nor exception.

In copy-up, the support for XATTR on the dst branch may differ from the
src branch. In this case, the copy-up operation will get an error and
the original user operation which triggered the copy-up will fail. It
can happen that even all copy-up will fail.
When both of src and dst branches support XATTR and if an error occurs
during copying XATTR, then the copy-up should fail obviously. That is a
good reason and aufs should return an error to userspace. But when only
the src branch support that XATTR, aufs should not return an error.
For example, the src branch supports ACL but the dst branch doesn't
because the dst branch may natively un-support it or temporary
un-support it due to "noacl" mount option. Of course, the dst branch fs
may NOT return an error even if the XATTR is not supported. It is
totally up to the branch fs.

Anyway when the aufs internal copy-up gets an error from the dst branch
fs, then aufs tries removing the just copied entry and returns the error
to the userspace. The worst case of this situation will be all copy-up
will fail.

For the copy-up operation, there two basic approaches.
- copy the specified XATTR only (by category above), and return the
  error unconditionally if it happens.
- copy all XATTR, and ignore the error on the specified category only.

In order to support XATTR and to implement the correct behaviour, aufs
chooses the latter approach and introduces some new branch attributes,
"icexsec", "icexsys", "icextr", "icexusr", and "icexoth".
They correspond to the XATTR namespaces (see above). Additionally, to be
convenient, "icex" is also provided which means all "icex*" attributes
are set (here the word "icex" stands for "ignore copy-error on XATTR").

The meaning of these attributes is to ignore the error from setting
XATTR on that branch.
Note that aufs tries copying all XATTR unconditionally, and ignores the
error from the dst branch according to the specified attributes.

Some XATTR may have its default value. The default value may come from
the parent dir or the environment. If the default value is set at the
file creating-time, it will be overwritten by copy-up.
Some contradiction may happen I am afraid.
Do we need another attribute to stop copying XATTR? I am unsure. For
now, aufs implements the branch attributes to ignore the error.