Re: peculiar disappearance of most audit rules

Peter Grandi
2014-04-22T20:53:27Z

>> I don't know what is managing your system, but its probably
>> deleting paths.

> I am the sole user (as far as I know...) of both systems, [
> ... ] None of the "disappeared" paths seems to have been
> modified in any way. [ ... ] Anyhow, I have now recorded the
> inos of the watched directories, and I shall also run
> 'inotifywait -m /' to catch if possible any changes in '/opt'
> and '/boot'.

I have done this and this morning during 'mlocate' treewalking
some of the usual paths disappeared; I verified the inos and
the 'inotify' output and no inos changed nor any of the watched
directories changed.

Since the list of directories that *do not* disappear is
usually:

LIST_RULES: exit,always dir=/bin (0x4) perm=wa key=pkg-s
LIST_RULES: exit,always dir=/etc (0x4) perm=wa key=pkg-s
LIST_RULES: exit,always dir=/lib (0x4) perm=wa key=pkg-s
LIST_RULES: exit,always dir=/usr (0x4) perm=wa key=pkg-s
LIST_RULES: exit,always dir=/fs/sozan/loc (0xd) perm=wa key=pkg-l
LIST_RULES: exit,always dir=/fs/sozan/com (0xd) perm=wa key=pkg-l

and those that disappear tend to be far less frequently used
directories like '/boot', '/opt', '/lib32'. Rereading this:

>> [ ... ] device and inode information. This is, technically,
>> what your watch is on. If the inode disappears, then the rule
>> is ejected. Rules can survive across renames but not deletions.

it appears that I misread earlier: this says "inode", not
"inum". Also it says "inode disappears", which is not
necessarily always because the on-disk inode is deleted.

Thus I have come up with a potential explanation:

* The 'audit' module does not identify the watched file and
directory by (device,ino) but by a pointer to an inode table
entry, a bit like a filesystem module would.
* During treewalks a lot of inodes get cached in the in-memory
inode table.
* This creates pressure on the inode tables and thus the least
used (in some sense) inodes get evicted, and this includes
those for the "disappearing directories".
* When these least used inodes are evicted the 'audit' module
sees it as if it was a removal of the inode.

If the above is the right explanation it is a pretty big deal,
because it means that a way to disable many/most 'audit' watches
on files is to create and access a lot of inodes, which is
pretty easy to do.