Am 03.02.2015 um 08:56 schrieb Al Viro:
On Tue, Feb 03, 2015 at 07:58:50AM +0100, Alexander Holler wrote:
Charming. Now, what exactly happens if two such syscalls overlap in time?
What do you think will happen? I assume you haven't looked at how I've
implemented set_secure_delete(). CHarming.
AFAICS, you get random unlink() happening at the same time hit by that
mess, whether they'd asked for it or not. What's more, this counter
of yours is *not* guaranteed to be elevated during the final iput() of the
inode you wanted to get - again, ls -lR racing with that syscall can
elevate the refcount of dentry, making d_delete() in vfs_unlink() just
remove that dentry from hash, while keeping it positive. If dentry
reference grabbed by stat(2) is released after both dput() and iput() in
do_unlinkat(), the final iput() will be done when stat(2) drops its
reference to dentry, triggering immediate dentry_kill() (since dentry
has already been unhashed) and dentry_iput() from it.
Thanks for the short explanation. I will see if I can make sense out of
it for me to get an idea how to solve that.
IOW, this counter is both too crude (it's fs-wide, for crying out loud)
*and* not guaranteed to cover enough. _IF_ you want that behaviour at
Sure it is crude.
But it keeps the patches simple. As I've written, unlinkat_s() isn't
meant for everyday usage, just for the rare case when one really wants
to get rid of some contents. Therefor execution speed or an i/o slowdown
while the "secure deletion" is in work is totally ignored
And that "rare case" doesn't include military security levels, it's just
meant for ordinary people which want make it much, much harder for other
ordinary people (or geeks or kernel maintainers) to read the deleted
content ever again. It's far too easy to use grep or something similiar
to find seemingly deleted stuff at device level again (after it was
deleted by what filesystems are offering nowadays). Especially if one
thinks at stuff like certificates and similiar which can be identified
by common patterns (bit sequences) they use.
Alexander Holler
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