[RFC PATCH 3/3] fs: detect that the i_rwsem has already been taken exclusively

[Mimi Zohar]

On Thu, 2017-09-28 at 16:39 -0700, Linus Torvalds wrote:
> On Thu, Sep 28, 2017 at 3:02 PM, Dave Chinner <david at fromorbit.com> wrote:
> > On Thu, Sep 28, 2017 at 08:39:33AM -0400, Mimi Zohar wrote:
> >> Don't attempt to take the i_rwsem, if it has already been taken
> >> exclusively.
> >>
> >> Signed-off-by:  Mimi Zohar <zohar at linux.vnet.ibm.com>
> >
> > That's bloody awful.
> >
> > The locking in filesystem IO paths is already complex enough without
> > adding a new IO path semantic that says "caller has already locked
> > the i_rwsem in some order and some dependencies that we have no idea
> > about".
> 
> I do have to admit that I never got a satisfactory answer on why IMA
> doesn't just use its own private per-inode lock for this all.
> 
> It isn't using the i_rwsem for file consistency reasons anyway, so it
> seems to be purely about serializing the actual signature generation
> with the xattr writing, but since IMA does those both, why isn't IMA
> just using its own lock (not the filesystem lock) to do that?

Originally IMA did define it's own lock, prior to IMA-appraisal.  IMA-
appraisal introduced writing the file hash as an xattr, which required
taking the i_mutex.  process_measurement() and ima_file_free() took
the iint->mutex first and then the i_mutex, while setxattr, chmod and
chown took the locks in reverse order.  To resolve the potential
deadlock, the iint->mutex was eliminated.

Mimi

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