On Fri, Nov 19, 2010 at 05:35:52PM -0500, J. Bruce Fields wrote:
> On Thu, Nov 18, 2010 at 08:52:55PM -0500, Josef Bacik wrote:
> > While trying to track down some NFS problems with BTRFS, I kept noticing I was
> > getting -EACCESS for no apparent reason. Eric Paris and printk() helped me
> > figure out that it was SELinux that was giving me grief, with the following
> > denial
> >
> > type=AVC msg=audit(1290013638.413:95): avc: denied { 0x800000 } for pid=1772
> > comm="nfsd" name="" dev=sda1 ino=256 scontext=system_u:system_r:kernel_t:s0
> > tcontext=system_u:object_r:unlabeled_t:s0 tclass=file
> >
> > Turns out this is because in d_obtain_alias if we can't find an alias we create
> > one and do all the normal instantiation stuff, but we don't do the
> > security_d_instantiate.
> >
> > Usually we are protected from getting a hashed dentry that hasn't yet run
> > security_d_instantiate() by the parent's i_mutex, but obviously this isn't an
> > option there, so in order to deal with the case that a second thread comes in
> > and finds our new dentry before we get to run security_d_instantiate(), we go
> > ahead and call it if we find a dentry already. Eric assures me that this is ok
> > as the code checks to see if the dentry has been initialized already so calling
> > security_d_instantiate() against the same dentry multiple times is ok. With
> > this patch I'm no longer getting errant -EACCESS values.
>
> Thanks, I can't see any reason that wouldn't work.
(FWIW, I also ran my usual nfs regression tests with this applied. They
don't exercise the problem you were seeing, but maybe it's at least some
sort of sanity check.)
--b.
>
> --b.
>
> >
> > Signed-off-by: Josef Bacik <josef@xxxxxxxxxx>
> > ---
> > V1->V2:
> > -added second security_d_instantiate() call
> >
> > fs/dcache.c | 3 +++
> > 1 files changed, 3 insertions(+), 0 deletions(-)
> >
> > diff --git a/fs/dcache.c b/fs/dcache.c
> > index 23702a9..119d489 100644
> > --- a/fs/dcache.c
> > +++ b/fs/dcache.c
> > @@ -1201,9 +1201,12 @@ struct dentry *d_obtain_alias(struct inode *inode)
> > spin_unlock(&tmp->d_lock);
> >
> > spin_unlock(&dcache_lock);
> > + security_d_instantiate(tmp, inode);
> > return tmp;
> >
> > out_iput:
> > + if (res && !IS_ERR(res))
> > + security_d_instantiate(res, inode);
> > iput(inode);
> > return res;
> > }
> > --
> > 1.6.6.1
> >
> --
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