Date: Fri, 14 Oct 2005 10:56:36 -0700 (PDT) From: Don Lewis <truckman@FreeBSD.org> To: jhb@FreeBSD.org Cc: mi+mx@aldan.algebra.com, freebsd-current@FreeBSD.org, re@FreeBSD.org, kris@obsecurity.org Subject: Re: 6.0 hangs (while building OOo) Message-ID: <200510141756.j9EHuahT054866@gw.catspoiler.org> In-Reply-To: <200510132325.j9DNPBif052696@gw.catspoiler.org>
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On 13 Oct, Don Lewis wrote:
> The bug is that once we have unlocked pdp, another thread can do a
> lookup and overwrite dp->i_ino, so instead of getting the vnode for the
> ".." directory entry, VFS_VGET() will return the vnode for a
> subdirectory of the current directory, and when we relock the current
> directory we'll have a lock order reversal.
>
> Even if this doesn't result in a deadlock, it looks like it has the
> potential for mucking up lookups that involve "..". I also don't
> currently see a way for this to become a vnode lock leak.
I think the leak happens when dp->i_ino gets overwritten by the inode
number for ".". This causes ufs_lookup() to recurse on the lock for the
current directory vnode (the lock is first acquired by VFS_VGET() and
then recursed by vn_lock()). This isn't expected by lookup(), which
compares whether the vnode returned by VOP_LOOKUP() is the same as the
directory vnode and uses this information to decide whether to call
vput() or vrele().
> The fix is to preserve a copy of dp->d_ino before unlocking pdp,
> and pass the saved value to VFS_VGET().
>
> Index: sys/ufs/ufs/ufs_lookup.c
> ===================================================================
> RCS file: /home/ncvs/src/sys/ufs/ufs/ufs_lookup.c,v
> retrieving revision 1.77
> diff -u -r1.77 ufs_lookup.c
> --- sys/ufs/ufs/ufs_lookup.c 13 Apr 2005 10:59:09 -0000 1.77
> +++ sys/ufs/ufs/ufs_lookup.c 13 Oct 2005 23:20:59 -0000
> @@ -153,6 +153,7 @@
> int flags = cnp->cn_flags;
> int nameiop = cnp->cn_nameiop;
> struct thread *td = cnp->cn_thread;
> + u_int32_t saved_ino;
>
> bp = NULL;
> slotoffset = -1;
> @@ -557,8 +558,9 @@
> */
> pdp = vdp;
> if (flags & ISDOTDOT) {
> + saved_ino = dp->i_ino;
> VOP_UNLOCK(pdp, 0, td); /* race to get the inode */
> - error = VFS_VGET(pdp->v_mount, dp->i_ino,
> + error = VFS_VGET(pdp->v_mount, saved_ino,
> cnp->cn_lkflags, &tdp);
> vn_lock(pdp, LK_EXCLUSIVE | LK_RETRY, td);
> if (error)
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