Re: [PATCH 4/7] autofs4 - fix indirect mount pending expire race

[Ian Kent]

On Fri, 2008-07-18 at 14:52 -0400, Jeff Moyer wrote:
> Ian Kent <raven@xxxxxxxxxx> writes:
> 
> > The selection of a dentry for expiration and the setting of the
> > AUTOFS_INF_EXPIRING flag isn't done atomically which can lead to
> > lookups walking into an expiring mount.
> >
> > What happens is that an expire is initiated by the daemon and
> > a dentry is selected for expire but, since there is no lock
> > held between the selection and setting of the expiring flag,
> > a process may find the flag clear and continue walking into
> > the mount tree at the same time the daemon attempts the expire
> > it.
> >
> > Signed-off-by: Ian Kent <raven@xxxxxxxxxx>
> >
> > ---
> 
> [...]
> >  static inline void autofs4_copy_atime(struct file *src, struct file *dst)
> > diff --git a/fs/autofs4/expire.c b/fs/autofs4/expire.c
> > index 894fee5..19f5bea 100644
> > --- a/fs/autofs4/expire.c
> > +++ b/fs/autofs4/expire.c
> > @@ -292,6 +292,8 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
> >  	struct list_head *next;
> >  	int do_now = how & AUTOFS_EXP_IMMEDIATE;
> >  	int exp_leaves = how & AUTOFS_EXP_LEAVES;
> > +	struct autofs_info *ino;
> > +	unsigned int ino_count;
> >  
> >  	if (!root)
> >  		return NULL;
> > @@ -316,6 +318,9 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
> >  		dentry = dget(dentry);
> >  		spin_unlock(&dcache_lock);
> >  
> > +		spin_lock(&sbi->fs_lock);
> > +		ino = autofs4_dentry_ino(dentry);
> > +
> >  		/*
> >  		 * Case 1: (i) indirect mount or top level pseudo direct mount
> >  		 *	   (autofs-4.1).
> > @@ -326,6 +331,11 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
> >  			DPRINTK("checking mountpoint %p %.*s",
> >  				dentry, (int)dentry->d_name.len, dentry->d_name.name);
> >  
> > +			/* Path walk currently on this dentry? */
> > +			ino_count = atomic_read(&ino->count) + 2;
> > +			if (atomic_read(&dentry->d_count) > ino_count)
> > +				goto next;
> > +
> 
> It would be nice to document the +2.  The +1s (below) may be evident
> given that we did a dget above, but still might merit mention.
> 
> >  			/* Can we umount this guy */
> >  			if (autofs4_mount_busy(mnt, dentry))
> >  				goto next;
> > @@ -343,23 +353,25 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
> >  
> >  		/* Case 2: tree mount, expire iff entire tree is not busy */
> >  		if (!exp_leaves) {
> > -			/* Lock the tree as we must expire as a whole */
> > -			spin_lock(&sbi->fs_lock);
> > -			if (!autofs4_tree_busy(mnt, dentry, timeout, do_now)) {
> > -				struct autofs_info *inf = autofs4_dentry_ino(dentry);
> > +			/* Path walk currently on this dentry? */
> > +			ino_count = atomic_read(&ino->count) + 1;
> > +			if (atomic_read(&dentry->d_count) > ino_count)
> > +				goto next;
> >  
> > -				/* Set this flag early to catch sys_chdir and the like */
> > -				inf->flags |= AUTOFS_INF_EXPIRING;
> > -				spin_unlock(&sbi->fs_lock);
> > +			if (!autofs4_tree_busy(mnt, dentry, timeout, do_now)) {
> >  				expired = dentry;
> >  				goto found;
> >  			}
> > -			spin_unlock(&sbi->fs_lock);
> >  		/*
> >  		 * Case 3: pseudo direct mount, expire individual leaves
> >  		 *	   (autofs-4.1).
> >  		 */
> >  		} else {
> > +			/* Path walk currently on this dentry? */
> > +			ino_count = atomic_read(&ino->count) + 1;
> > +			if (atomic_read(&dentry->d_count) > ino_count)
> > +				goto next;
> > +
> >  			expired = autofs4_check_leaves(mnt, dentry, timeout, do_now);
> >  			if (expired) {
> >  				dput(dentry);

The expired dentry may be a different dentry!

> > @@ -367,6 +379,7 @@ static struct dentry *autofs4_expire_indirect(struct super_block *sb,
> >  			}
> >  		}
> >  next:
> > +		spin_unlock(&sbi->fs_lock);
> >  		dput(dentry);
> >  		spin_lock(&dcache_lock);
> >  		next = next->next;
> > @@ -377,6 +390,9 @@ next:
> >  found:
> >  	DPRINTK("returning %p %.*s",
> >  		expired, (int)expired->d_name.len, expired->d_name.name);
> > +	ino = autofs4_dentry_ino(expired);
> 
> If we get here, ino is already set to the autofs4_dentry_ino(expired),
> so this statement is redundant.

Almost but see above.

> 
> [...]
> > diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
> > index 2944b28..2ed2a51 100644
> > --- a/fs/autofs4/root.c
> > +++ b/fs/autofs4/root.c
> > @@ -133,7 +133,10 @@ static int try_to_fill_dentry(struct dentry *dentry, int flags)
> >  	/* Block on any pending expiry here; invalidate the dentry
> >             when expiration is done to trigger mount request with a new
> >             dentry */
> > -	if (ino && (ino->flags & AUTOFS_INF_EXPIRING)) {
> > +	spin_lock(&sbi->fs_lock);
> > +	if (ino->flags & AUTOFS_INF_EXPIRING) {
> > +		spin_unlock(&sbi->fs_lock);
> > +
> >  		DPRINTK("waiting for expire %p name=%.*s",
> >  			 dentry, dentry->d_name.len, dentry->d_name.name);
> 
> This is okay, since we wait on the AUTOFS_INF_EXPIRING flag in
> validate_request.  That check is done outside the lock, but I doubt
> there are issues with not seeing an update since you perform a schedule
> there.
> 
> > @@ -149,8 +152,11 @@ static int try_to_fill_dentry(struct dentry *dentry, int flags)
> >  		status = d_invalidate(dentry);
> >  		if (status != -EBUSY)
> >  			return -EAGAIN;
> > -	}
> >  
> > +		goto cont;
> > +	}
> > +	spin_unlock(&sbi->fs_lock);
> > +cont:
> >  	DPRINTK("dentry=%p %.*s ino=%p",
> >  		 dentry, dentry->d_name.len, dentry->d_name.name, dentry->d_inode);
> >  
> > @@ -229,15 +235,21 @@ static void *autofs4_follow_link(struct dentry *dentry, struct nameidata *nd)
> >  		goto done;
> >  
> >  	/* If an expire request is pending wait for it. */
> > -	if (ino && (ino->flags & AUTOFS_INF_EXPIRING)) {
> > +	spin_lock(&sbi->fs_lock);
> > +	if (ino->flags & AUTOFS_INF_EXPIRING) {
> > +		spin_unlock(&sbi->fs_lock);
> > +
> >  		DPRINTK("waiting for active request %p name=%.*s",
> >  			dentry, dentry->d_name.len, dentry->d_name.name);
> >  
> >  		status = autofs4_wait(sbi, dentry, NFY_NONE);
> >  
> >  		DPRINTK("request done status=%d", status);
> > -	}
> >  
> > +		goto cont;
> > +	}
> > +	spin_unlock(&sbi->fs_lock);
> > +cont:
> 
> could've done an:
> } else
> 	spin_unlock(&sbi->fs_lock);
> 
> But, whatever...
> 
> Reviewed-by: Jeff Moyer <jmoyer@xxxxxxxxxx>

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