[RFC,1/1] shiftfs: uid/gid shifting bind mount
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Message ID 1487352280.4351.19.camel@HansenPartnership.com
State New
Headers show

Commit Message

James Bottomley Feb. 17, 2017, 5:24 p.m. UTC
On Fri, 2017-02-17 at 02:29 +0000, Al Viro wrote:
> On Sat, Feb 04, 2017 at 11:19:32AM -0800, James Bottomley wrote:
> 
> > +static const struct dentry_operations shiftfs_dentry_ops = {
> > +	.d_release	= shiftfs_d_release,
> > +	.d_real		= shiftfs_d_real,
> > +};
> 
> In other words, those dentries are *never* revalidated.  Nevermind 
> that underlying fs might be mounted elsewhere and be actively 
> modified under you.
> 
> > +static struct dentry *shiftfs_lookup(struct inode *dir, struct
> > dentry *dentry,
> > +				     unsigned int flags)
> > +{
> > +	struct dentry *real = dir->i_private, *new;
> > +	struct inode *reali = real->d_inode, *newi;
> > +	const struct cred *oldcred, *newcred;
> > +
> > +	inode_lock(reali);
> > +	oldcred = shiftfs_new_creds(&newcred, dentry->d_sb);
> > +	new = lookup_one_len(dentry->d_name.name, real, dentry
> > ->d_name.len);
> > +	shiftfs_old_creds(oldcred, &newcred);
> > +	inode_unlock(reali);
> > +
> > +	if (IS_ERR(new))
> > +		return new;
> > +
> > +	dentry->d_fsdata = new;
> > +
> > +	if (!new->d_inode)
> > +		return NULL;
> 
> What happens when somebody comes along and creates the damn thing on 
> the underlying fs?  _Not_ via your code, that is - using the 
> underlying fs mounted elsewhere.

Point taken.  This, I think fixes the dcache revalidation issue.

James

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Comments

Al Viro Feb. 17, 2017, 5:51 p.m. UTC | #1
On Fri, Feb 17, 2017 at 09:24:40AM -0800, James Bottomley wrote:

> > What happens when somebody comes along and creates the damn thing on 
> > the underlying fs?  _Not_ via your code, that is - using the 
> > underlying fs mounted elsewhere.
> 
> Point taken.  This, I think fixes the dcache revalidation issue.

No, it doesn't.  Consider a local filesystem.  Those do not have any
->d_revalidate() - the kernel bloody well knows what happens to
directories.  If e.g. a previously absent file gets created, it's
been done by the kernel itself and dentry has been made positive; if
a previously existing file has been removed, dentry has either become
negative or, if it had been pinned (e.g. file was opened at the time,
or your code had been holding a reference to it, etc.) it will be unhashed
so that new lookups won't find it, etc.  No need to revalidate anything.

Now, consider your code.  You've done a lookup in the underlying fs.
It has, at the time, come negative, so you have your (negative) dentry
pointing to that on the underlying fs.  If somebody comes and does
e.g. mkdir() via your fs, it will call vfs_mkdir() on the underlying
sucker, hopefully turning it positive and associate a new in-core inode
with your previously negative dentry.  But what happens if mkdir is done
via underlying fs, or via another instance of yours over the same tree?
Underlying dentry goes positive; yours is still negative.  The underlying
fs either doesn't have ->d_revalidate() or, if there is one it says that
the underlying dentry is valid, thank you very much, no need to invalidate
anything.

In other words, your patch does nothing for object getting created.
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Vivek Goyal Feb. 17, 2017, 8:27 p.m. UTC | #2
On Fri, Feb 17, 2017 at 05:51:18PM +0000, Al Viro wrote:
> On Fri, Feb 17, 2017 at 09:24:40AM -0800, James Bottomley wrote:
> 
> > > What happens when somebody comes along and creates the damn thing on 
> > > the underlying fs?  _Not_ via your code, that is - using the 
> > > underlying fs mounted elsewhere.
> > 
> > Point taken.  This, I think fixes the dcache revalidation issue.
> 
> No, it doesn't.  Consider a local filesystem.  Those do not have any
> ->d_revalidate() - the kernel bloody well knows what happens to
> directories.  If e.g. a previously absent file gets created, it's
> been done by the kernel itself and dentry has been made positive; if
> a previously existing file has been removed, dentry has either become
> negative or, if it had been pinned (e.g. file was opened at the time,
> or your code had been holding a reference to it, etc.) it will be unhashed
> so that new lookups won't find it, etc.  No need to revalidate anything.
> 
> Now, consider your code.  You've done a lookup in the underlying fs.
> It has, at the time, come negative, so you have your (negative) dentry
> pointing to that on the underlying fs.  If somebody comes and does
> e.g. mkdir() via your fs, it will call vfs_mkdir() on the underlying
> sucker, hopefully turning it positive and associate a new in-core inode
> with your previously negative dentry.  But what happens if mkdir is done
> via underlying fs, or via another instance of yours over the same tree?
> Underlying dentry goes positive; yours is still negative.  The underlying
> fs either doesn't have ->d_revalidate() or, if there is one it says that
> the underlying dentry is valid, thank you very much, no need to invalidate
> anything.
> 
> In other words, your patch does nothing for object getting created.

I thought assumption here is that underlying subtree is not changed
outside of shiftfs. IIUC, overlayfs has the same assumption.

Two shiftfs instances writing to same dir will be a problem though.

Vivek
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Patch
diff mbox

diff --git a/fs/shiftfs.c b/fs/shiftfs.c
index a4a1f98..1e71efe 100644
--- a/fs/shiftfs.c
+++ b/fs/shiftfs.c
@@ -118,9 +118,43 @@  static struct dentry *shiftfs_d_real(struct dentry *dentry,
 	return real;
 }
 
+static int shiftfs_d_weak_revalidate(struct dentry *dentry, unsigned int flags)
+{
+	struct dentry *real = dentry->d_fsdata;
+
+	if (d_unhashed(real))
+		return 0;
+
+	if (!(real->d_flags & DCACHE_OP_WEAK_REVALIDATE))
+		return 1;
+
+	return real->d_op->d_weak_revalidate(real, flags);
+}
+
+static int shiftfs_d_revalidate(struct dentry *dentry, unsigned int flags)
+{
+	struct dentry *real = dentry->d_fsdata;
+	int ret;
+
+	if (d_unhashed(real))
+		return 0;
+
+	if (!(real->d_flags & DCACHE_OP_REVALIDATE))
+		return 1;
+
+	ret = real->d_op->d_revalidate(real, flags);
+
+	if (ret == 0 && !(flags & LOOKUP_RCU))
+		d_invalidate(real);
+
+	return ret;
+}
+
 static const struct dentry_operations shiftfs_dentry_ops = {
 	.d_release	= shiftfs_d_release,
 	.d_real		= shiftfs_d_real,
+	.d_revalidate	= shiftfs_d_revalidate,
+	.d_weak_revalidate = shiftfs_d_weak_revalidate,
 };
 
 static int shiftfs_readlink(struct dentry *dentry, char __user *data,
@@ -431,9 +465,7 @@  static struct dentry *shiftfs_lookup(struct inode *dir, struct dentry *dentry,
 		return ERR_PTR(-ENOMEM);
 	}
 
-	d_splice_alias(newi, dentry);
-
-	return NULL;
+	return d_splice_alias(newi, dentry);
 }
 
 static int shiftfs_permission(struct inode *inode, int mask)