[PATCH] fs: Detect API misuse of i_op->lookup

[PATCH] fs: Detect API misuse of i_op->lookup

[Richard Weinberger]

Hunting down a refcount issue in an out-of-tree filesystem uncovered
that its lookup function directly returned the dentry which was passed to it.
Add a BUG_ON() to detect such misuse in future.

Signed-off-by: Richard Weinberger <richard@nod.at>
---
 fs/namei.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/namei.c b/fs/namei.c
index bc35b02..f8ccae6 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1370,6 +1370,7 @@ static struct dentry *lookup_real(struct inode *dir, struct dentry *dentry,
 
 	old = dir->i_op->lookup(dir, dentry, flags);
 	if (unlikely(old)) {
+		BUG_ON(old == dentry);
 		dput(dentry);
 		dentry = old;
 	}
-- 
2.2.2

Re: [PATCH] fs: Detect API misuse of i_op->lookup

[Al Viro]

On Sun, Jan 25, 2015 at 10:57:40PM +0100, Richard Weinberger wrote:
> Hunting down a refcount issue in an out-of-tree filesystem uncovered
> that its lookup function directly returned the dentry which was passed to it.
> Add a BUG_ON() to detect such misuse in future.

In principle, it's not a bug.  You *must* grab a reference to dentry before
returning it, but you can very well decide to return the one you've got -
any place where have ->lookup() return NULL has every right to become
return dget(dentry).

Re: [PATCH] fs: Detect API misuse of i_op->lookup

[Richard Weinberger]

Am 25.01.2015 um 23:16 schrieb Al Viro:
> On Sun, Jan 25, 2015 at 10:57:40PM +0100, Richard Weinberger wrote:
>> Hunting down a refcount issue in an out-of-tree filesystem uncovered
>> that its lookup function directly returned the dentry which was passed to it.
>> Add a BUG_ON() to detect such misuse in future.
> 
> In principle, it's not a bug.  You *must* grab a reference to dentry before
> returning it, but you can very well decide to return the one you've got -
> any place where have ->lookup() return NULL has every right to become
> return dget(dentry).

Thanks for the clarification! The filesystem in question did of course
not grab a reference before returning the same dentry.

Thanks,
//richard