Re: since when does linkat() on deleted /proc/$PID/fd/$num returnENOENT ?

From: Alexey Dobriyan
Date: Sat Mar 31 2012 - 02:53:47 EST

On Fri, Mar 30, 2012 at 12:21:21PM +0200, David Madore wrote:
> It used to be the case (last time I checked was around late 2008 or
> early 2009) that deleted entries from /proc/$PID/fd/ could be linked
> back to the filesystem by using linkat(,,,,AT_SYMLINK_FOLLOW).
> Now this just returns ENOENT.
> I'd like to understand when, how and why this change took place. What
> commit introduced it and was it a deliberate move (e.g., because the
> feature was a security issue of itself, or came into conflict with
> something else) or was it accidental?

It was explicitly prohibited since 2.6.39:

commit aae8a97d3ec30788790d1720b71d76fd8eb44b73
Author: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
Date: Sat Jan 29 18:43:27 2011 +0530

fs: Don't allow to create hardlink for deleted file

Add inode->i_nlink == 0 check in VFS. Some of the file systems
do this internally. A followup patch will remove those instance.
This is needed to ensure that with link by handle we don't allow
to create hardlink of an unlinked file. The check also prevent a race
between unlink and link

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

diff --git a/fs/namei.c b/fs/namei.c
index 83e92ba..33be51a 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2906,7 +2906,11 @@ int vfs_link(struct dentry *old_dentry, struct inode *dir, struct dentry *new_de
return error;

- error = dir->i_op->link(old_dentry, dir, new_dentry);
+ /* Make sure we don't allow creating hardlink to an unlinked file */
+ if (inode->i_nlink == 0)
+ error = -ENOENT;
+ else
+ error = dir->i_op->link(old_dentry, dir, new_dentry);
if (!error)
fsnotify_link(dir, inode, new_dentry);
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