procfs: fix race between symlink removals and traversals

use_pde()/unuse_pde() in ->follow_link()/->put_link() resp.

Cc: [e-mail]
Signed-off-by: Al Viro <[e-mail]>

fs/proc/generic.c

···
 #include <linux/mount.h>
 #include <linux/init.h>
 #include <linux/idr.h>
-#include <linux/namei.h>
 #include <linux/bitops.h>
 #include <linux/spinlock.h>
 #include <linux/completion.h>
···
 	ida_remove(&proc_inum_ida, inum - PROC_DYNAMIC_FIRST);
 	spin_unlock_irqrestore(&proc_inum_lock, flags);
 }
-
-static void *proc_follow_link(struct dentry *dentry, struct nameidata *nd)
-{
-	nd_set_link(nd, __PDE_DATA(dentry->d_inode));
-	return NULL;
-}
-
-static const struct inode_operations proc_link_inode_operations = {
-	.readlink	= generic_readlink,
-	.follow_link	= proc_follow_link,
-};
 
 /*
  * Don't create negative dentries here, return -ENOENT by hand

fs/proc/inode.c

···
 #include <linux/slab.h>
 #include <linux/mount.h>
 #include <linux/magic.h>
+#include <linux/namei.h>
 
 #include <asm/uaccess.h>
 
···
 	.release	= proc_reg_release,
 };
 #endif
+
+static void *proc_follow_link(struct dentry *dentry, struct nameidata *nd)
+{
+	struct proc_dir_entry *pde = PDE(dentry->d_inode);
+	if (unlikely(!use_pde(pde)))
+		return ERR_PTR(-EINVAL);
+	nd_set_link(nd, pde->data);
+	return pde;
+}
+
+static void proc_put_link(struct dentry *dentry, struct nameidata *nd, void *p)
+{
+	unuse_pde(p);
+}
+
+const struct inode_operations proc_link_inode_operations = {
+	.readlink	= generic_readlink,
+	.follow_link	= proc_follow_link,
+	.put_link	= proc_put_link,
+};
 
 struct inode *proc_get_inode(struct super_block *sb, struct proc_dir_entry *de)
 {

fs/proc/internal.h

···
 	int closing;
 	struct completion *c;
 };
+extern const struct inode_operations proc_link_inode_operations;
 
 extern const struct inode_operations proc_pid_link_inode_operations;