[PATCH RFC 37/38] proc: don't pin a symlink's entry while readlink() copies it out

proc_get_link() pins the entry with use_pde() and the delayed call
unpins it. vfs_readlink() copies the link to userspace in between. So a
page fault in that copy holds up remove_proc_entry() just like the
copies of reads and writes do, and the callers hold locks while they
wait. /proc/asound/<id> is removed with snd_card_mutex and info_mutex
held when a card changes its id.

The pin doesn't keep anything alive. proc_symlink() copies the target
and pde_free() frees it with the entry, and the inode holds a reference
to the entry. What the pin does is keep a removed symlink from being
followed. So check ->in_use instead and drop the delayed call. That
also takes an atomic pair off every traversal of /proc/net and
/proc/mounts.

Fixes: 7e0e953bb0cf ("procfs: fix race between symlink removals and traversals")
Cc: <redacted>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 fs/proc/inode.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index 9933f03ab9fb..2d65f3c8ec03 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c

@@ -629,19 +629,19 @@ static const struct file_operations proc_iter_file_ops_compat = {
 };
 #endif
 
-static void proc_put_link(void *p)
-{
-	unuse_pde(p);
-}
-
 static const char *proc_get_link(struct dentry *dentry,
 				 struct inode *inode,
 				 struct delayed_call *done)
 {
 	struct proc_dir_entry *pde = PDE(inode);
-	if (!use_pde(pde))
+
+	/*
+	 * No pin. The link lives with the entry, which the inode holds, and a
+	 * reader could fault while vfs_readlink() copies it out. Just don't
+	 * follow a removed one.
+	 */
+	if (unlikely(atomic_read(&pde->in_use) < 0))
 		return ERR_PTR(-EINVAL);
-	set_delayed_call(done, proc_put_link, pde);
 	return pde->data;
 }
 

-- 
2.53.0