proc 1/2: do PDE usecounting even for ->read_proc, ->write_proc

struct proc_dir_entry::owner is going to be removed. Now it's only necessary
to protect PDEs which are using ->read_proc, ->write_proc hooks.

However, ->owner assignments are racy and make it very easy for someone to switch
->owner on live PDE (as some subsystems do) without fixing refcounts and so on.

http://bugzilla.kernel.org/show_bug.cgi?id=12454

So, ->owner is on death row.

Proxy file operations exist already (proc_file_operations), just bump usecount
when necessary.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>

3 files changed, 40 insertions, 11 deletions

diff --git a/fs/proc/generic.c b/fs/proc/generic.c
index 8c68bbe2b61e..fa678abc9db1 100644
--- a/fs/proc/generic.c
+++ b/fs/proc/generic.c
@@ -37,7 +37,7 @@ static int proc_match(int len, const char *name, struct proc_dir_entry *de)
 #define PROC_BLOCK_SIZE (PAGE_SIZE - 1024)
 
 static ssize_t
-proc_file_read(struct file *file, char __user *buf, size_t nbytes,
+__proc_file_read(struct file *file, char __user *buf, size_t nbytes,
                loff_t *ppos)
 {
         struct inode * inode = file->f_path.dentry->d_inode;
@@ -183,19 +183,47 @@ proc_file_read(struct file *file, char __user *buf, size_t nbytes,
 }
 
 static ssize_t
+proc_file_read(struct file *file, char __user *buf, size_t nbytes,
+               loff_t *ppos)
+{
+        struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
+        ssize_t rv = -EIO;
+
+        spin_lock(&pde->pde_unload_lock);
+        if (!pde->proc_fops) {
+                spin_unlock(&pde->pde_unload_lock);
+                return rv;
+        }
+        pde->pde_users++;
+        spin_unlock(&pde->pde_unload_lock);
+
+        rv = __proc_file_read(file, buf, nbytes, ppos);
+
+        pde_users_dec(pde);
+        return rv;
+}
+
+static ssize_t
 proc_file_write(struct file *file, const char __user *buffer,
                 size_t count, loff_t *ppos)
 {
-        struct inode *inode = file->f_path.dentry->d_inode;
-        struct proc_dir_entry * dp;
-        
-        dp = PDE(inode);
-
-        if (!dp->write_proc)
-                return -EIO;
+        struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
+        ssize_t rv = -EIO;
+
+        if (pde->write_proc) {
+                spin_lock(&pde->pde_unload_lock);
+                if (!pde->proc_fops) {
+                        spin_unlock(&pde->pde_unload_lock);
+                        return rv;
+                }
+                pde->pde_users++;
+                spin_unlock(&pde->pde_unload_lock);
 
-        /* FIXME: does this routine need ppos?  probably... */
-        return dp->write_proc(file, buffer, count, dp->data);
+                /* FIXME: does this routine need ppos?  probably... */
+                rv = pde->write_proc(file, buffer, count, pde->data);
+                pde_users_dec(pde);
+        }
+        return rv;
 }
 
 
diff --git a/fs/proc/inode.c b/fs/proc/inode.c
index d8bb5c671f42..e11dc22c6511 100644
--- a/fs/proc/inode.c
+++ b/fs/proc/inode.c
@@ -127,7 +127,7 @@ static void __pde_users_dec(struct proc_dir_entry *pde)
                 complete(pde->pde_unload_completion);
 }
 
-static void pde_users_dec(struct proc_dir_entry *pde)
+void pde_users_dec(struct proc_dir_entry *pde)
 {
         spin_lock(&pde->pde_unload_lock);
         __pde_users_dec(pde);
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index cd53ff838498..f6db9618a888 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -91,3 +91,4 @@ struct pde_opener {
         int (*release)(struct inode *, struct file *);
         struct list_head lh;
 };
+void pde_users_dec(struct proc_dir_entry *pde);