debugfs: prevent access to possibly dead file_operations at file open

[cascardo/linux.git] / fs / debugfs / file.c

diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
index d2ba12e..736ab3c 100644 (file)
--- a/fs/debugfs/file.c
+++ b/fs/debugfs/file.c
@@ -22,6 +22,9 @@
 #include <linux/slab.h>
 #include <linux/atomic.h>
 #include <linux/device.h>
+#include <linux/srcu.h>
+
+#include "internal.h"
 
 static ssize_t default_read_file(struct file *file, char __user *buf,
                                 size_t count, loff_t *ppos)
@@ -35,13 +38,99 @@ static ssize_t default_write_file(struct file *file, const char __user *buf,
        return count;
 }
 
-const struct file_operations debugfs_file_operations = {
+const struct file_operations debugfs_noop_file_operations = {
        .read =         default_read_file,
        .write =        default_write_file,
        .open =         simple_open,
        .llseek =       noop_llseek,
 };
 
+/**
+ * debugfs_use_file_start - mark the beginning of file data access
+ * @dentry: the dentry object whose data is being accessed.
+ * @srcu_idx: a pointer to some memory to store a SRCU index in.
+ *
+ * Up to a matching call to debugfs_use_file_finish(), any
+ * successive call into the file removing functions debugfs_remove()
+ * and debugfs_remove_recursive() will block. Since associated private
+ * file data may only get freed after a successful return of any of
+ * the removal functions, you may safely access it after a successful
+ * call to debugfs_use_file_start() without worrying about
+ * lifetime issues.
+ *
+ * If -%EIO is returned, the file has already been removed and thus,
+ * it is not safe to access any of its data. If, on the other hand,
+ * it is allowed to access the file data, zero is returned.
+ *
+ * Regardless of the return code, any call to
+ * debugfs_use_file_start() must be followed by a matching call
+ * to debugfs_use_file_finish().
+ */
+static int debugfs_use_file_start(const struct dentry *dentry, int *srcu_idx)
+       __acquires(&debugfs_srcu)
+{
+       *srcu_idx = srcu_read_lock(&debugfs_srcu);
+       barrier();
+       if (d_unlinked(dentry))
+               return -EIO;
+       return 0;
+}
+
+/**
+ * debugfs_use_file_finish - mark the end of file data access
+ * @srcu_idx: the SRCU index "created" by a former call to
+ *            debugfs_use_file_start().
+ *
+ * Allow any ongoing concurrent call into debugfs_remove() or
+ * debugfs_remove_recursive() blocked by a former call to
+ * debugfs_use_file_start() to proceed and return to its caller.
+ */
+static void debugfs_use_file_finish(int srcu_idx) __releases(&debugfs_srcu)
+{
+       srcu_read_unlock(&debugfs_srcu, srcu_idx);
+}
+
+#define F_DENTRY(filp) ((filp)->f_path.dentry)
+
+#define REAL_FOPS_DEREF(dentry)                                        \
+       ((const struct file_operations *)(dentry)->d_fsdata)
+
+static int open_proxy_open(struct inode *inode, struct file *filp)
+{
+       const struct dentry *dentry = F_DENTRY(filp);
+       const struct file_operations *real_fops = NULL;
+       int srcu_idx, r;
+
+       r = debugfs_use_file_start(dentry, &srcu_idx);
+       if (r) {
+               r = -ENOENT;
+               goto out;
+       }
+
+       real_fops = REAL_FOPS_DEREF(dentry);
+       real_fops = fops_get(real_fops);
+       if (!real_fops) {
+               /* Huh? Module did not clean up after itself at exit? */
+               WARN(1, "debugfs file owner did not clean up at exit: %pd",
+                       dentry);
+               r = -ENXIO;
+               goto out;
+       }
+       replace_fops(filp, real_fops);
+
+       if (real_fops->open)
+               r = real_fops->open(inode, filp);
+
+out:
+       fops_put(real_fops);
+       debugfs_use_file_finish(srcu_idx);
+       return r;
+}
+
+const struct file_operations debugfs_open_proxy_file_operations = {
+       .open = open_proxy_open,
+};
+
 static struct dentry *debugfs_create_mode(const char *name, umode_t mode,
                                          struct dentry *parent, void *value,
                                          const struct file_operations *fops,