[RFC PATCH 1/2] fs: allow filesystems to directly pass an existing struct file

["Enrico Weigelt, metux IT consult" <info@metux.net>]

In some scenarios, file systems might want to pass an already opened
struct file instance on an open() call, instead of opening a new one.

This allows similar techniques like the already well known file descriptor
passing via Unix domain sockets, but now also for plain open() calls.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
---
 fs/Kconfig         |  3 +++
 fs/internal.h      |  6 ++++++
 fs/namei.c         |  2 +-
 fs/open.c          | 42 +++++++++++++++++++++++++++++++++++++++++-
 include/linux/fs.h |  9 +++++++++
 5 files changed, 60 insertions(+), 2 deletions(-)

diff --git a/fs/Kconfig b/fs/Kconfig
index 141a856c50e7..b8b7a77b656c 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -357,4 +357,7 @@ source "fs/unicode/Kconfig"
 config IO_WQ
 	bool
 
+config FS_BOXED_FILE
+	bool
+
 endmenu
diff --git a/fs/internal.h b/fs/internal.h
index 6aeae7ef3380..e5e9cf038a24 100644
--- a/fs/internal.h
+++ b/fs/internal.h
@@ -142,6 +142,12 @@ int do_fchownat(int dfd, const char __user *filename, uid_t user, gid_t group,
 int chown_common(const struct path *path, uid_t user, gid_t group);
 extern int vfs_open(const struct path *, struct file *);
 
+#ifdef CONFIG_FS_BOXED_FILE
+extern struct file *unbox_file(struct file *);
+#else
+static inline struct file *unbox_file(struct file *f) { return f; }
+#endif
+
 /*
  * inode.c
  */
diff --git a/fs/namei.c b/fs/namei.c
index 79b0ff9b151e..b186d2d75b63 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3496,7 +3496,7 @@ static struct file *path_openat(struct nameidata *nd,
 	}
 	if (likely(!error)) {
 		if (likely(file->f_mode & FMODE_OPENED))
-			return file;
+			return unbox_file(file);
 		WARN_ON(1);
 		error = -EINVAL;
 	}
diff --git a/fs/open.c b/fs/open.c
index e53af13b5835..88daf09ffeb4 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -769,6 +769,46 @@ SYSCALL_DEFINE3(fchown, unsigned int, fd, uid_t, user, gid_t, group)
 	return ksys_fchown(fd, user, group);
 }
 
+#ifdef CONFIG_FS_BOXED_FILE
+/*
+ * Finish up an open procedure before returning the file to the caller.
+ * in case the the fs returns some unusual things like directly passing
+ * another file, this will be handled here.
+ *
+ * This function is only supposed to be called by functions like dentry_open()
+ * and path_openat() that allocate a new struct file and finally pass it to
+ * vfs_open() - the struct file should not have been used in any ways in the
+ * meantime, or unpleasant things may happen.
+ */
+struct file *unbox_file(struct file *f)
+{
+	struct file *boxed;
+
+	if (unlikely(!f))
+		return NULL;
+
+	if (IS_ERR(f))
+		return f;
+
+	if (likely(!f->boxed_file))
+		return f;
+
+	/* the fs returned another struct file (f->lower_file) that should be
+	   directly passed to our callers instead of the one that had been newly
+	   created for the open procedure.
+
+	   the lower_file is already ref'ed, so we keep the refcount.
+	   since the upper file (f) just had been opened, and no further access,
+	   we can just call fput() on it.
+	*/
+
+	boxed = f->boxed_file;
+	fput(f);
+
+	return boxed;
+}
+#endif /* CONFIG_FS_BOXED_FILE */
+
 static int do_dentry_open(struct file *f,
 			  struct inode *inode,
 			  int (*open)(struct inode *, struct file *))
@@ -959,7 +999,7 @@ struct file *dentry_open(const struct path *path, int flags,
 			f = ERR_PTR(error);
 		}
 	}
-	return f;
+	return unbox_file(f);
 }
 EXPORT_SYMBOL(dentry_open);
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index c3c88fdb9b2a..a778c5c057ab 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -955,6 +955,15 @@ struct file {
 	struct address_space	*f_mapping;
 	errseq_t		f_wb_err;
 	errseq_t		f_sb_err; /* for syncfs */
+
+#ifdef CONFIG_FS_BOXED_FILE
+	/* Only for file systems that wanna pass an *existing* file to the
+	   caller of open() instead of the newly created one. This has similar
+	   semantics like passing an fd via unix socket, but instead via some
+	   open() call.
+	*/
+	struct file		*boxed_file;
+#endif
 } __randomize_layout
   __attribute__((aligned(4)));	/* lest something weird decides that 2 is OK */
 
-- 
2.20.1