[RFC][PATCH 4/4] fs: factor out backing_file_mmap() helper

[Amir Goldstein <amir73il@gmail.com>]

Assert that the file object is allocated in a backing_file container
so that file_user_path() could be used to display the user path and
not the backing file's path in /proc/<pid>/maps.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/backing-file.c            | 27 +++++++++++++++++++++++++++
 fs/overlayfs/file.c          | 23 ++++++-----------------
 include/linux/backing-file.h |  2 ++
 3 files changed, 35 insertions(+), 17 deletions(-)

diff --git a/fs/backing-file.c b/fs/backing-file.c
index 46488de821a2..1ad8c252ec8d 100644
--- a/fs/backing-file.c
+++ b/fs/backing-file.c
@@ -11,6 +11,7 @@
 #include <linux/fs.h>
 #include <linux/backing-file.h>
 #include <linux/splice.h>
+#include <linux/mm.h>
 
 #include "internal.h"
 
@@ -284,6 +285,32 @@ ssize_t backing_file_splice_write(struct pipe_inode_info *pipe,
 }
 EXPORT_SYMBOL_GPL(backing_file_splice_write);
 
+int backing_file_mmap(struct file *file, struct vm_area_struct *vma,
+		      struct backing_file_ctx *ctx)
+{
+	const struct cred *old_cred;
+	int ret;
+
+	if (WARN_ON_ONCE(!(file->f_mode & FMODE_BACKING)) ||
+	    WARN_ON_ONCE(ctx->user_file != vma->vm_file))
+		return -EIO;
+
+	if (!file->f_op->mmap)
+		return -ENODEV;
+
+	vma_set_file(vma, file);
+
+	old_cred = override_creds(ctx->cred);
+	ret = call_mmap(vma->vm_file, vma);
+	revert_creds(old_cred);
+
+	if (ctx->accessed)
+		ctx->accessed(ctx->user_file);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(backing_file_mmap);
+
 static int __init backing_aio_init(void)
 {
 	backing_aio_cachep = kmem_cache_create("backing_aio",
diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 69b52d2f9c74..05536964d37f 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -10,7 +10,6 @@
 #include <linux/uio.h>
 #include <linux/uaccess.h>
 #include <linux/security.h>
-#include <linux/mm.h>
 #include <linux/fs.h>
 #include <linux/backing-file.h>
 #include "overlayfs.h"
@@ -415,23 +414,13 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync)
 static int ovl_mmap(struct file *file, struct vm_area_struct *vma)
 {
 	struct file *realfile = file->private_data;
-	const struct cred *old_cred;
-	int ret;
-
-	if (!realfile->f_op->mmap)
-		return -ENODEV;
-
-	if (WARN_ON(file != vma->vm_file))
-		return -EIO;
-
-	vma_set_file(vma, realfile);
-
-	old_cred = ovl_override_creds(file_inode(file)->i_sb);
-	ret = call_mmap(vma->vm_file, vma);
-	revert_creds(old_cred);
-	ovl_file_accessed(file);
+	struct backing_file_ctx ctx = {
+		.cred = ovl_creds(file_inode(file)->i_sb),
+		.user_file = file,
+		.accessed = ovl_file_accessed,
+	};
 
-	return ret;
+	return backing_file_mmap(realfile, vma, &ctx);
 }
 
 static long ovl_fallocate(struct file *file, int mode, loff_t offset, loff_t len)
diff --git a/include/linux/backing-file.h b/include/linux/backing-file.h
index 0546d5b1c9f5..3f1fe1774f1b 100644
--- a/include/linux/backing-file.h
+++ b/include/linux/backing-file.h
@@ -36,5 +36,7 @@ ssize_t backing_file_splice_write(struct pipe_inode_info *pipe,
 				  struct file *out, loff_t *ppos, size_t len,
 				  unsigned int flags,
 				  struct backing_file_ctx *ctx);
+int backing_file_mmap(struct file *file, struct vm_area_struct *vma,
+		      struct backing_file_ctx *ctx);
 
 #endif /* _LINUX_BACKING_FILE_H */
-- 
2.34.1