fs: factor out backing_file_mmap() helper

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>

1 files changed, 27 insertions, 0 deletions

diff --git a/fs/backing-file.c b/fs/backing-file.c
index ddd35c1d6c71..a681f38d84d8 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"
 
@@ -296,6 +297,32 @@ 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",