[PATCH v6 5/5] fs, fcntl: add F_MAP_DIRECT

[Dan Williams <dan.j.williams-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

A common pattern for granting a privilege to an unprivileged process is
to pass it an established file descriptor over a unix domain socket.
This enables fine grained access to the MAP_DIRECT mechanism instead of
requiring the mapping process have CAP_LINUX_IMMUTABLE.

Cc: Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>
Cc: Jeff Moyer <jmoyer-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
Cc: Dave Chinner <david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org>
Cc: Alexander Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org>
Cc: "Darrick J. Wong" <darrick.wong-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>
Cc: Ross Zwisler <ross.zwisler-VuQAYsv1563Yd54FQh9/CA@public.gmane.org>
Signed-off-by: Dan Williams <dan.j.williams-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
---
 fs/fcntl.c                 |   15 +++++++++++++++
 include/linux/fs.h         |    5 +++--
 include/uapi/linux/fcntl.h |    5 +++++
 mm/mmap.c                  |    3 ++-
 4 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index 3b01b646e528..f2375c406e6f 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -318,6 +318,18 @@ static long fcntl_rw_hint(struct file *file, unsigned int cmd,
 	}
 }
 
+static int fcntl_map_direct(struct file *filp)
+{
+	if (!capable(CAP_LINUX_IMMUTABLE))
+		return -EACCES;
+
+	spin_lock(&filp->f_lock);
+	filp->f_map_direct = 1;
+	spin_unlock(&filp->f_lock);
+
+	return 0;
+}
+
 static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 		struct file *filp)
 {
@@ -425,6 +437,9 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 	case F_SET_FILE_RW_HINT:
 		err = fcntl_rw_hint(filp, cmd, arg);
 		break;
+	case F_MAP_DIRECT:
+		err = fcntl_map_direct(filp);
+		break;
 	default:
 		break;
 	}
diff --git a/include/linux/fs.h b/include/linux/fs.h
index db42da9f98c4..ec2e1d6bf22c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -855,11 +855,12 @@ struct file {
 	const struct file_operations	*f_op;
 
 	/*
-	 * Protects f_ep_links, f_flags.
+	 * Protects f_ep_links, f_flags, f_write_hint, and f_map_direct.
 	 * Must not be taken from IRQ context.
 	 */
 	spinlock_t		f_lock;
-	enum rw_hint		f_write_hint;
+	enum rw_hint		f_write_hint:3;
+	unsigned int		f_map_direct:1;
 	atomic_long_t		f_count;
 	unsigned int 		f_flags;
 	fmode_t			f_mode;
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index ec69d55bcec7..2a57a503174e 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -53,6 +53,11 @@
 #define F_SET_FILE_RW_HINT	(F_LINUX_SPECIFIC_BASE + 14)
 
 /*
+ * Enable MAP_DIRECT on the file without CAP_LINUX_IMMUTABLE
+ */
+#define F_MAP_DIRECT		(F_LINUX_SPECIFIC_BASE + 15)
+
+/*
  * Valid hint values for F_{GET,SET}_RW_HINT. 0 is "not set", or can be
  * used to clear any hints previously set.
  */
diff --git a/mm/mmap.c b/mm/mmap.c
index 32417b2a668c..cf5e0cb7d0e3 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1399,7 +1399,8 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 			if (flags & MAP_DIRECT) {
 				if (!(prot & PROT_WRITE))
 					return -EACCES;
-				if (!capable(CAP_LINUX_IMMUTABLE))
+				if (!file->f_map_direct
+						&& !capable(CAP_LINUX_IMMUTABLE))
 					return -EACCES;
 			}