[PATCH v2 1/2] mm/memfd: add ioctl(MEMFD_CHECK_IF_ORIGINAL)

[Michal Clapinski <mclapinski@google.com>]

Add a way to check if an fd points to the memfd's original open fd
(the one created by memfd_create).
Useful because only the original open fd can be both writable and
executable.

Signed-off-by: Michal Clapinski <mclapinski@google.com>
---
 Documentation/userspace-api/ioctl/ioctl-number.rst |  1 +
 fs/hugetlbfs/inode.c                               |  9 +++++++++
 include/linux/memfd.h                              | 12 ++++++++++++
 mm/memfd.c                                         |  9 +++++++++
 mm/shmem.c                                         |  9 +++++++++
 5 files changed, 40 insertions(+)

diff --git a/Documentation/userspace-api/ioctl/ioctl-number.rst b/Documentation/userspace-api/ioctl/ioctl-number.rst
index 4ea5b837399a..9a0782116ac2 100644
--- a/Documentation/userspace-api/ioctl/ioctl-number.rst
+++ b/Documentation/userspace-api/ioctl/ioctl-number.rst
@@ -355,6 +355,7 @@ Code  Seq#    Include File                                           Comments
 0xB6  all    linux/fpga-dfl.h
 0xB7  all    uapi/linux/remoteproc_cdev.h                            <mailto:linux-remoteproc@vger.kernel.org>
 0xB7  all    uapi/linux/nsfs.h                                       <mailto:Andrei Vagin <avagin@openvz.org>>
+0xB8  00     linux/memfd.h
 0xC0  00-0F  linux/usb/iowarrior.h
 0xCA  00-0F  uapi/misc/cxl.h
 0xCA  10-2F  uapi/misc/ocxl.h
diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index 316c4cebd3f3..89ff46f7ac54 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -35,6 +35,7 @@
 #include <linux/magic.h>
 #include <linux/migrate.h>
 #include <linux/uio.h>
+#include <linux/memfd.h>
 
 #include <linux/uaccess.h>
 #include <linux/sched/mm.h>
@@ -1324,6 +1325,12 @@ static void init_once(void *foo)
 	inode_init_once(&ei->vfs_inode);
 }
 
+static long hugetlbfs_file_ioctl(struct file *file, unsigned int cmd,
+				 unsigned long arg)
+{
+	return memfd_ioctl(file, cmd, arg);
+}
+
 const struct file_operations hugetlbfs_file_operations = {
 	.read_iter		= hugetlbfs_read_iter,
 	.mmap			= hugetlbfs_file_mmap,
@@ -1331,6 +1338,8 @@ const struct file_operations hugetlbfs_file_operations = {
 	.get_unmapped_area	= hugetlb_get_unmapped_area,
 	.llseek			= default_llseek,
 	.fallocate		= hugetlbfs_fallocate,
+	.unlocked_ioctl		= hugetlbfs_file_ioctl,
+	.compat_ioctl		= hugetlbfs_file_ioctl,
 };
 
 static const struct inode_operations hugetlbfs_dir_inode_operations = {
diff --git a/include/linux/memfd.h b/include/linux/memfd.h
index e7abf6fa4c52..50f512624c92 100644
--- a/include/linux/memfd.h
+++ b/include/linux/memfd.h
@@ -3,14 +3,26 @@
 #define __LINUX_MEMFD_H
 
 #include <linux/file.h>
+#include <linux/ioctl.h>
 
 #ifdef CONFIG_MEMFD_CREATE
 extern long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg);
+extern long memfd_ioctl(struct file *file, unsigned int cmd, unsigned int arg);
 #else
 static inline long memfd_fcntl(struct file *f, unsigned int c, unsigned int a)
 {
 	return -EINVAL;
 }
+static inline long memfd_ioctl(struct file *f, unsigned int c, unsigned int a)
+{
+	return -EINVAL;
+}
 #endif
 
+/*
+ * Return 1 if the memfd is original (i.e. was created by memfd_create,
+ * not reopened), 0 otherwise.
+ */
+#define MEMFD_CHECK_IF_ORIGINAL		_IOR(0xB8, 0, int)
+
 #endif /* __LINUX_MEMFD_H */
diff --git a/mm/memfd.c b/mm/memfd.c
index 1cad1904fc26..06bcb970c387 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -262,6 +262,15 @@ long memfd_fcntl(struct file *file, unsigned int cmd, unsigned int arg)
 	return error;
 }
 
+long memfd_ioctl(struct file *file, unsigned int cmd, unsigned int arg)
+{
+	if (cmd == MEMFD_CHECK_IF_ORIGINAL)
+		return (file->f_mode & FMODE_WRITE) &&
+		       !(file->f_mode & FMODE_WRITER);
+
+	return -EINVAL;
+}
+
 #define MFD_NAME_PREFIX "memfd:"
 #define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1)
 #define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN)
diff --git a/mm/shmem.c b/mm/shmem.c
index 02e62fccc80d..347fcba15fb7 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -79,6 +79,7 @@ static struct vfsmount *shm_mnt;
 #include <linux/rmap.h>
 #include <linux/uuid.h>
 #include <linux/quotaops.h>
+#include <linux/memfd.h>
 
 #include <linux/uaccess.h>
 
@@ -4459,6 +4460,12 @@ const struct address_space_operations shmem_aops = {
 };
 EXPORT_SYMBOL(shmem_aops);
 
+static long shmem_file_ioctl(struct file *file, unsigned int cmd,
+			     unsigned long arg)
+{
+	return memfd_ioctl(file, cmd, arg);
+}
+
 static const struct file_operations shmem_file_operations = {
 	.mmap		= shmem_mmap,
 	.open		= shmem_file_open,
@@ -4471,6 +4478,8 @@ static const struct file_operations shmem_file_operations = {
 	.splice_read	= shmem_file_splice_read,
 	.splice_write	= iter_file_splice_write,
 	.fallocate	= shmem_fallocate,
+	.unlocked_ioctl = shmem_file_ioctl,
+	.compat_ioctl   = shmem_file_ioctl,
 #endif
 };
 
-- 
2.42.0.283.g2d96d420d3-goog