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

Michal Clapinski
2023-09-08T17:58:10Z

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