[RFC,v3,03/12] fs: add RWF_ENCODED for reading/writing compressed data
diff mbox series

Message ID 07f9cc1969052e94818fa50019e7589d206d1d18.1574273658.git.osandov@fb.com
State New
Headers show
Series
  • fs: interface for directly reading/writing compressed data
Related show

Commit Message

Omar Sandoval Nov. 20, 2019, 6:24 p.m. UTC
From: Omar Sandoval <osandov@fb.com>

Btrfs supports transparent compression: data written by the user can be
compressed when written to disk and decompressed when read back.
However, we'd like to add an interface to write pre-compressed data
directly to the filesystem, and the matching interface to read
compressed data without decompressing it. This adds support for
so-called "encoded I/O" via preadv2() and pwritev2().

A new RWF_ENCODED flags indicates that a read or write is "encoded". If
this flag is set, iov[0].iov_base points to a struct encoded_iov which
is used for metadata: namely, the compression algorithm, unencoded
(i.e., decompressed) length, and what subrange of the unencoded data
should be used (needed for truncated or hole-punched extents and when
reading in the middle of an extent). For reads, the filesystem returns
this information; for writes, the caller provides it to the filesystem.
iov[0].iov_len must be set to sizeof(struct encoded_iov), which can be
used to extend the interface in the future a la copy_struct_from_user().
The remaining iovecs contain the encoded extent.

This adds the VFS helpers for supporting encoded I/O and documentation
for filesystem support.

Signed-off-by: Omar Sandoval <osandov@fb.com>
---
 Documentation/filesystems/encoded_io.rst |  79 +++++++++++
 Documentation/filesystems/index.rst      |   1 +
 include/linux/fs.h                       |  16 +++
 include/uapi/linux/fs.h                  |  33 ++++-
 mm/filemap.c                             | 165 +++++++++++++++++++++--
 5 files changed, 280 insertions(+), 14 deletions(-)
 create mode 100644 Documentation/filesystems/encoded_io.rst

Comments

Nikolay Borisov Nov. 26, 2019, 1:53 p.m. UTC | #1
On 20.11.19 г. 20:24 ч., Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>

<snip>

>  
> +enum {
> +	ENCODED_IOV_COMPRESSION_NONE,
> +#define ENCODED_IOV_COMPRESSION_NONE ENCODED_IOV_COMPRESSION_NONE
> +	ENCODED_IOV_COMPRESSION_ZLIB,
> +#define ENCODED_IOV_COMPRESSION_ZLIB ENCODED_IOV_COMPRESSION_ZLIB
> +	ENCODED_IOV_COMPRESSION_LZO,
> +#define ENCODED_IOV_COMPRESSION_LZO ENCODED_IOV_COMPRESSION_LZO
> +	ENCODED_IOV_COMPRESSION_ZSTD,
> +#define ENCODED_IOV_COMPRESSION_ZSTD ENCODED_IOV_COMPRESSION_ZSTD
> +	ENCODED_IOV_COMPRESSION_TYPES = ENCODED_IOV_COMPRESSION_ZSTD,

This looks very dodgy, what am I missing?

<snip>
Omar Sandoval Nov. 26, 2019, 5:36 p.m. UTC | #2
On Tue, Nov 26, 2019 at 03:53:02PM +0200, Nikolay Borisov wrote:
> 
> 
> On 20.11.19 г. 20:24 ч., Omar Sandoval wrote:
> > From: Omar Sandoval <osandov@fb.com>
> 
> <snip>
> 
> >  
> > +enum {
> > +	ENCODED_IOV_COMPRESSION_NONE,
> > +#define ENCODED_IOV_COMPRESSION_NONE ENCODED_IOV_COMPRESSION_NONE
> > +	ENCODED_IOV_COMPRESSION_ZLIB,
> > +#define ENCODED_IOV_COMPRESSION_ZLIB ENCODED_IOV_COMPRESSION_ZLIB
> > +	ENCODED_IOV_COMPRESSION_LZO,
> > +#define ENCODED_IOV_COMPRESSION_LZO ENCODED_IOV_COMPRESSION_LZO
> > +	ENCODED_IOV_COMPRESSION_ZSTD,
> > +#define ENCODED_IOV_COMPRESSION_ZSTD ENCODED_IOV_COMPRESSION_ZSTD
> > +	ENCODED_IOV_COMPRESSION_TYPES = ENCODED_IOV_COMPRESSION_ZSTD,
> 
> This looks very dodgy, what am I missing?

This is a somewhat common trick so that enum values can be checked for
with ifdef/ifndef. See include/uapi/linux.in.h, for example.
Nikolay Borisov Nov. 27, 2019, 9 a.m. UTC | #3
On 26.11.19 г. 19:36 ч., Omar Sandoval wrote:
> On Tue, Nov 26, 2019 at 03:53:02PM +0200, Nikolay Borisov wrote:
>>
>>
>> On 20.11.19 г. 20:24 ч., Omar Sandoval wrote:
>>> From: Omar Sandoval <osandov@fb.com>
>>
>> <snip>
>>
>>>  
>>> +enum {
>>> +	ENCODED_IOV_COMPRESSION_NONE,
>>> +#define ENCODED_IOV_COMPRESSION_NONE ENCODED_IOV_COMPRESSION_NONE
>>> +	ENCODED_IOV_COMPRESSION_ZLIB,
>>> +#define ENCODED_IOV_COMPRESSION_ZLIB ENCODED_IOV_COMPRESSION_ZLIB
>>> +	ENCODED_IOV_COMPRESSION_LZO,
>>> +#define ENCODED_IOV_COMPRESSION_LZO ENCODED_IOV_COMPRESSION_LZO
>>> +	ENCODED_IOV_COMPRESSION_ZSTD,
>>> +#define ENCODED_IOV_COMPRESSION_ZSTD ENCODED_IOV_COMPRESSION_ZSTD
>>> +	ENCODED_IOV_COMPRESSION_TYPES = ENCODED_IOV_COMPRESSION_ZSTD,
>>
>> This looks very dodgy, what am I missing?
> 
> This is a somewhat common trick so that enum values can be checked for
> with ifdef/ifndef. See include/uapi/linux.in.h, for example.

I cannot seem to have this file on my system (or any .in.h file for that
matter in the kernel source dir).

>
Omar Sandoval Nov. 27, 2019, 9:10 a.m. UTC | #4
On Wed, Nov 27, 2019 at 11:00:15AM +0200, Nikolay Borisov wrote:
> 
> 
> On 26.11.19 г. 19:36 ч., Omar Sandoval wrote:
> > On Tue, Nov 26, 2019 at 03:53:02PM +0200, Nikolay Borisov wrote:
> >>
> >>
> >> On 20.11.19 г. 20:24 ч., Omar Sandoval wrote:
> >>> From: Omar Sandoval <osandov@fb.com>
> >>
> >> <snip>
> >>
> >>>  
> >>> +enum {
> >>> +	ENCODED_IOV_COMPRESSION_NONE,
> >>> +#define ENCODED_IOV_COMPRESSION_NONE ENCODED_IOV_COMPRESSION_NONE
> >>> +	ENCODED_IOV_COMPRESSION_ZLIB,
> >>> +#define ENCODED_IOV_COMPRESSION_ZLIB ENCODED_IOV_COMPRESSION_ZLIB
> >>> +	ENCODED_IOV_COMPRESSION_LZO,
> >>> +#define ENCODED_IOV_COMPRESSION_LZO ENCODED_IOV_COMPRESSION_LZO
> >>> +	ENCODED_IOV_COMPRESSION_ZSTD,
> >>> +#define ENCODED_IOV_COMPRESSION_ZSTD ENCODED_IOV_COMPRESSION_ZSTD
> >>> +	ENCODED_IOV_COMPRESSION_TYPES = ENCODED_IOV_COMPRESSION_ZSTD,
> >>
> >> This looks very dodgy, what am I missing?
> > 
> > This is a somewhat common trick so that enum values can be checked for
> > with ifdef/ifndef. See include/uapi/linux.in.h, for example.
> 
> I cannot seem to have this file on my system (or any .in.h file for that
> matter in the kernel source dir).

Whoops, that should be include/uapi/linux/in.h

Patch
diff mbox series

diff --git a/Documentation/filesystems/encoded_io.rst b/Documentation/filesystems/encoded_io.rst
new file mode 100644
index 000000000000..3ed1ba6e34de
--- /dev/null
+++ b/Documentation/filesystems/encoded_io.rst
@@ -0,0 +1,79 @@ 
+===========
+Encoded I/O
+===========
+
+Encoded I/O is a mechanism for reading and writing encoded (e.g., compressed
+and/or encrypted) data directly from/to the filesystem. The userspace interface
+is thoroughly described in the :manpage:`encoded_io(7)` man page; this document
+describes the requirements for filesystem support.
+
+First of all, a filesystem supporting encoded I/O must indicate this by setting
+the ``FMODE_ENCODED_IO`` flag in its ``file_open`` file operation::
+
+    static int foo_file_open(struct inode *inode, struct file *filp)
+    {
+            ...
+            filep->f_mode |= FMODE_ENCODED_IO;
+            ...
+    }
+
+Encoded I/O goes through ``read_iter`` and ``write_iter``, designated by the
+``IOCB_ENCODED`` flag in ``kiocb->ki_flags``.
+
+Reads
+=====
+
+Encoded ``read_iter`` should:
+
+1. Call ``generic_encoded_read_checks()`` to validate the file and buffers
+   provided by userspace.
+2. Initialize the ``encoded_iov`` appropriately.
+3. Copy it to the user with ``copy_encoded_iov_to_iter()``.
+4. Copy the encoded data to the user.
+5. Advance ``kiocb->ki_pos`` by ``encoded_iov->len``.
+6. Return the size of the encoded data read, not including the ``encoded_iov``.
+
+There are a few details to be aware of:
+
+* Encoded ``read_iter`` should support reading unencoded data if the extent is
+  not encoded.
+* If the buffers provided by the user are not large enough to contain an entire
+  encoded extent, then ``read_iter`` should return ``-ENOBUFS``. This is to
+  avoid confusing userspace with truncated data that cannot be properly
+  decoded.
+* Reads in the middle of an encoded extent can be returned by setting
+  ``encoded_iov->unencoded_offset`` to non-zero.
+* Truncated unencoded data (e.g., because the file does not end on a block
+  boundary) may be returned by setting ``encoded_iov->len`` to a value smaller
+  value than ``encoded_iov->unencoded_len - encoded_iov->unencoded_offset``.
+* If ``encoded_iov->len`` is greater than ``encoded_iov->unencoded_len -
+  encoded_iov->unencoded_offset``, then the user will treat the remainder as
+  zero-filled. Therefore, holes may be returned by setting ``encoded_iov->len``
+  to the size of the hole, setting ``encoded_iov->unencoded_len`` to zero, and
+  copying no data to the user.
+
+Writes
+======
+
+Encoded ``write_iter`` should (in addition to the usual accounting/checks done
+by ``write_iter``):
+
+1. Call ``copy_encoded_iov_from_iter()`` to get and validate the
+   ``encoded_iov``.
+2. Call ``generic_encoded_write_checks()`` instead of
+   ``generic_write_checks()``.
+3. Check that the provided encoding in ``encoded_iov`` is supported.
+4. Advance ``kiocb->ki_pos`` by ``encoded_iov->len``.
+5. Return the size of the encoded data written.
+
+Again, there are a few details:
+
+* Encoded ``write_iter`` doesn't need to support writing unencoded data.
+* ``write_iter`` should either write all of the encoded data or none of it; it
+  must not do partial writes.
+* ``write_iter`` doesn't need to validate the encoded data; a subsequent read
+  may return, e.g., ``-EIO`` if the data is not valid.
+* The user may lie about the unencoded size of the data; a subsequent read
+  should truncate or zero-extend the unencoded data rather than returning an
+  error.
+* Be careful of page cache coherency.
diff --git a/Documentation/filesystems/index.rst b/Documentation/filesystems/index.rst
index 2c3a9f761205..ac9a46fc6120 100644
--- a/Documentation/filesystems/index.rst
+++ b/Documentation/filesystems/index.rst
@@ -37,6 +37,7 @@  filesystem implementations.
    journalling
    fscrypt
    fsverity
+   encoded_io
 
 Filesystems
 ===========
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e0d909d35763..b4f8949d3588 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -175,6 +175,9 @@  typedef int (dio_iodone_t)(struct kiocb *iocb, loff_t offset,
 /* File does not contribute to nr_files count */
 #define FMODE_NOACCOUNT		((__force fmode_t)0x20000000)
 
+/* File supports encoded IO */
+#define FMODE_ENCODED_IO	((__force fmode_t)0x40000000)
+
 /*
  * Flag for rw_copy_check_uvector and compat_rw_copy_check_uvector
  * that indicates that they should check the contents of the iovec are
@@ -314,6 +317,7 @@  enum rw_hint {
 #define IOCB_SYNC		(1 << 5)
 #define IOCB_WRITE		(1 << 6)
 #define IOCB_NOWAIT		(1 << 7)
+#define IOCB_ENCODED		(1 << 8)
 
 struct kiocb {
 	struct file		*ki_filp;
@@ -3088,6 +3092,13 @@  extern int sb_min_blocksize(struct super_block *, int);
 extern int generic_file_mmap(struct file *, struct vm_area_struct *);
 extern int generic_file_readonly_mmap(struct file *, struct vm_area_struct *);
 extern ssize_t generic_write_checks(struct kiocb *, struct iov_iter *);
+struct encoded_iov;
+extern int generic_encoded_write_checks(struct kiocb *,
+					const struct encoded_iov *);
+extern int copy_encoded_iov_from_iter(struct encoded_iov *, struct iov_iter *);
+extern ssize_t generic_encoded_read_checks(struct kiocb *, struct iov_iter *);
+extern int copy_encoded_iov_to_iter(const struct encoded_iov *,
+				    struct iov_iter *);
 extern int generic_remap_checks(struct file *file_in, loff_t pos_in,
 				struct file *file_out, loff_t pos_out,
 				loff_t *count, unsigned int remap_flags);
@@ -3403,6 +3414,11 @@  static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags)
 			return -EOPNOTSUPP;
 		ki->ki_flags |= IOCB_NOWAIT;
 	}
+	if (flags & RWF_ENCODED) {
+		if (!(ki->ki_filp->f_mode & FMODE_ENCODED_IO))
+			return -EOPNOTSUPP;
+		ki->ki_flags |= IOCB_ENCODED;
+	}
 	if (flags & RWF_HIPRI)
 		ki->ki_flags |= IOCB_HIPRI;
 	if (flags & RWF_DSYNC)
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index 379a612f8f1d..775d18bb0efc 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -284,6 +284,34 @@  struct fsxattr {
 
 typedef int __bitwise __kernel_rwf_t;
 
+enum {
+	ENCODED_IOV_COMPRESSION_NONE,
+#define ENCODED_IOV_COMPRESSION_NONE ENCODED_IOV_COMPRESSION_NONE
+	ENCODED_IOV_COMPRESSION_ZLIB,
+#define ENCODED_IOV_COMPRESSION_ZLIB ENCODED_IOV_COMPRESSION_ZLIB
+	ENCODED_IOV_COMPRESSION_LZO,
+#define ENCODED_IOV_COMPRESSION_LZO ENCODED_IOV_COMPRESSION_LZO
+	ENCODED_IOV_COMPRESSION_ZSTD,
+#define ENCODED_IOV_COMPRESSION_ZSTD ENCODED_IOV_COMPRESSION_ZSTD
+	ENCODED_IOV_COMPRESSION_TYPES = ENCODED_IOV_COMPRESSION_ZSTD,
+};
+
+enum {
+	ENCODED_IOV_ENCRYPTION_NONE,
+#define ENCODED_IOV_ENCRYPTION_NONE ENCODED_IOV_ENCRYPTION_NONE
+	ENCODED_IOV_ENCRYPTION_TYPES = ENCODED_IOV_ENCRYPTION_NONE,
+};
+
+struct encoded_iov {
+	__aligned_u64 len;
+	__aligned_u64 unencoded_len;
+	__aligned_u64 unencoded_offset;
+	__u32 compression;
+	__u32 encryption;
+};
+
+#define ENCODED_IOV_SIZE_VER0 32
+
 /* high priority request, poll if possible */
 #define RWF_HIPRI	((__force __kernel_rwf_t)0x00000001)
 
@@ -299,8 +327,11 @@  typedef int __bitwise __kernel_rwf_t;
 /* per-IO O_APPEND */
 #define RWF_APPEND	((__force __kernel_rwf_t)0x00000010)
 
+/* encoded (e.g., compressed and/or encrypted) IO */
+#define RWF_ENCODED	((__force __kernel_rwf_t)0x00000020)
+
 /* mask of flags supported by the kernel */
 #define RWF_SUPPORTED	(RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\
-			 RWF_APPEND)
+			 RWF_APPEND | RWF_ENCODED)
 
 #endif /* _UAPI_LINUX_FS_H */
diff --git a/mm/filemap.c b/mm/filemap.c
index 85b7d087eb45..78b8535b58dd 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2949,24 +2949,15 @@  static int generic_write_check_limits(struct file *file, loff_t pos,
 	return 0;
 }
 
-/*
- * Performs necessary checks before doing a write
- *
- * Can adjust writing position or amount of bytes to write.
- * Returns appropriate error code that caller should return or
- * zero in case that write should be allowed.
- */
-inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
+static int generic_write_checks_common(struct kiocb *iocb, loff_t *count)
 {
 	struct file *file = iocb->ki_filp;
 	struct inode *inode = file->f_mapping->host;
-	loff_t count;
-	int ret;
 
 	if (IS_SWAPFILE(inode))
 		return -ETXTBSY;
 
-	if (!iov_iter_count(from))
+	if (!*count)
 		return 0;
 
 	/* FIXME: this is for backwards compatibility with 2.4 */
@@ -2976,8 +2967,21 @@  inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
 	if ((iocb->ki_flags & IOCB_NOWAIT) && !(iocb->ki_flags & IOCB_DIRECT))
 		return -EINVAL;
 
-	count = iov_iter_count(from);
-	ret = generic_write_check_limits(file, iocb->ki_pos, &count);
+	return generic_write_check_limits(iocb->ki_filp, iocb->ki_pos, count);
+}
+
+/*
+ * Performs necessary checks before doing a write
+ *
+ * Can adjust writing position or amount of bytes to write.
+ * Returns a negative errno or the new number of bytes to write.
+ */
+inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
+{
+	loff_t count = iov_iter_count(from);
+	int ret;
+
+	ret = generic_write_checks_common(iocb, &count);
 	if (ret)
 		return ret;
 
@@ -2986,6 +2990,141 @@  inline ssize_t generic_write_checks(struct kiocb *iocb, struct iov_iter *from)
 }
 EXPORT_SYMBOL(generic_write_checks);
 
+/**
+ * generic_encoded_write_checks() - check an encoded write
+ * @iocb: I/O context.
+ * @encoded: Encoding metadata.
+ *
+ * This should be called by RWF_ENCODED write implementations rather than
+ * generic_write_checks(). Unlike generic_write_checks(), it returns -EFBIG
+ * instead of adjusting the size of the write.
+ *
+ * Return: 0 on success, -errno on error.
+ */
+int generic_encoded_write_checks(struct kiocb *iocb,
+				 const struct encoded_iov *encoded)
+{
+	loff_t count = encoded->len;
+	int ret;
+
+	if (!(iocb->ki_filp->f_flags & O_ALLOW_ENCODED))
+		return -EPERM;
+
+	ret = generic_write_checks_common(iocb, &count);
+	if (ret)
+		return ret;
+
+	if (count != encoded->len) {
+		/*
+		 * The write got truncated by generic_write_checks_common(). We
+		 * can't do a partial encoded write.
+		 */
+		return -EFBIG;
+	}
+	return 0;
+}
+EXPORT_SYMBOL(generic_encoded_write_checks);
+
+/**
+ * copy_encoded_iov_from_iter() - copy a &struct encoded_iov from userspace
+ * @encoded: Returned encoding metadata.
+ * @from: Source iterator.
+ *
+ * This copies in the &struct encoded_iov and does some basic sanity checks.
+ * This should always be used rather than a plain copy_from_iter(), as it does
+ * the proper handling for backward- and forward-compatibility.
+ *
+ * Return: 0 on success, -EFAULT if access to userspace failed, -E2BIG if the
+ *         copied structure contained non-zero fields that this kernel doesn't
+ *         support, -EINVAL if the copied structure was invalid.
+ */
+int copy_encoded_iov_from_iter(struct encoded_iov *encoded,
+			       struct iov_iter *from)
+{
+	size_t usize;
+	int ret;
+
+	usize = iov_iter_single_seg_count(from);
+	if (usize > PAGE_SIZE)
+		return -E2BIG;
+	if (usize < ENCODED_IOV_SIZE_VER0)
+		return -EINVAL;
+	ret = copy_struct_from_iter(encoded, sizeof(*encoded), from, usize);
+	if (ret)
+		return ret;
+
+	if (encoded->compression == ENCODED_IOV_COMPRESSION_NONE &&
+	    encoded->encryption == ENCODED_IOV_ENCRYPTION_NONE)
+		return -EINVAL;
+	if (encoded->compression > ENCODED_IOV_COMPRESSION_TYPES ||
+	    encoded->encryption > ENCODED_IOV_ENCRYPTION_TYPES)
+		return -EINVAL;
+	if (encoded->unencoded_len &&
+	    encoded->unencoded_offset >= encoded->unencoded_len)
+		return -EINVAL;
+	return 0;
+}
+EXPORT_SYMBOL(copy_encoded_iov_from_iter);
+
+/**
+ * generic_encoded_read_checks() - sanity check an RWF_ENCODED read
+ * @iocb: I/O context.
+ * @iter: Destination iterator for read.
+ *
+ * This should always be called by RWF_ENCODED read implementations before
+ * returning any data.
+ *
+ * Return: Number of bytes available to return encoded data in @iter on success,
+ *         -EPERM if the file was not opened with O_ALLOW_ENCODED, -EINVAL if
+ *         the size of the &struct encoded_iov iovec is invalid.
+ */
+ssize_t generic_encoded_read_checks(struct kiocb *iocb, struct iov_iter *iter)
+{
+	size_t usize;
+
+	if (!(iocb->ki_filp->f_flags & O_ALLOW_ENCODED))
+		return -EPERM;
+	usize = iov_iter_single_seg_count(iter);
+	if (usize > PAGE_SIZE || usize < ENCODED_IOV_SIZE_VER0)
+		return -EINVAL;
+	return iov_iter_count(iter) - usize;
+}
+EXPORT_SYMBOL(generic_encoded_read_checks);
+
+/**
+ * copy_encoded_iov_to_iter() - copy a &struct encoded_iov to userspace
+ * @encoded: Encoding metadata to return.
+ * @to: Destination iterator.
+ *
+ * This should always be used by RWF_ENCODED read implementations rather than a
+ * plain copy_to_iter(), as it does the proper handling for backward- and
+ * forward-compatibility. The iterator must be sanity-checked with
+ * generic_encoded_read_checks() before this is called.
+ *
+ * Return: 0 on success, -EFAULT if access to userspace failed, -E2BIG if there
+ *         were non-zero fields in @encoded that the user buffer could not
+ *         accommodate.
+ */
+int copy_encoded_iov_to_iter(const struct encoded_iov *encoded,
+			     struct iov_iter *to)
+{
+	size_t ksize = sizeof(*encoded);
+	size_t usize = iov_iter_single_seg_count(to);
+	size_t size = min(ksize, usize);
+
+	/* We already sanity-checked usize in generic_encoded_read_checks(). */
+
+	if (usize < ksize &&
+	    memchr_inv((char *)encoded + usize, 0, ksize - usize))
+		return -E2BIG;
+	if (copy_to_iter(encoded, size, to) != size ||
+	    (usize > ksize &&
+	     iov_iter_zero(usize - ksize, to) != usize - ksize))
+		return -EFAULT;
+	return 0;
+}
+EXPORT_SYMBOL(copy_encoded_iov_to_iter);
+
 /*
  * Performs necessary checks before doing a clone.
  *