[PATCH v8 00/10] fs: interface for directly reading/writing compressed data

@@ -217,6 +217,7 @@ in
 .I arg
 are ignored.
 On Linux, this command can change only the
+.BR O_ALLOW_ENCODED ,
 .BR O_APPEND ,
 .BR O_ASYNC ,
 .BR O_DIRECT ,

@@ -1815,6 +1816,13 @@ and the soft or hard user pipe limit has been reached; see
 .BR pipe (7).
 .TP
 .B EPERM
+Attempted to set the
+.B O_ALLOW_ENCODED
+flag and the calling process did not have the
+.B CAP_SYS_ADMIN
+capability.
+.TP
+.B EPERM
 Attempted to clear the
 .B O_APPEND
 flag on a file that has the append-only attribute set.

@@ -180,6 +180,14 @@ for details.
 .PP
 The full list of file creation flags and file status flags is as follows:
 .TP
+.B O_ALLOW_ENCODED
+Open the file with encoded I/O permissions;
+see
+.BR encoded_io (7).
+The caller must have the
+.B CAP_SYS_ADMIN
+capability.
+.TP
 .B O_APPEND
 The file is opened in append mode.
 Before each

@@ -1232,6 +1240,11 @@ did not match the owner of the file and the caller was not privileged.
 The operation was prevented by a file seal; see
 .BR fcntl (2).
 .TP
+.B EPERM
+The
+.B O_ALLOW_ENCODED
+flag was specified, but the caller was not privileged.
+.TP
 .B EROFS
 .I pathname
 refers to a file on a read-only filesystem and write access was

@@ -263,6 +263,11 @@ the data is always appended to the end of the file.
 However, if the
 .I offset
 argument is \-1, the current file offset is updated.
+.TP
+.BR RWF_ENCODED " (since Linux 5.13)"
+Read or write encoded (e.g., compressed) data.
+See
+.BR encoded_io (7).
 .SH RETURN VALUE
 On success,
 .BR readv (),

@@ -282,6 +287,12 @@ than requested (see
 and
 .BR write (2)).
 .PP
+If
+.B RWF_ENCODED
+was specified in
+.IR flags ,
+then the return value is the number of encoded bytes.
+.PP
 On error, \-1 is returned, and \fIerrno\fP is set to indicate the error.
 .SH ERRORS
 The errors are as given for

@@ -312,6 +323,64 @@ is less than zero or greater than the permitted maximum.
 .TP
 .B EOPNOTSUPP
 An unknown flag is specified in \fIflags\fP.
+.TP
+.B EOPNOTSUPP
+.B RWF_ENCODED
+is specified in
+.I flags
+and the filesystem does not implement encoded I/O.
+.TP
+.B EPERM
+.B RWF_ENCODED
+is specified in
+.I flags
+and the file was not opened with the
+.B O_ALLOW_ENCODED
+flag.
+.PP
+.BR preadv2 ()
+can additionally fail for the following reasons:
+.TP
+.B E2BIG
+.B RWF_ENCODED
+is specified in
+.I flags
+and
+.I iov[0]
+is not large enough to return the encoding metadata.
+.TP
+.B ENOBUFS
+.B RWF_ENCODED
+is specified in
+.I flags
+and the buffers in
+.I iov
+are not big enough to return the encoded data.
+.PP
+.BR pwritev2 ()
+can additionally fail for the following reasons:
+.TP
+.B E2BIG
+.B RWF_ENCODED
+is specified in
+.I flags
+and
+.I iov[0]
+contains non-zero fields
+after the kernel's
+.IR "sizeof(struct encoded_iov)" .
+.TP
+.B EINVAL
+.B RWF_ENCODED
+is specified in
+.I flags
+and the encoding is unknown or not supported by the filesystem.
+.TP
+.B EINVAL
+.B RWF_ENCODED
+is specified in
+.I flags
+and the alignment and/or size requirements are not met.
 .SH VERSIONS
 .BR preadv ()
 and

@@ -0,0 +1,369 @@
+.\" Copyright (c) 2020 by Omar Sandoval <osandov@fb.com>
+.\"
+.\" %%%LICENSE_START(VERBATIM)
+.\" Permission is granted to make and distribute verbatim copies of this
+.\" manual provided the copyright notice and this permission notice are
+.\" preserved on all copies.
+.\"
+.\" Permission is granted to copy and distribute modified versions of this
+.\" manual under the conditions for verbatim copying, provided that the
+.\" entire resulting derived work is distributed under the terms of a
+.\" permission notice identical to this one.
+.\"
+.\" Since the Linux kernel and libraries are constantly changing, this
+.\" manual page may be incorrect or out-of-date.  The author(s) assume no
+.\" responsibility for errors or omissions, or for damages resulting from
+.\" the use of the information contained herein.  The author(s) may not
+.\" have taken the same level of care in the production of this manual,
+.\" which is licensed free of charge, as they might when working
+.\" professionally.
+.\"
+.\" Formatted or processed versions of this manual, if unaccompanied by
+.\" the source, must acknowledge the copyright and authors of this work.
+.\" %%%LICENSE_END
+.\"
+.\"
+.TH ENCODED_IO  7 2020-11-11 "Linux" "Linux Programmer's Manual"
+.SH NAME
+encoded_io \- overview of encoded I/O
+.SH DESCRIPTION
+Several filesystems (e.g., Btrfs) support transparent encoding
+(e.g., compression, encryption) of data on disk:
+written data is encoded by the kernel before it is written to disk,
+and read data is decoded before being returned to the user.
+In some cases, it is useful to skip this encoding step.
+For example, the user may want to read the compressed contents of a file
+or write pre-compressed data directly to a file.
+This is referred to as "encoded I/O".
+.SS Encoded I/O API
+Encoded I/O is specified with the
+.B RWF_ENCODED
+flag to
+.BR preadv2 (2)
+and
+.BR pwritev2 (2).
+If
+.B RWF_ENCODED
+is specified, then
+.I iov[0].iov_base
+points to an
+.I encoded_iov
+structure, defined in
+.I <linux/fs.h>
+as:
+.PP
+.in +4n
+.EX
+struct encoded_iov {
+    __aligned_u64 len;
+    __aligned_u64 unencoded_len;
+    __aligned_u64 unencoded_offset;
+    __u32 compression;
+    __u32 encryption;
+};
+.EE
+.in
+.PP
+This may be extended in the future, so
+.I iov[0].iov_len
+must be set to
+.I sizeof(struct encoded_iov)
+for forward/backward compatibility.
+The remaining buffers contain the encoded data.
+.PP
+.I compression
+and
+.I encryption
+are the encoding fields.
+.I compression
+is
+.B ENCODED_IOV_COMPRESSION_NONE
+(zero)
+or a filesystem-specific
+.B ENCODED_IOV_COMPRESSION_*
+constant;
+see
+.B "Filesystem support"
+below.
+.I encryption
+is currently always
+.B ENCODED_IOV_ENCRYPTION_NONE
+(zero).
+.PP
+.I unencoded_len
+is the length of the unencoded (i.e., decrypted and decompressed) data.
+.I unencoded_offset
+is the offset from the first byte of the unencoded data
+to the first byte of logical data in the file
+(less than or equal to
+.IR unencoded_len ).
+.I len
+is the length of the data in the file
+(less than or equal to
+.I unencoded_len
+-
+.IR unencoded_offset ).
+See
+.B Extent layout
+below for some examples.
+.PP
+If the unencoded data is actually longer than
+.IR unencoded_len ,
+then it is truncated;
+if it is shorter, then it is extended with zeroes.
+.PP
+.BR pwritev2 (2)
+uses the metadata specified in
+.IR iov[0] ,
+writes the encoded data from the remaining buffers,
+and returns the number of encoded bytes written
+(that is, the sum of
+.I iov[n].iov_len
+for 1 <=
+.I n
+<
+.IR iovcnt ;
+partial writes will not occur).
+At least one encoding field must be non-zero.
+Note that the encoded data is not validated when it is written;
+if it is not valid (e.g., it cannot be decompressed),
+then a subsequent read may return an error.
+If the
+.I offset
+argument to
+.BR pwritev2 (2)
+is -1, then the file offset is incremented by
+.IR len .
+If
+.I iov[0].iov_len
+is less than
+.I sizeof(struct encoded_iov)
+in the kernel,
+then any fields unknown to user space are treated as if they were zero;
+if it is greater and any fields unknown to the kernel are non-zero,
+then this returns -1 and sets
+.I errno
+to
+.BR E2BIG .
+.PP
+.BR preadv2 (2)
+populates the metadata in
+.IR iov[0] ,
+the encoded data in the remaining buffers,
+and returns the number of encoded bytes read.
+This will only return one extent per call.
+This can also read data which is not encoded;
+all encoding fields will be zero in that case.
+If the
+.I offset
+argument to
+.BR preadv2 (2)
+is -1, then the file offset is incremented by
+.IR len .
+If
+.I iov[0].iov_len
+is less than
+.I sizeof(struct encoded_iov)
+in the kernel and any fields unknown to user space are non-zero,
+then
+.BR preadv2 (2)
+returns -1 and sets
+.I errno
+to
+.BR E2BIG ;
+if it is greater,
+then any fields unknown to the kernel are returned as zero.
+If the provided buffers are not large enough
+to return an entire encoded extent,
+then
+.BR preadv2 (2)
+returns -1 and sets
+.I errno
+to
+.BR ENOBUFS .
+.PP
+As the filesystem page cache typically contains decoded data,
+encoded I/O bypasses the page cache.
+.SS Extent layout
+By using
+.IR len ,
+.IR unencoded_len ,
+and
+.IR unencoded_offset ,
+it is possible to refer to a subset of an unencoded extent.
+.PP
+In the simplest case,
+.I len
+is equal to
+.I unencoded_len
+and
+.I unencoded_offset
+is zero.
+This means that the entire unencoded extent is used.
+.PP
+However, suppose we read 50 bytes into a file
+which contains a single compressed extent.
+The filesystem must still return the entire compressed extent
+for us to be able to decompress it,
+so
+.I unencoded_len
+would be the length of the entire decompressed extent.
+However, because the read was at offset 50,
+the first 50 bytes should be ignored.
+Therefore,
+.I unencoded_offset
+would be 50,
+and
+.I len
+would accordingly be
+.I unencoded_len
+- 50.
+.PP
+Additionally, suppose we want to create an encrypted file with length 500,
+but the file is encrypted with a block cipher using a block size of 4096.
+The unencoded data would therefore include the appropriate padding,
+and
+.I unencoded_len
+would be 4096.
+However, to represent the logical size of the file,
+.I len
+would be 500
+(and
+.I unencoded_offset
+would be 0).
+.PP
+Similar situations can arise in other cases:
+.IP * 3
+If the filesystem pads data to the filesystem block size before compressing,
+then compressed files with a size unaligned to the filesystem block size
+will end with an extent with
+.I len
+<
+.IR unencoded_len .
+.IP *
+Extents cloned from the middle of a larger encoded extent with
+.B FICLONERANGE
+may have a non-zero
+.I unencoded_offset
+and/or
+.I len
+<
+.IR unencoded_len .
+.IP *
+If the middle of an encoded extent is overwritten,
+the filesystem may create extents with a non-zero
+.I unencoded_offset
+and/or
+.I len
+<
+.I unencoded_len
+for the parts that were not overwritten.
+.SS Security
+Encoded I/O creates the potential for some security issues:
+.IP * 3
+Encoded writes allow writing arbitrary data
+which the kernel will decode on a subsequent read.
+Decompression algorithms are complex
+and may have bugs which can be exploited by maliciously crafted data.
+.IP *
+Encoded reads may return data which is not logically present in the file
+(see the discussion of
+.I len
+vs
+.I unencoded_len
+above).
+It may not be intended for this data to be readable.
+.PP
+Therefore, encoded I/O requires privilege.
+Namely, the
+.B RWF_ENCODED
+flag may only be used if the file description has the
+.B O_ALLOW_ENCODED
+file status flag set,
+and the
+.B O_ALLOW_ENCODED
+flag may only be set by a thread with the
+.B CAP_SYS_ADMIN
+capability.
+The
+.B O_ALLOW_ENCODED
+flag can be set by
+.BR open (2)
+or
+.BR fcntl (2).
+It can also be cleared by
+.BR fcntl (2);
+clearing it does not require
+.B CAP_SYS_ADMIN.
+Note that it is not cleared on
+.BR fork (2)
+or
+.BR execve (2).
+One may wish to use
+.B O_CLOEXEC
+with
+.BR O_ALLOW_ENCODED .
+.SS Filesystem support
+Encoded I/O is supported on the following filesystems:
+.TP
+Btrfs (since Linux 5.13)
+.IP
+Btrfs supports encoded reads and writes of compressed data.
+The data is encoded as follows:
+.RS
+.IP * 3
+If
+.I compression
+is
+.BR ENCODED_IOV_COMPRESSION_BTRFS_ZLIB ,
+then the encoded data is a single zlib stream.
+.IP *
+If
+.I compression
+is
+.BR ENCODED_IOV_COMPRESSION_BTRFS_ZSTD ,
+then the encoded data is a single zstd frame compressed with the
+.I windowLog
+compression parameter set to no more than 17.
+.IP *
+If
+.I compression
+is one of
+.BR ENCODED_IOV_COMPRESSION_BTRFS_LZO_4K ,
+.BR ENCODED_IOV_COMPRESSION_BTRFS_LZO_8K ,
+.BR ENCODED_IOV_COMPRESSION_BTRFS_LZO_16K ,
+.BR ENCODED_IOV_COMPRESSION_BTRFS_LZO_32K ,
+or
+.BR ENCODED_IOV_COMPRESSION_BTRFS_LZO_64K ,
+then the encoded data is compressed page by page
+(using the page size indicated by the name of the constant)
+with LZO1X
+and wrapped in the format documented in the Linux kernel source file
+.IR fs/btrfs/lzo.c .
+.RE
+.IP
+Additionally, there are some restrictions on
+.BR pwritev2 (2):
+.RS
+.IP * 3
+.I offset
+(or the current file offset if
+.I offset
+is -1) must be aligned to the sector size of the filesystem.
+.IP *
+.I len
+must be aligned to the sector size of the filesystem
+unless the data ends at or beyond the current end of the file.
+.IP *
+.I unencoded_len
+and the length of the encoded data must each be no more than 128 KiB.
+This limit may increase in the future.
+.IP *
+The length of the encoded data must be less than or equal to
+.IR unencoded_len .
+.IP *
+If using LZO, the filesystem's page size must match the compression page size.
+.RE
+.SH SEE ALSO
+.BR open (2),
+.BR preadv2 (2)

@@ -121,6 +121,8 @@ size_t copy_page_to_iter(struct page *page, size_t offset, size_t bytes,
 			 struct iov_iter *i);
 size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
 			 struct iov_iter *i);
+int copy_struct_from_iter(void *dst, size_t ksize, struct iov_iter *i,
+			  size_t usize);
 
 size_t _copy_to_iter(const void *addr, size_t bytes, struct iov_iter *i);
 size_t _copy_from_iter(void *addr, size_t bytes, struct iov_iter *i);

@@ -948,6 +948,88 @@ size_t copy_page_from_iter(struct page *page, size_t offset, size_t bytes,
 }
 EXPORT_SYMBOL(copy_page_from_iter);
 
+/**
+ * copy_struct_from_iter - copy a struct from an iov_iter
+ * @dst: Destination buffer.
+ * @ksize: Size of @dst struct.
+ * @i: Source iterator.
+ * @usize: (Alleged) size of struct in @i.
+ *
+ * Copies a struct from an iov_iter in a way that guarantees
+ * backwards-compatibility for struct arguments in an iovec (as long as the
+ * rules for copy_struct_from_user() are followed).
+ *
+ * The recommended usage is that @usize be taken from the current segment:
+ *
+ *   int do_foo(struct iov_iter *i)
+ *   {
+ *     size_t usize = iov_iter_single_seg_count(i);
+ *     struct foo karg;
+ *     int err;
+ *
+ *     if (usize > PAGE_SIZE)
+ *       return -E2BIG;
+ *     if (usize < FOO_SIZE_VER0)
+ *       return -EINVAL;
+ *     err = copy_struct_from_iter(&karg, sizeof(karg), i, usize);
+ *     if (err)
+ *       return err;
+ *
+ *     // ...
+ *   }
+ *
+ * Return: 0 on success, -errno on error (see copy_struct_from_user()).
+ *
+ * On success, the iterator is advanced @usize bytes. On error, the iterator is
+ * not advanced.
+ */
+int copy_struct_from_iter(void *dst, size_t ksize, struct iov_iter *i,
+			  size_t usize)
+{
+	if (usize <= ksize) {
+		if (!copy_from_iter_full(dst, usize, i))
+			return -EFAULT;
+		memset(dst + usize, 0, ksize - usize);
+	} else {
+		size_t copied = 0, copy;
+		int ret;
+
+		if (WARN_ON(iov_iter_is_pipe(i)) || unlikely(i->count < usize))
+			return -EFAULT;
+		if (iter_is_iovec(i))
+			might_fault();
+		iterate_all_kinds(i, usize, v, ({
+			copy = min(ksize - copied, v.iov_len);
+			if (copy && copyin(dst + copied, v.iov_base, copy))
+				return -EFAULT;
+			copied += copy;
+			ret = check_zeroed_user(v.iov_base + copy,
+						v.iov_len - copy);
+			if (ret <= 0)
+				return ret ?: -E2BIG;
+			0;}), ({
+			char *addr = kmap_atomic(v.bv_page);
+			copy = min_t(size_t, ksize - copied, v.bv_len);
+			memcpy(dst + copied, addr + v.bv_offset, copy);
+			copied += copy;
+			ret = memchr_inv(addr + v.bv_offset + copy, 0,
+					 v.bv_len - copy) ? -E2BIG : 0;
+			kunmap_atomic(addr);
+			if (ret)
+				return ret;
+			}), ({
+			copy = min(ksize - copied, v.iov_len);
+			memcpy(dst + copied, v.iov_base, copy);
+			if (memchr_inv(v.iov_base, 0, v.iov_len))
+				return -E2BIG;
+			})
+		)
+		iov_iter_advance(i, usize);
+	}
+	return 0;
+}
+EXPORT_SYMBOL_GPL(copy_struct_from_iter);
+
 static size_t pipe_zero(size_t bytes, struct iov_iter *i)
 {
 	struct pipe_inode_info *pipe = i->pipe;

@@ -34,6 +34,7 @@
 
 #define O_PATH		040000000
 #define __O_TMPFILE	0100000000
+#define O_ALLOW_ENCODED	0200000000
 
 #define F_GETLK		7
 #define F_SETLK		8

@@ -19,6 +19,7 @@
 
 #define O_PATH		020000000
 #define __O_TMPFILE	040000000
+#define O_ALLOW_ENCODED	100000000
 
 #define F_GETLK64	8
 #define F_SETLK64	9

@@ -37,6 +37,7 @@
 
 #define O_PATH		0x1000000
 #define __O_TMPFILE	0x2000000
+#define O_ALLOW_ENCODED	0x8000000
 
 #define F_GETOWN	5	/*  for sockets. */
 #define F_SETOWN	6	/*  for sockets. */

@@ -30,7 +30,8 @@
 #include <asm/siginfo.h>
 #include <linux/uaccess.h>
 
-#define SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | O_DIRECT | O_NOATIME)
+#define SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | O_DIRECT | O_NOATIME | \
+		    O_ALLOW_ENCODED)
 
 static int setfl(int fd, struct file * filp, unsigned long arg)
 {

@@ -49,6 +50,11 @@ static int setfl(int fd, struct file * filp, unsigned long arg)
 		if (!inode_owner_or_capable(inode))
 			return -EPERM;
 
+	/* O_ALLOW_ENCODED can only be set by superuser */
+	if ((arg & O_ALLOW_ENCODED) && !(filp->f_flags & O_ALLOW_ENCODED) &&
+	    !capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
 	/* required for strict SunOS emulation */
 	if (O_NONBLOCK != O_NDELAY)
 	       if (arg & O_NDELAY)

@@ -1035,7 +1041,7 @@ static int __init fcntl_init(void)
 	 * Exceptions: O_NONBLOCK is a two bit define on parisc; O_NDELAY
 	 * is defined as O_NONBLOCK on some platforms and not on others.
 	 */
-	BUILD_BUG_ON(21 - 1 /* for O_RDONLY being 0 */ !=
+	BUILD_BUG_ON(22 - 1 /* for O_RDONLY being 0 */ !=
 		HWEIGHT32(
 			(VALID_OPEN_FLAGS & ~(O_NONBLOCK | O_NDELAY)) |
 			__FMODE_EXEC | __FMODE_NONOTIFY));

@@ -2892,6 +2892,10 @@ static int may_open(const struct path *path, int acc_mode, int flag)
 	if (flag & O_NOATIME && !inode_owner_or_capable(inode))
 		return -EPERM;
 
+	/* O_ALLOW_ENCODED can only be set by superuser */
+	if ((flag & O_ALLOW_ENCODED) && !capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
 	return 0;
 }
 

@@ -10,7 +10,7 @@
 	(O_RDONLY | O_WRONLY | O_RDWR | O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC | \
 	 O_APPEND | O_NDELAY | O_NONBLOCK | __O_SYNC | O_DSYNC | \
 	 FASYNC	| O_DIRECT | O_LARGEFILE | O_DIRECTORY | O_NOFOLLOW | \
-	 O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE)
+	 O_NOATIME | O_CLOEXEC | O_PATH | __O_TMPFILE | O_ALLOW_ENCODED)
 
 /* List of all valid flags for the how->upgrade_mask argument: */
 #define VALID_UPGRADE_FLAGS \

@@ -89,6 +89,10 @@
 #define __O_TMPFILE	020000000
 #endif
 
+#ifndef O_ALLOW_ENCODED
+#define O_ALLOW_ENCODED	040000000
+#endif
+
 /* a horrid kludge trying to make sure that this will fail on old kernels */
 #define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
 #define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT)      

@@ -0,0 +1,74 @@
+===========
+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``.
+
+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.

@@ -53,6 +53,7 @@ filesystem implementations.
    journalling
    fscrypt
    fsverity
+   encoded_io
 
 Filesystems
 ===========

@@ -20,6 +20,7 @@
 #include <linux/compat.h>
 #include <linux/mount.h>
 #include <linux/fs.h>
+#include <linux/encoded_io.h>
 #include "internal.h"
 
 #include <linux/uaccess.h>

@@ -1625,24 +1626,15 @@ int generic_write_check_limits(struct file *file, loff_t pos, loff_t *count)
 	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.
- */
-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 */

@@ -1652,8 +1644,22 @@ 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 appropriate error code that caller should return or
+ * zero in case that write should be allowed.
+ */
+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;
 

@@ -1684,3 +1690,139 @@ int generic_file_rw_checks(struct file *file_in, struct file *file_out)
 
 	return 0;
 }
+
+/**
+ * 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_offset > encoded->unencoded_len)
+		return -EINVAL;
+	if (encoded->len > encoded->unencoded_len - encoded->unencoded_offset)
+		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);