Re: [PATCH man-pages v4] Document encoded I/O

Re: [PATCH man-pages v4] Document encoded I/O

[Michael Kerrisk (man-pages)]

Hello Omar,

(Unless you CC both me and mtk.manpages@gmail.com, it's easily
possible that I will miss your man-pages patches.)

What's the status here? I presume the features documented here are not
yet merged, right? Is the aim still to have them merged in the future?

Thanks,

Michael

On Sat, 29 Feb 2020 at 00:16, Omar Sandoval <osandov@osandov.com> wrote:
>
> From: Omar Sandoval <osandov@fb.com>
>
> This adds a new page, encoded_io(7), providing an overview of encoded
> I/O and updates fcntl(2), open(2), and preadv2(2)/pwritev2(2) to
> reference it.
>
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
>  man2/fcntl.2      |  10 +-
>  man2/open.2       |  13 ++
>  man2/readv.2      |  64 ++++++++++
>  man7/encoded_io.7 | 298 ++++++++++++++++++++++++++++++++++++++++++++++
>  4 files changed, 384 insertions(+), 1 deletion(-)
>  create mode 100644 man7/encoded_io.7
>
> diff --git a/man2/fcntl.2 b/man2/fcntl.2
> index bb1ac1f5d..15a1010a6 100644
> --- a/man2/fcntl.2
> +++ b/man2/fcntl.2
> @@ -222,8 +222,9 @@ On Linux, this command can change only the
>  .BR O_ASYNC ,
>  .BR O_DIRECT ,
>  .BR O_NOATIME ,
> +.BR O_NONBLOCK ,
>  and
> -.B O_NONBLOCK
> +.B O_ALLOW_ENCODED
>  flags.
>  It is not possible to change the
>  .BR O_DSYNC
> @@ -1821,6 +1822,13 @@ Attempted to clear the
>  flag on a file that has the append-only attribute set.
>  .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
>  .I cmd
>  was
>  .BR F_ADD_SEALS ,
> diff --git a/man2/open.2 b/man2/open.2
> index 3ab4ee17b..256cb4247 100644
> --- a/man2/open.2
> +++ b/man2/open.2
> @@ -421,6 +421,14 @@ was followed by a call to
>  .BR fdatasync (2)).
>  .IR "See NOTES below" .
>  .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_EXCL
>  Ensure that this call creates the file:
>  if this flag is specified in conjunction with
> @@ -1176,6 +1184,11 @@ 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
> diff --git a/man2/readv.2 b/man2/readv.2
> index af27aa63e..8b5458023 100644
> --- a/man2/readv.2
> +++ b/man2/readv.2
> @@ -265,6 +265,11 @@ 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 5.7)"
> +Read or write encoded (e.g., compressed) data.
> +See
> +.BR encoded_io (7).
>  .SH RETURN VALUE
>  On success,
>  .BR readv (),
> @@ -284,6 +289,13 @@ 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 appropriately.
>  .SH ERRORS
>  The errors are as given for
> @@ -314,6 +326,58 @@ 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 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 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 alignment and/or size requirements are not met.
>  .SH VERSIONS
>  .BR preadv ()
>  and
> diff --git a/man7/encoded_io.7 b/man7/encoded_io.7
> new file mode 100644
> index 000000000..72b40353f
> --- /dev/null
> +++ b/man7/encoded_io.7
> @@ -0,0 +1,298 @@
> +.\" Copyright (c) 2019 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 2019-10-14 "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 one of
> +.B ENCODED_IOV_COMPRESSION_NONE
> +(zero),
> +.BR ENCODED_IOV_COMPRESSION_ZLIB ,
> +.BR ENCODED_IOV_COMPRESSION_LZO ,
> +or
> +.BR ENCODED_IOV_COMPRESSION_ZSTD .
> +.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 into the unencoded data where the data in the file begins
> +(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 ).
> +.I
> +.PP
> +In most cases,
> +.I len
> +is equal to
> +.I unencoded_len
> +and
> +.I unencoded_offset
> +is zero.
> +However, it may be necessary to refer to a subset of the unencoded data,
> +usually because a read occurred in the middle of an encoded extent,
> +because part of an extent was overwritten or deallocated in some
> +way (e.g., with
> +.BR write (2),
> +.BR truncate (2),
> +or
> +.BR fallocate (2))
> +or because part of an extent was added to the file (e.g., with
> +.BR ioctl_ficlonerange (2)
> +or
> +.BR ioctl_fideduperange (2)).
> +For example, if
> +.I len
> +is 300,
> +.I unencoded_len
> +is 1000,
> +and
> +.I unencoded_offset
> +is 600,
> +then the encoded data is 1000 bytes long when decoded,
> +of which only the 300 bytes starting at offset 600 are used;
> +the first 600 and last 100 bytes should be ignored.
> +.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
> +For
> +.BR pwritev2 (),
> +the metadata should be specified in
> +.IR iov[0] .
> +If
> +.I iov[0].iov_len
> +is less than
> +.I "sizeof(struct\ encoded_iov)"
> +in the kernel,
> +then any fields unknown to userspace 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 .
> +The encoded data should be passed in the remaining buffers.
> +This 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).
> +If the
> +.I offset
> +argument to
> +.BR pwritev2 ()
> +is -1, then the file offset is incremented by
> +.IR len .
> +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.
> +.PP
> +For
> +.BR preadv2 (),
> +the metadata is returned in
> +.IR iov[0] .
> +If
> +.I iov[0].iov_len
> +is less than
> +.I "sizeof(struct\ encoded_iov)"
> +in the kernel and any fields unknown to userspace are non-zero,
> +then this returns -1 and sets
> +.I errno
> +to
> +.BR E2BIG ;
> +if it is greater,
> +then any fields unknown to the kernel are returned as zero.
> +The encoded data is returned in the remaining buffers.
> +If the provided buffers are not large enough to return an entire encoded
> +extent,
> +then this returns -1 and sets
> +.I errno
> +to
> +.BR ENOBUFS .
> +This returns the number of encoded bytes read.
> +If the
> +.I offset
> +argument to
> +.BR preadv2 ()
> +is -1, then the file offset is incremented by
> +.IR len .
> +This will only return one encoded extent per call.
> +This can also read data which is not encoded;
> +all encoding fields will be zero in that case.
> +.PP
> +As the filesystem page cache typically contains decoded data,
> +encoded I/O bypasses the page cache.
> +.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 when the file was opened with the
> +.B O_ALLOW_ENCODED
> +flag to
> +.BR open (2),
> +which requires the
> +.B CAP_SYS_ADMIN
> +capability.
> +.B O_ALLOW_ENCODED
> +may be set and cleared with
> +.BR fcntl (2).
> +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 5.8)
> +.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_ZLIB ,
> +then the encoded data is a single zlib stream.
> +.IP *
> +If
> +.I compression
> +is
> +.BR ENCODED_IOV_COMPRESSION_LZO ,
> +then the encoded data is compressed page by page with LZO1X
> +and wrapped in the format documented in the Linux kernel source file
> +.IR fs/btrfs/lzo.c .
> +.IP *
> +If
> +.I compression
> +is
> +.BR ENCODED_IOV_COMPRESSION_ZSTD ,
> +then the encoded data is a single zstd frame compressed with the
> +.I windowLog
> +compression parameter set to no more than 17.
> +.RE
> +.IP
> +Additionally, there are some restrictions on
> +.BR pwritev2 ():
> +.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 .
> +.RE
> --
> 2.25.1
>


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

Re: [PATCH man-pages v4] Document encoded I/O

[Omar Sandoval]

On Thu, Apr 16, 2020 at 02:26:01PM +0200, Michael Kerrisk (man-pages) wrote:
> Hello Omar,
> 
> (Unless you CC both me and mtk.manpages@gmail.com, it's easily
> possible that I will miss your man-pages patches.)

That's good to know, thanks. Do you mind being CCd on man-pages for
features that haven't been finalized yet?

> What's the status here? I presume the features documented here are not
> yet merged, right? Is the aim still to have them merged in the future?

They're not yet merged but I'm still working on having them merged. I'm
still waiting for VFS review.

Thanks!

Re: [PATCH man-pages v4] Document encoded I/O

[Michael Kerrisk (man-pages)]

Hello Omar,

On Thu, 16 Apr 2020 at 19:02, Omar Sandoval <osandov@osandov.com> wrote:
>
> On Thu, Apr 16, 2020 at 02:26:01PM +0200, Michael Kerrisk (man-pages) wrote:
> > Hello Omar,
> >
> > (Unless you CC both me and mtk.manpages@gmail.com, it's easily
> > possible that I will miss your man-pages patches.)
>
> That's good to know, thanks. Do you mind being CCd on man-pages for
> features that haven't been finalized yet?

Please do CC me and linux-man@ on such patches. Just make sure that
the patch notes that the feature is not yet upstream.
>
> > What's the status here? I presume the features documented here are not
> > yet merged, right? Is the aim still to have them merged in the future?
>
> They're not yet merged but I'm still working on having them merged. I'm
> still waiting for VFS review.

Okay.

Thanks,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/