* Re: [PATCH man-pages v4] Document encoded I/O
[not found] ` <00f86ed7c25418599e6067cb1dfb186c90ce7bf3.1582931488.git.osandov@fb.com>
@ 2020-04-16 12:26 ` Michael Kerrisk (man-pages)
2020-04-16 17:02 ` Omar Sandoval
0 siblings, 1 reply; 3+ messages in thread
From: Michael Kerrisk (man-pages) @ 2020-04-16 12:26 UTC (permalink / raw)
To: Omar Sandoval
Cc: linux-fsdevel@vger.kernel.org, Linux btrfs Developers List,
Al Viro, Christoph Hellwig, Dave Chinner, Jann Horn,
Amir Goldstein, Aleksa Sarai, Linux API, Kernel Team,
Michael Kerrisk, linux-man
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/
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