From patchwork Fri Aug 21 07:38:31 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Omar Sandoval X-Patchwork-Id: 11728419 Return-Path: Received: from mail.kernel.org (pdx-korg-mail-1.web.codeaurora.org [172.30.200.123]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 770341575 for ; Fri, 21 Aug 2020 07:39:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 54527207DF for ; Fri, 21 Aug 2020 07:39:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=osandov-com.20150623.gappssmtp.com header.i=@osandov-com.20150623.gappssmtp.com header.b="uucMpO60" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725908AbgHUHjC (ORCPT ); Fri, 21 Aug 2020 03:39:02 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:36924 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728036AbgHUHix (ORCPT ); Fri, 21 Aug 2020 03:38:53 -0400 Received: from mail-pg1-x52e.google.com (mail-pg1-x52e.google.com [IPv6:2607:f8b0:4864:20::52e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C3B6FC061388 for ; Fri, 21 Aug 2020 00:38:53 -0700 (PDT) Received: by mail-pg1-x52e.google.com with SMTP id m34so602680pgl.11 for ; Fri, 21 Aug 2020 00:38:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=osandov-com.20150623.gappssmtp.com; s=20150623; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=OuOQkpJ+s8y7Py7BHshRC8sGkmjsoFJukPaR2Amb08g=; b=uucMpO60tYPbHYue1thgRZF0+vFXF3+UQYYL/nerj0E/orLh3f0eUl4BZd894S1Ekd OOIeCQYVMBK8oVpjgCckOWc+QGfQzsXAlS0E7V1ujmDt7I8bfgiQgGKvjy1nC9LC2UYV vTd8GjtzWk7D7bin/FWmiWJW/SyKDLI+jTBY/OOnUJtlkeOJDileRxjrlIOa+rLkTvju J6JakhZrwXO1NQGlLz4rQ64R5p6ujUm29fpBxO7D2jcIZEBwidaxoXpFTXlfhJZ5LgKI XS6zktbPVwoUYjHquCuPjXe6xyYYYH7HCsCdrFthY1bXBpXQArkgobpoqej3+8033x5T Mpmg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=OuOQkpJ+s8y7Py7BHshRC8sGkmjsoFJukPaR2Amb08g=; b=DS4q7rJw0Xp3SbMit/kmZ6BYw+XKVwoYXsn5LR6LipRSVqF23pAQbfr+zIUtI39dbX gCaza+mRLtCg3e58t0m+IjcKdvAATGOodGfcwFkxCe5sStiJTeclzkLX1CJZgtTGHxVW TiT53kIsb4F17zIDputGqp8qxfKfHYqPeg8qaj89rP/9GnkImHRpaVUTtMrLBDcNDR+d gFCat1HS5hO/SkdhrEdToN1ynGadPRrhMfRVGrW6IjVeyDNvTz6nPDlksggGNdi+I8an 50vVnYEIQ5dEKFN5tFYb6gQce9NOQscGnm1s/IjnxSrOypfU8RD9aPLjv9cwmDmWpP+o 3Psg== X-Gm-Message-State: AOAM530xFxxbfiJVDk+2YOJ2t2WbGU7EDRqqPmRBGUUh2lgUb+f5VOvU AChVykM0l85tLMbGIW0ttPWO8A== X-Google-Smtp-Source: ABdhPJwVuN7DSUI2iMZNO3RcEWKAHFDmKUoBzPK7A3/LYjX3ns4+qFDCYZucRMG1kvb2WcUsX9gyjg== X-Received: by 2002:a63:545a:: with SMTP id e26mr1484120pgm.60.1597995531244; Fri, 21 Aug 2020 00:38:51 -0700 (PDT) Received: from exodia.tfbnw.net ([2620:10d:c090:400::5:f2a4]) by smtp.gmail.com with ESMTPSA id t10sm1220867pgp.15.2020.08.21.00.38.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 21 Aug 2020 00:38:50 -0700 (PDT) From: Omar Sandoval To: linux-fsdevel@vger.kernel.org, linux-btrfs@vger.kernel.org, Al Viro , Christoph Hellwig Cc: Dave Chinner , Jann Horn , Amir Goldstein , Aleksa Sarai , linux-api@vger.kernel.org, kernel-team@fb.com, Michael Kerrisk , linux-man Subject: [PATCH man-pages v5] Document encoded I/O Date: Fri, 21 Aug 2020 00:38:31 -0700 Message-Id: <64cc229872230dc6998a3dbf2264513870a8a6f6.1597994017.git.osandov@osandov.com> X-Mailer: git-send-email 2.28.0 In-Reply-To: References: MIME-Version: 1.0 Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org From: Omar Sandoval 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. Cc: Michael Kerrisk Cc: linux-man Signed-off-by: Omar Sandoval --- This feature is not yet upstream. man2/fcntl.2 | 10 +- man2/open.2 | 13 ++ man2/readv.2 | 64 +++++++++ man7/encoded_io.7 | 347 ++++++++++++++++++++++++++++++++++++++++++++++ 4 files changed, 433 insertions(+), 1 deletion(-) create mode 100644 man7/encoded_io.7 diff --git a/man2/fcntl.2 b/man2/fcntl.2 index 874cf2826..bf937260a 100644 --- a/man2/fcntl.2 +++ b/man2/fcntl.2 @@ -221,8 +221,9 @@ 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 @@ -1820,6 +1821,13 @@ 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 21ed2db22..7d36fdcc8 100644 --- a/man2/open.2 +++ b/man2/open.2 @@ -437,6 +437,14 @@ 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 @@ -1230,6 +1238,11 @@ 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 40b52964d..6ddd81930 100644 --- a/man2/readv.2 +++ b/man2/readv.2 @@ -264,6 +264,11 @@ 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 5.8)" +Read or write encoded (e.g., compressed) data. +See +.BR encoded_io (7). .SH RETURN VALUE On success, .BR readv (), @@ -283,6 +288,13 @@ 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 @@ -313,6 +325,58 @@ 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..ad0542c38 --- /dev/null +++ b/man7/encoded_io.7 @@ -0,0 +1,347 @@ +.\" Copyright (c) 2019 by Omar Sandoval +.\" +.\" %%%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 +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 ). +See +.B Extent layout +below for some examples. +.I +.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 () +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 () +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 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 . +.PP +.BR preadv2 () +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 () +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 userspace are non-zero, +then +.BR preadv2 () +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 () +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 +.IR 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 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 5.10) +.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