From patchwork Fri Feb 28 23:13:52 2020 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Omar Sandoval X-Patchwork-Id: 11413437 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 3CFE41395 for ; Fri, 28 Feb 2020 23:14:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 136622469C for ; Fri, 28 Feb 2020 23:14:36 +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="kyHvBnhn" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726642AbgB1XOO (ORCPT ); Fri, 28 Feb 2020 18:14:14 -0500 Received: from mail-pl1-f196.google.com ([209.85.214.196]:42619 "EHLO mail-pl1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726366AbgB1XON (ORCPT ); Fri, 28 Feb 2020 18:14:13 -0500 Received: by mail-pl1-f196.google.com with SMTP id u3so1802490plr.9 for ; Fri, 28 Feb 2020 15:14:12 -0800 (PST) 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=J5RxgP7fRri7oxbT5fgIZfBiRPHAOrRUAOI4Y0HgwrE=; b=kyHvBnhnGj/S5pJ9hnc69XQRhbNZTWAh5A7OfOpimkDIOrnSUbAT3X6ZDlw1eIQ4nZ AIGUVqJjBhF1sKrGyfE9XWFslI7KsPoQnfiGmvv2EYUV5LydSJQ0TUvjEjdrdpHzLTfX u2ZdnE9omblfzSV3n3sPYLCuWb2dGw/JExHrvDCOOWG7d+3OXCWuE8+TmhzeN9QXtTQJ 8aYi0SfxQKMGSxIsiRjPMLCE+4kjcISw86qeg/ZlD2RGoR7WnGTe1/OOk/jw0e3vhCYT xEpQ7R6zHozVTwA2GVexm7wfZaHrsTDUsRvh3PJTo9L/Qq2icOnJ9WgtPD9QqUTu4Pz5 M+NA== 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=J5RxgP7fRri7oxbT5fgIZfBiRPHAOrRUAOI4Y0HgwrE=; b=Cgl/b3G+g/0CzekPQqJxptxQyzDHNxamg41AJzrQq6T/p6IZ+gqegDv05SoXtOyVGK LSfxjzOHbb9tyGvl5SdXFWv++dh7Ups2FsMNHp8eu0cATdEtkW7zRKwiQbpE7uQ9YP0P BGP2z3BzcHisEhYtzklyWgP1c7yVFAAKbdWX9vQpolI9SLo2rQcSCT3AKvQ+BFv5udL6 HmY7+SISE+d8+MIlkO78z9rdHZjd/CNr/eKHZdQrIY7gSY7PJe9mwEDsu8/Ky0Y6v0LT wy3r9basZEAeX2rGZtXUaXlakFn4QPA5rB7ale7/L5G6BAPy4W+688FxaaVkLV4UfJzL 8oLg== X-Gm-Message-State: APjAAAWybMkmcQhe775pFDzlIIHJY+edUeyGqi2Fz9wdxh4QVbox9G1c vlEO9ElNsgEGszUNJnSSx1e1KrNgWBo= X-Google-Smtp-Source: APXvYqz36U8+H74m5wQe84lOxacXJTav+oBBsiyNPcpdCXNdo/vvAAKuYPCQ7LZU+fDfmb6cUnXMxA== X-Received: by 2002:a17:902:8647:: with SMTP id y7mr6249296plt.224.1582931651096; Fri, 28 Feb 2020 15:14:11 -0800 (PST) Received: from vader.thefacebook.com ([2620:10d:c090:500::6:1714]) by smtp.gmail.com with ESMTPSA id q7sm11421878pgk.62.2020.02.28.15.14.09 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 28 Feb 2020 15:14:10 -0800 (PST) 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 Subject: [PATCH man-pages v4] Document encoded I/O Date: Fri, 28 Feb 2020 15:13:52 -0800 Message-Id: <00f86ed7c25418599e6067cb1dfb186c90ce7bf3.1582931488.git.osandov@fb.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: References: MIME-Version: 1.0 Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@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. Signed-off-by: Omar Sandoval --- 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 +.\" +.\" %%%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 ). +.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