From patchwork Fri Jan 22 20:46:47 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Omar Sandoval X-Patchwork-Id: 12040637 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-16.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id EFE44C433DB for ; Fri, 22 Jan 2021 21:16:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C2A5E23AF8 for ; Fri, 22 Jan 2021 21:16:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728456AbhAVVQd (ORCPT ); Fri, 22 Jan 2021 16:16:33 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49278 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730936AbhAVUtJ (ORCPT ); Fri, 22 Jan 2021 15:49:09 -0500 Received: from mail-pf1-x42e.google.com (mail-pf1-x42e.google.com [IPv6:2607:f8b0:4864:20::42e]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 198FCC0617A7 for ; Fri, 22 Jan 2021 12:47:20 -0800 (PST) Received: by mail-pf1-x42e.google.com with SMTP id o20so4633772pfu.0 for ; Fri, 22 Jan 2021 12:47:20 -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=vbu9QLmVwyatXp0OfhFPNy2zQtieZiKeMZxlJn9Kfv4=; b=GOgaOsTp+FUPgWB4mcGr8eF5RKOyQcGmI7XEnHl4Jcfk2vKy4Aod4Fbpj2D5sQZXN6 u7A5aH3ncaigNKvMgOUN7ajXUNpJdPwkpaYHw8d9xW+KjxxTcdqjuk0/uMFC4ks8w9ue K3Wa2DozeExWFVhV/GHQgNAwPgi4SkiKE1MxzHn2RXHWJD52tAyKXvRHZ6g4PQ5T/GS+ cEcxIFBwYJa2RyCrych1I4K1N8itAHcAiyimRYm9kstpAdNZSTcOweW6izOg5uuyFvl8 rIKuSxUTArkewvViRerRryiq/Fe0gqwqkGnQ9JWt09I6dre6sQzoF+zIAvpTDWBn8Sb/ +nQw== 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=vbu9QLmVwyatXp0OfhFPNy2zQtieZiKeMZxlJn9Kfv4=; b=KUl9r/9cpJj95QHwYKtDCcNoT7UWXva/Hr8j9TLu3nNdtecUN9/0E/hd3SDrhHXLWM gge4HaIPgKXhMBz6jRn1pC9j7RvX3aOgfO+G0Y/1Xijf5sxFSSqkNM+JKGDrN8Txkvk1 tixu1ZfuAYH9Yid7UkCYO+Ij0dg70c+q8WMZF9j+cpSOAQ68cqyxBdLZniPH+h2zLi/D wZyRglg4DBaHQu52YsDNE1skiXIbhQnSHEX8MG4q1ZmPmwzww++MeYu1JTjyQyDFkqJ1 6JHj6J4YkUjxLLUcf3eBESMR30qfGKn2nyDnKTbo80Rd1E13HY2ZpxJ7SfA1+tcBE2I8 62DA== X-Gm-Message-State: AOAM53225HU+Z2GIJmsSM42LXNp+aOjyxD8yLgkFx11nzaZtGwIrbrMw lZkoL5Vm2Gfe7hkUFTaf6qn2iVJ0KLdUOA== X-Google-Smtp-Source: ABdhPJz3kXjLdoq+PAq+YZqCDHze64La1WV72OnjCILeDtnT0eorLoB9/YkMnFglfzquhK1WlZTXNg== X-Received: by 2002:a65:6116:: with SMTP id z22mr6450538pgu.264.1611348438257; Fri, 22 Jan 2021 12:47:18 -0800 (PST) Received: from relinquished.tfbnw.net ([2620:10d:c090:400::5:ea88]) by smtp.gmail.com with ESMTPSA id j18sm4092900pfc.99.2021.01.22.12.47.15 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 22 Jan 2021 12:47:16 -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, Michael Kerrisk , linux-man Subject: [PATCH man-pages v7] Document encoded I/O Date: Fri, 22 Jan 2021 12:46:47 -0800 Message-Id: X-Mailer: git-send-email 2.30.0 In-Reply-To: References: MIME-Version: 1.0 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. Cc: Michael Kerrisk Cc: linux-man Signed-off-by: Omar Sandoval --- man2/fcntl.2 | 8 + man2/open.2 | 13 ++ man2/readv.2 | 69 +++++++++ man7/encoded_io.7 | 369 ++++++++++++++++++++++++++++++++++++++++++++++ 4 files changed, 459 insertions(+) create mode 100644 man7/encoded_io.7 diff --git a/man2/fcntl.2 b/man2/fcntl.2 index a15f467ef..1081cb70f 100644 --- a/man2/fcntl.2 +++ b/man2/fcntl.2 @@ -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. diff --git a/man2/open.2 b/man2/open.2 index b30dc1532..6aab44b6d 100644 --- a/man2/open.2 +++ b/man2/open.2 @@ -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 diff --git a/man2/readv.2 b/man2/readv.2 index 472adcf73..c7aec9fa7 100644 --- a/man2/readv.2 +++ b/man2/readv.2 @@ -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 diff --git a/man7/encoded_io.7 b/man7/encoded_io.7 new file mode 100644 index 000000000..1f5e6a510 --- /dev/null +++ b/man7/encoded_io.7 @@ -0,0 +1,369 @@ +.\" Copyright (c) 2020 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 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 +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)