From mboxrd@z Thu Jan 1 00:00:00 1970 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=-19.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 3D9BAC4338F for ; Sun, 15 Aug 2021 06:46:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 06D7B60ED5 for ; Sun, 15 Aug 2021 06:46:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233827AbhHOGrY (ORCPT ); Sun, 15 Aug 2021 02:47:24 -0400 Received: from mail.kernel.org ([198.145.29.99]:50624 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233396AbhHOGrY (ORCPT ); Sun, 15 Aug 2021 02:47:24 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 0B31B60ED5; Sun, 15 Aug 2021 06:46:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1629010014; bh=LAtpauFu7/3vCZAAmCRXbHpjI8kOYu9rgDW+qKmjYDo=; h=From:To:Cc:Subject:Date:From; b=jX4xOweGvcKDeQbyEGmDvxz85gNi8ckT4A6Ys/VzCscZHaPi+K+mGuKmo2EnTL2S2 yVWGwboZrGcNmZps/JLUJWOIw0kmPSQQHeiDnzl1sU48qXSJHL7N0wBTi8nDqb6ZwI eI0icKpRrMWBkewfrSjptFCQyKo4mYKie/tZ9CZBtXiJgKUym/w8ZIrwHHPVzbo/T9 hmVpmmS/+rQv2fxzAcRVpAWsoeGzfOQgNUuDcKc3E9cm2E6f57tFXT/jnQq2Qe+kLP pVJJKt35DgOslPdWx0AtcAELftFqMWkLt736g+X4Y1NNPHHxuVzmWYuBU3vQhfHW7a lRNBRsDFO2M2A== From: Mike Rapoport To: Michael Kerrisk Cc: Alejandro Colomar , Mike Rapoport , Mike Rapoport , linux-man@vger.kernel.org Subject: [PATCH v3] man2: new page describing memfd_secret() system call Date: Sun, 15 Aug 2021 09:46:48 +0300 Message-Id: <20210815064648.300529-1-rppt@kernel.org> X-Mailer: git-send-email 2.31.1 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-man@vger.kernel.org From: Mike Rapoport Signed-off-by: Mike Rapoport --- v3: * Incorporate Michael's feedback except the extensive NOTES section. v2: https://lore.kernel.org/linux-api/20210729082900.1581359-1-rppt@kernel.org Address Alex's comments: * update synopsis to match new style for syscalls without a wrapper * drop note about absence of glibc wrapper * update formatting v1: https://lore.kernel.org/linux-api/20210727124140.1487079-1-rppt@kernel.org man2/memfd_secret.2 | 154 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 154 insertions(+) create mode 100644 man2/memfd_secret.2 diff --git a/man2/memfd_secret.2 b/man2/memfd_secret.2 new file mode 100644 index 000000000..188c547bf --- /dev/null +++ b/man2/memfd_secret.2 @@ -0,0 +1,154 @@ +.\" Copyright (c) 2021, IBM Corporation. +.\" Written by Mike Rapoport +.\" +.\" Based on memfd_create(2) man page +.\" Copyright (C) 2014 Michael Kerrisk +.\" and Copyright (C) 2014 David Herrmann +.\" +.\" %%%LICENSE_START(GPLv2+) +.\" +.\" This program is free software; you can redistribute it and/or modify +.\" it under the terms of the GNU General Public License as published by +.\" the Free Software Foundation; either version 2 of the License, or +.\" (at your option) any later version. +.\" +.\" This program is distributed in the hope that it will be useful, +.\" but WITHOUT ANY WARRANTY; without even the implied warranty of +.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +.\" GNU General Public License for more details. +.\" +.\" You should have received a copy of the GNU General Public +.\" License along with this manual; if not, see +.\" . +.\" %%%LICENSE_END +.\" +.TH MEMFD_SECRET 2 2020-08-02 Linux "Linux Programmer's Manual" +.SH NAME +memfd_secret \- create an anonymous RAM-based file +to access secret memory regions +.SH SYNOPSIS +.nf +.PP +.BR "#include " " /* Definition of " SYS_* " constants */" +.B #include +.PP +.BI "int syscall(SYS_memfd_secret, unsigned int " flags ); +.fi +.PP +.IR Note : +glibc provides no wrapper for +.BR memfd_secret (), +necessitating the use of +.BR syscall (2). +.SH DESCRIPTION +.BR memfd_secret () +creates an anonymous file and returns a file descriptor that refers to it. +The file provides a way to create and access memory regions +with stronger protection than usual RAM-based files and +anonymous memory mappings. +Once all open references to the file are closed, +it is automatically released. +The initial size of the file is set to 0. +Following the call, the file size should be set using +.BR ftruncate (2). +.PP +The memory areas backing the file created with +.BR memfd_create(2) +are visible only to the processes that have access to the file descriptor. +The memory region is removed from the kernel page tables +and only the page tables of the processes holding the file descriptor +map the corresponding physical memory. +(Thus, the pages in the region can't be accessed by the kernel itself, +so that, for example, pointers to the region can't be passed to +system calls.) +.PP +The following values may be bitwise ORed in +.I flags +to control the behavior of +.BR memfd_secret (2): +.TP +.B FD_CLOEXEC +Set the close-on-exec flag on the new file descriptor, +which causes the region to be removed from the process on +.BR execve (2). +See the description of the +.B O_CLOEXEC +flag in +.BR open (2) +.PP +As its return value, +.BR memfd_secret () +returns a new file descriptor that refers to an anonymous file. +This file descriptor is opened for both reading and writing +.RB ( O_RDWR ) +and +.B O_LARGEFILE +is set for the file descriptor. +.PP +With respect to +.BR fork (2) +and +.BR execve (2), +the usual semantics apply for the file descriptor created by +.BR memfd_secret (). +A copy of the file descriptor is inherited by the child produced by +.BR fork (2) +and refers to the same file. +The file descriptor is preserved across +.BR execve (2), +unless the close-on-exec flag has been set. +.PP +The memory region is locked into memory in the same way as with +.BR mlock (2), +so that it will never be written into swap. +However the implementation of +.BR memfd_secret (2) +will not try to populate the whole range during the +.BR mmap (2) +call that attaches the region into the process's address space; +instead, the pages are only actually allocated +as they are faulted in. +The amount of memory allowed for memory mappings +of the file descriptor obeys the same rules as +.BR mlock (2) +and cannot exceed +.BR RLIMIT_MEMLOCK . +.SH RETURN VALUE +On success, +.BR memfd_secret (2) +returns a new file descriptor. +On error, \-1 is returned and +.I errno +is set to indicate the error. +.SH ERRORS +.TP +.B EINVAL +.I flags +included unknown bits. +.TP +.B EMFILE +The per-process limit on the number of open file descriptors has been reached. +.TP +.B EMFILE +The system-wide limit on the total number of open files has been reached. +.TP +.B ENOMEM +There was insufficient memory to create a new anonymous file. +.TP +.B ENOSYS +.BR memfd_secret () +is not implemented on this architecture. +.SH VERSIONS +The +.BR memfd_secret (2) +system call first appeared in Linux 5.14. +.SH CONFORMING TO +The +.BR memfd_secret (2) +system call is Linux-specific. +.SH SEE ALSO +.BR fcntl (2), +.BR ftruncate (2), +.BR mlock (2), +.BR mmap (2), +.BR setrlimit (2) -- 2.31.1