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From: Mike Rapoport <>
To: Michael Kerrisk <>,
	Alejandro Colomar <>
Cc: Mike Rapoport <>,
	Mike Rapoport <>,,,
Subject: [PATCH v2] man2: new page describing memfd_secret() system call
Date: Thu, 29 Jul 2021 11:29:00 +0300	[thread overview]
Message-ID: <> (raw)

From: Mike Rapoport <>

Signed-off-by: Mike Rapoport <>

v2: address Alex's comments:
* update synopsis to match new style for syscalls without a wrapper
* drop note about absence of glibc wrapper
* update formatting


 man2/memfd_secret.2 | 147 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 147 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..5a70cb5d2
--- /dev/null
+++ b/man2/memfd_secret.2
@@ -0,0 +1,147 @@
+.\" 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 <>
+.\" 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
+.\" 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
+.\" <>.
+.TH MEMFD_SECRET 2 2020-08-02 Linux "Linux Programmer's Manual"
+memfd_secret \- create an anonymous file to access secret memory regions
+.BR "#include <sys/syscall.h>" "      /* Definition of " SYS_* " constants */"
+.B #include <unistd.h>
+.BI "int syscall(SYS_memfd_secret, int " cmd ", unsigned int " flags \
+", int " cpu_id );
+.IR Note :
+glibc provides no wrapper for
+.BR memfd_secret (),
+necessitating the use of
+.BR syscall (2).
+.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 references to the file are dropped, 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).
+The memory areas backing the file created with
+.BR memfd_create(2)
+are visible only to the contexts that have access to the file descriptor.
+These areas are removed from the kernel page tables
+and only the page tables of the processes holding the file descriptor
+map the corresponding physical memory.
+The following values may be bitwise ORed in
+.I flags
+to control the behavior of
+.BR memfd_secret (2):
+Set the close-on-exec flag on the new file descriptor.
+See the description of the
+flag in
+.BR open (2)
+for reasons why this may be useful.
+As its return value,
+.BR memfd_secret ()
+returns a new file descriptor that can be used to refer to an anonymous file.
+This file descriptor is opened for both reading and writing
+.RB ( O_RDWR )
+is set for the file descriptor.
+With respect to
+.BR fork (2)
+.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.
+The memory regions backed with
+.BR memfd_secret ()
+are locked in the same way as
+.BR mlock (2),
+however the implementation will not try to
+populate the whole range during the
+.BR mmap (2)
+The amount of memory allowed for memory mappings
+of the file descriptor obeys the same rules as
+.BR mlock (2)
+and cannot exceed
+On success,
+.BR memfd_secret ()
+returns a new file descriptor.
+On error, \-1 is returned and
+.I errno
+is set to indicate the error.
+.I flags
+included unknown bits.
+The per-process limit on the number of open file descriptors has been reached.
+The system-wide limit on the total number of open files has been reached.
+There was insufficient memory to create a new anonymous file.
+.BR memfd_secret ()
+is not implemented on this architecture.
+.BR memfd_secret (2)
+system call first appeared in Linux 5.14.
+.BR memfd_secret (2)
+system call is Linux-specific.
+.BR fcntl (2),
+.BR ftruncate (2),
+.BR mlock (2),
+.BR mmap (2),
+.BR setrlimit (2)

             reply	other threads:[~2021-07-29  8:29 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-29  8:29 Mike Rapoport [this message]
2021-07-29 10:12 ` Alejandro Colomar (man-pages)
2021-07-29 10:30   ` Mike Rapoport
2021-07-29 11:07     ` Alejandro Colomar (man-pages)

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