[PATCH v3] man2: new page describing memfd_secret() system call

Mike Rapoport
2021-08-15T06:46:56Z

From: Mike Rapoport <rppt@linux.ibm.com>

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
---
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 <rppt@linux.ibm.com>
+.\"
+.\" Based on memfd_create(2) man page
+.\" Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
+.\" and Copyright (C) 2014 David Herrmann <dh.herrmann@gmail.com>
+.\"
+.\" %%%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
+.\" <http://www.gnu.org/licenses/>.
+.\" %%%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 <sys/syscall.h>" " /* Definition of " SYS_* " constants */"
+.B #include <unistd.h>
+.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