mirror of https://github.com/mkerrisk/man-pages
memfd_create.2: Add EXAMPLE programs
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk
parent
e8a0dfae91
commit
878cc34886
|
|
@ -1,5 +1,5 @@
|
|||
.\" Copyright (C) 2014 David Herrmann <dh.herrmann@gmail.com>
|
||||
.\" and Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
|
||||
.\" Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
|
||||
.\" and Copyright (C) 2014 David Herrmann <dh.herrmann@gmail.com>
|
||||
.\"
|
||||
.\" %%%LICENSE_START(GPLv2+_SW_3_PARA)
|
||||
.\"
|
||||
|
|
@ -268,8 +268,199 @@ If desired, the second process can apply further seals
|
|||
to impose additional restrictions (so long as the
|
||||
.BR F_SEAL_SEAL
|
||||
seal has not yet been applied).
|
||||
.\"
|
||||
.\" FIXME Do we have any nice example program that could go in the man page?
|
||||
.\" FIXME I added the EXAMPLE section below. Please review.
|
||||
.SH EXAMPLE
|
||||
Below are shown two example programs that demonstrate the use of
|
||||
.BR memfd_create ()
|
||||
and the file sealing API.
|
||||
|
||||
The first program,
|
||||
.IR t_memfd_create.c ,
|
||||
creates a
|
||||
.I tmpfs
|
||||
file using
|
||||
.BR memfd_create (),
|
||||
sets a size for the file, maps it into memory,
|
||||
and optionally places some seals on the file.
|
||||
The program accepts up to three command-line arguments,
|
||||
of which the first two are required.
|
||||
The first argument is the name to associate with the file,
|
||||
the second argument is the size to be set for the file,
|
||||
and the optional third is a string of characters that specify
|
||||
seals to be set on file.
|
||||
|
||||
The second program,
|
||||
.IR t_get_seals.c ,
|
||||
can be used to open an existing file that was created via
|
||||
.BR memfd_create ()
|
||||
and inspect the set of seals that have been applied to that file.
|
||||
|
||||
The following shell session demonstrates the use of these programs.
|
||||
First we create a
|
||||
.I tmpfs
|
||||
file and set some seals on it:
|
||||
|
||||
.in +4n
|
||||
.nf
|
||||
$ \fB./t_memfd_create my_memfd_file 4096 sw &\fP
|
||||
[1] 11775
|
||||
PID: 11775; fd: 3; /proc/11775/fd/3
|
||||
.fi
|
||||
.in
|
||||
|
||||
At this point, the
|
||||
.I t_memfd_create
|
||||
program continues to run in the background.
|
||||
From another program, we can obtain a file descriptor for the
|
||||
memfd file by opening the
|
||||
.IR /proc/PID/fd
|
||||
file that corresponds to the descriptor opened by
|
||||
.BR memfd_create ().
|
||||
Using that pathname, we inspect the content of the
|
||||
.IR /proc/PID/fd
|
||||
symbolic link, and use our
|
||||
.I t_get_seals
|
||||
program to view the seals that have been placed on the file:
|
||||
|
||||
.in +4n
|
||||
.nf
|
||||
$ \fBreadlink /proc/11775/fd/3\fP
|
||||
/memfd:my_memfd_file (deleted)
|
||||
$ \fB./t_get_seals /proc/11775/fd/3\fP
|
||||
Existing seals: WRITE SHRINK
|
||||
.fi
|
||||
.in
|
||||
.SS Program source: t_memfd_create.c
|
||||
\&
|
||||
.nf
|
||||
#include <sys/memfd.h>
|
||||
#include <fcntl.h>
|
||||
#include <stdlib.h>
|
||||
#include <unistd.h>
|
||||
#include <string.h>
|
||||
#include <stdio.h>
|
||||
|
||||
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \\
|
||||
} while (0)
|
||||
|
||||
int
|
||||
main(int argc, char *argv[])
|
||||
{
|
||||
int fd;
|
||||
unsigned int seals;
|
||||
char *addr;
|
||||
char *name, *seals_arg;
|
||||
ssize_t len;
|
||||
|
||||
if (argc < 3) {
|
||||
fprintf(stderr, "%s name size [seals]\\n", argv[0]);
|
||||
fprintf(stderr, "\\t\(aqseals\(aq can contain any of the "
|
||||
"following characters:\\n");
|
||||
fprintf(stderr, "\\t\\tg \- F_SEAL_GROW\\n");
|
||||
fprintf(stderr, "\\t\\ts \- F_SEAL_SHRINK\\n");
|
||||
fprintf(stderr, "\\t\\tw \- F_SEAL_WRITE\\n");
|
||||
fprintf(stderr, "\\t\\tS \- F_SEAL_SEAL\\n");
|
||||
exit(EXIT_FAILURE);
|
||||
}
|
||||
|
||||
name = argv[1];
|
||||
len = atoi(argv[2]);
|
||||
seals_arg = argv[3];
|
||||
|
||||
/* Create an anonymous file in tmpfs; allow seals to be
|
||||
placed on the file */
|
||||
|
||||
fd = memfd_create(name, MFD_ALLOW_SEALING);
|
||||
if (fd == \-1)
|
||||
errExit("memfd_create");
|
||||
|
||||
/* Size the file as specified on the command line */
|
||||
|
||||
if (ftruncate(fd, len) == \-1)
|
||||
errExit("truncate");
|
||||
|
||||
printf("PID: %ld; fd: %d; /proc/%ld/fd/%d\\n",
|
||||
(long) getpid(), fd, (long) getpid(), fd);
|
||||
|
||||
/* Code to map the file and populate the mapping with data
|
||||
omitted */
|
||||
|
||||
/* If a \(aqseals\(aq command\-line argument was supplied, set some
|
||||
seals on the file */
|
||||
|
||||
if (seals_arg != NULL) {
|
||||
seals = 0;
|
||||
|
||||
if (strchr(seals_arg, \(aqg\(aq) != NULL)
|
||||
seals |= F_SEAL_GROW;
|
||||
if (strchr(seals_arg, \(aqs\(aq) != NULL)
|
||||
seals |= F_SEAL_SHRINK;
|
||||
if (strchr(seals_arg, \(aqw\(aq) != NULL)
|
||||
seals |= F_SEAL_WRITE;
|
||||
if (strchr(seals_arg, \(aqS\(aq) != NULL)
|
||||
seals |= F_SEAL_SEAL;
|
||||
|
||||
if (fcntl(fd, F_ADD_SEALS, seals) == \-1)
|
||||
errExit("fcntl");
|
||||
}
|
||||
|
||||
/* Keep running, so that the file created by memfd_create()
|
||||
continues to exist */
|
||||
|
||||
pause();
|
||||
|
||||
exit(EXIT_SUCCESS);
|
||||
}
|
||||
.fi
|
||||
.SS Program source: t_get_seals.c
|
||||
\&
|
||||
.nf
|
||||
#include <sys/memfd.h>
|
||||
#include <fcntl.h>
|
||||
#include <unistd.h>
|
||||
#include <stdlib.h>
|
||||
#include <string.h>
|
||||
#include <stdio.h>
|
||||
|
||||
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \\
|
||||
} while (0)
|
||||
|
||||
int
|
||||
main(int argc, char *argv[])
|
||||
{
|
||||
int fd;
|
||||
unsigned int seals;
|
||||
|
||||
if (argc != 2) {
|
||||
fprintf(stderr, "%s /proc/PID/fd/FD\\n", argv[0]);
|
||||
exit(EXIT_FAILURE);
|
||||
}
|
||||
|
||||
fd = open(argv[1], O_RDWR);
|
||||
if (fd == \-1)
|
||||
errExit("open");
|
||||
|
||||
seals = fcntl(fd, F_GET_SEALS);
|
||||
if (seals == \-1)
|
||||
errExit("fcntl");
|
||||
|
||||
printf("Existing seals:");
|
||||
if (seals & F_SEAL_SEAL)
|
||||
printf(" SEAL");
|
||||
if (seals & F_SEAL_GROW)
|
||||
printf(" GROW");
|
||||
if (seals & F_SEAL_WRITE)
|
||||
printf(" WRITE");
|
||||
if (seals & F_SEAL_SHRINK)
|
||||
printf(" SHRINK");
|
||||
printf("\\n");
|
||||
|
||||
/* Code to map the file and access the contents of the
|
||||
resulting mapping omitted */
|
||||
|
||||
exit(EXIT_SUCCESS);
|
||||
}
|
||||
.fi
|
||||
.SH SEE ALSO
|
||||
.BR fcntl (2),
|
||||
.BR ftruncate (2),
|
||||
|
|
|
|||
Loading…
Reference in New Issue