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namespaces.7: Remove content split out into ioctl_ns(2) 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2017-01-09 07:22:28 +13:00
parent 008f58d5e8
commit e0ab72cb98
1 changed files with 3 additions and 259 deletions
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262
man7/namespaces.7
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@ -323,272 +323,16 @@ and
Use of UTS namespaces requires a kernel that is configured with the
.B CONFIG_UTS_NS
option.
.\"
.\" ============================================================
.\"
.SS Discovering namespace relationships
Since Linux 4.9,
.\" commit bcac25a58bfc6bd79191ac5d7afb49bea96da8c9
.\" commit 6786741dbf99e44fb0c0ed85a37582b8a26f1c3b
.\" commit a7306ed8d94af729ecef8b6e37506a1c6fc14788
.\" commit 6ad92bf63e45f97e306da48cd1cbce6e4fef1e5d
two
.BR ioctl (2)
operations are provided to allow discovery of namespace relationships
(see
.BR user_namespaces (7)
and
.BR pid_namespaces (7)).
The form of the calls is:
new_fd = ioctl(fd, request);
In each case,
.I fd
refers to a
.IR /proc/[pid]/ns/*
file.
Both operations return a new file descriptor on success.
.TP
.BR NS_GET_USERNS
Returns a file descriptor that refers to the owning user namespace
for the namespace referred to by
.IR fd .
.TP
.BR NS_GET_PARENT
Returns a file descriptor that refers to the parent namespace of
the namespace referred to by
.IR fd .
This operation is valid only for hierarchical namespaces
(i.e., PID and user namespaces).
For user namespaces,
.BR NS_GET_PARENT
is synonymous with
.BR NS_GET_USERNS .
.PP
The new file descriptor returned by these operations is opened with the
.BR O_RDONLY
and
.BR O_CLOEXEC
(close-on-exec; see
.BR fcntl (2))
flags.
.PP
By applying
.BR fstat (2)
to the returned file descriptor, one obtains a
.I stat
structure whose
.I st_dev
(resident device) and
.I st_ino
(inode number) fields together identify the owning/parent namespace.
This inode number can be matched with the inode number of another
.IR /proc/[pid]/ns/{pid,user}
file to determine whether that is the owning/parent namespace.
Either of these
.BR ioctl (2)
operations can fail with the following errors:
.TP
.B EPERM
The requested namespace is outside of the caller's namespace scope.
This error can occur if, for example, the owning user namespace is an
ancestor of the caller's current user namespace.
It can also occur on attempts to obtain the parent of the initial
user or PID namespace.
.TP
.B ENOTTY
The operation is not supported by this kernel version.
.PP
Additionally, the
.B NS_GET_PARENT
operation can fail with the following error:
.TP
.B EINVAL
.I fd
refers to a nonhierarchical namespace.
.PP
See the EXAMPLE section for an example of the use of these operations.
.SH CONFORMING TO
Namespaces are a Linux-specific feature.
.fi
.SH EXAMPLE
For one example,
See
.BR user_namespaces (7).
The example shown below uses the
.BR ioctl (2)
operations described above to perform simple
discovery of namespace relationships.
The following shell sessions show various examples of the use
of this program.
Trying to get the parent of the initial user namespace fails,
since it has no parent:
.nf
.in +4n
$ \fB./ns_show /proc/self/ns/user p\fP
The parent namespace is outside your namespace scope
.in
.fi
Create a process running
.BR sleep (1)
that resides in new user and UTS namespaces,
and show that the new UTS namespace is associated with the new user namespace:
.nf
.in +4n
$ \fBunshare \-Uu sleep 1000 &\fP
[1] 23235
$ \fB./ns_show /proc/23235/ns/uts u\fP
Device/Inode of owning user namespace is: [0,3] / 4026532448
$ \fBreadlink /proc/23235/ns/user \fP
user:[4026532448]
.in
.fi
Then show that the parent of the new user namespace in the preceding
example is the initial user namespace:
.nf
.in +4n
$ \fBreadlink /proc/self/ns/user\fP
user:[4026531837]
$ \fB./ns_show /proc/23235/ns/user p\fP
Device/Inode of parent namespace is: [0,3] / 4026531837
.in
.fi
Start a shell in a new user namespace, and show that from within
this shell, the parent user namespace can't be discovered.
Similarly, the UTS namespace
(which is associated with the initial user namespace)
can't be discovered.
.nf
.in +4n
$ \fBPS1="sh2$ " unshare \-U bash\fP
sh2$ \fB./ns_show /proc/self/ns/user p\fP
The parent namespace is outside your namespace scope
sh2$ \fB./ns_show /proc/self/ns/uts u\fP
The owning user namespace is outside your namespace scope
.in
.fi
.SS Program source
\&
.nf
/* ns_show.c
Licensed under the GNU General Public License v2 or later.
*/
#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/ioctl.h>
#include <errno.h>
#include <sys/sysmacros.h>
#ifndef NS_GET_USERNS
#define NSIO 0xb7
#define NS_GET_USERNS _IO(NSIO, 0x1)
#define NS_GET_PARENT _IO(NSIO, 0x2)
#endif
int
main(int argc, char *argv[])
{
int fd, userns_fd, parent_fd;
struct stat sb;
if (argc < 2) {
fprintf(stderr, "Usage: %s /proc/[pid]/ns/[file] [p|u]\\n",
argv[0]);
fprintf(stderr, "\\nDisplay the result of one or both "
"of NS_GET_USERNS (u) or NS_GET_PARENT (p)\\n"
"for the specified /proc/[pid]/ns/[file]. If neither "
"\(aqp\(aq nor \(aqu\(aq is specified,\\n"
"NS_GET_USERNS is the default.\\n");
exit(EXIT_FAILURE);
}
/* Obtain a file descriptor for the \(aqns\(aq file specified
in argv[1] */
fd = open(argv[1], O_RDONLY);
if (fd == \-1) {
perror("open");
exit(EXIT_FAILURE);
}
/* Obtain a file descriptor for the owning user namespace and
then obtain and display the inode number of that namespace */
if (argc < 3 || strchr(argv[2], \(aqu\(aq)) {
userns_fd = ioctl(fd, NS_GET_USERNS);
if (userns_fd == \-1) {
if (errno == EPERM)
printf("The owning user namespace is outside "
"your namespace scope\\n");
else
perror("ioctl\-NS_GET_USERNS");
exit(EXIT_FAILURE);
}
if (fstat(userns_fd, &sb) == \-1) {
perror("fstat\-userns");
exit(EXIT_FAILURE);
}
printf("Device/Inode of owning user namespace is: "
"[%lx,%lx] / %ld\\n",
(long) major(sb.st_dev), (long) minor(sb.st_dev),
(long) sb.st_ino);
close(userns_fd);
}
/* Obtain a file descriptor for the parent namespace and
then obtain and display the inode number of that namespace */
if (argc > 2 && strchr(argv[2], \(aqp\(aq)) {
parent_fd = ioctl(fd, NS_GET_PARENT);
if (parent_fd == \-1) {
if (errno == EINVAL)
printf("Can\(aq get parent namespace of a "
"nonhierarchical namespace\\n");
else if (errno == EPERM)
printf("The parent namespace is outside "
"your namespace scope\\n");
else
perror("ioctl\-NS_GET_PARENT");
exit(EXIT_FAILURE);
}
if (fstat(parent_fd, &sb) == \-1) {
perror("fstat\-parentns");
exit(EXIT_FAILURE);
}
printf("Device/Inode of parent namespace is: [%lx,%lx] / %ld\\n",
(long) major(sb.st_dev), (long) minor(sb.st_dev),
(long) sb.st_ino);
close(parent_fd);
}
exit(EXIT_SUCCESS);
}
.fi
.SH SEE ALSO
.BR nsenter (1),
.BR readlink (1),
.BR unshare (1),
.BR clone (2),
.BR ioctl_ns (2),
.BR setns (2),
.BR unshare (2),
.BR proc (5),