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Linus Torvalds
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Jan 3, 2009
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--- | ||
refs/heads/master: b15dd79ea06b04a7ecee95f62ce7b6a3547dbb0a | ||
refs/heads/master: f60a0a79846abed04ad5abddb5dafd14b66e1ab0 |
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To support containers, we now allow multiple instances of devpts filesystem, | ||
such that indices of ptys allocated in one instance are independent of indices | ||
allocated in other instances of devpts. | ||
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To preserve backward compatibility, this support for multiple instances is | ||
enabled only if: | ||
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- CONFIG_DEVPTS_MULTIPLE_INSTANCES=y, and | ||
- '-o newinstance' mount option is specified while mounting devpts | ||
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IOW, devpts now supports both single-instance and multi-instance semantics. | ||
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If CONFIG_DEVPTS_MULTIPLE_INSTANCES=n, there is no change in behavior and | ||
this referred to as the "legacy" mode. In this mode, the new mount options | ||
(-o newinstance and -o ptmxmode) will be ignored with a 'bogus option' message | ||
on console. | ||
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If CONFIG_DEVPTS_MULTIPLE_INSTANCES=y and devpts is mounted without the | ||
'newinstance' option (as in current start-up scripts) the new mount binds | ||
to the initial kernel mount of devpts. This mode is referred to as the | ||
'single-instance' mode and the current, single-instance semantics are | ||
preserved, i.e PTYs are common across the system. | ||
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The only difference between this single-instance mode and the legacy mode | ||
is the presence of new, '/dev/pts/ptmx' node with permissions 0000, which | ||
can safely be ignored. | ||
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If CONFIG_DEVPTS_MULTIPLE_INSTANCES=y and 'newinstance' option is specified, | ||
the mount is considered to be in the multi-instance mode and a new instance | ||
of the devpts fs is created. Any ptys created in this instance are independent | ||
of ptys in other instances of devpts. Like in the single-instance mode, the | ||
/dev/pts/ptmx node is present. To effectively use the multi-instance mode, | ||
open of /dev/ptmx must be a redirected to '/dev/pts/ptmx' using a symlink or | ||
bind-mount. | ||
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Eg: A container startup script could do the following: | ||
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$ chmod 0666 /dev/pts/ptmx | ||
$ rm /dev/ptmx | ||
$ ln -s pts/ptmx /dev/ptmx | ||
$ ns_exec -cm /bin/bash | ||
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# We are now in new container | ||
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$ umount /dev/pts | ||
$ mount -t devpts -o newinstance lxcpts /dev/pts | ||
$ sshd -p 1234 | ||
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where 'ns_exec -cm /bin/bash' calls clone() with CLONE_NEWNS flag and execs | ||
/bin/bash in the child process. A pty created by the sshd is not visible in | ||
the original mount of /dev/pts. | ||
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User-space changes | ||
------------------ | ||
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In multi-instance mode (i.e '-o newinstance' mount option is specified at least | ||
once), following user-space issues should be noted. | ||
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1. If -o newinstance mount option is never used, /dev/pts/ptmx can be ignored | ||
and no change is needed to system-startup scripts. | ||
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2. To effectively use multi-instance mode (i.e -o newinstance is specified) | ||
administrators or startup scripts should "redirect" open of /dev/ptmx to | ||
/dev/pts/ptmx using either a bind mount or symlink. | ||
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$ mount -t devpts -o newinstance devpts /dev/pts | ||
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followed by either | ||
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$ rm /dev/ptmx | ||
$ ln -s pts/ptmx /dev/ptmx | ||
$ chmod 666 /dev/pts/ptmx | ||
or | ||
$ mount -o bind /dev/pts/ptmx /dev/ptmx | ||
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3. The '/dev/ptmx -> pts/ptmx' symlink is the preferred method since it | ||
enables better error-reporting and treats both single-instance and | ||
multi-instance mounts similarly. | ||
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But this method requires that system-startup scripts set the mode of | ||
/dev/pts/ptmx correctly (default mode is 0000). The scripts can set the | ||
mode by, either | ||
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- adding ptmxmode mount option to devpts entry in /etc/fstab, or | ||
- using 'chmod 0666 /dev/pts/ptmx' | ||
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4. If multi-instance mode mount is needed for containers, but the system | ||
startup scripts have not yet been updated, container-startup scripts | ||
should bind mount /dev/ptmx to /dev/pts/ptmx to avoid breaking single- | ||
instance mounts. | ||
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Or, in general, container-startup scripts should use: | ||
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mount -t devpts -o newinstance -o ptmxmode=0666 devpts /dev/pts | ||
if [ ! -L /dev/ptmx ]; then | ||
mount -o bind /dev/pts/ptmx /dev/ptmx | ||
fi | ||
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When all devpts mounts are multi-instance, /dev/ptmx can permanently be | ||
a symlink to pts/ptmx and the bind mount can be ignored. | ||
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5. A multi-instance mount that is not accompanied by the /dev/ptmx to | ||
/dev/pts/ptmx redirection would result in an unusable/unreachable pty. | ||
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mount -t devpts -o newinstance lxcpts /dev/pts | ||
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immediately followed by: | ||
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open("/dev/ptmx") | ||
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would create a pty, say /dev/pts/7, in the initial kernel mount. | ||
But /dev/pts/7 would be invisible in the new mount. | ||
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6. The permissions for /dev/pts/ptmx node should be specified when mounting | ||
/dev/pts, using the '-o ptmxmode=%o' mount option (default is 0000). | ||
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mount -t devpts -o newinstance -o ptmxmode=0644 devpts /dev/pts | ||
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The permissions can be later be changed as usual with 'chmod'. | ||
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chmod 666 /dev/pts/ptmx | ||
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7. A mount of devpts without the 'newinstance' option results in binding to | ||
initial kernel mount. This behavior while preserving legacy semantics, | ||
does not provide strict isolation in a container environment. i.e by | ||
mounting devpts without the 'newinstance' option, a container could | ||
get visibility into the 'host' or root container's devpts. | ||
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To workaround this and have strict isolation, all mounts of devpts, | ||
including the mount in the root container, should use the newinstance | ||
option. |
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Original file line number | Diff line number | Diff line change |
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Environment variables | ||
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KCPPFLAGS | ||
-------------------------------------------------- | ||
Additional options to pass when preprocessing. The preprocessing options | ||
will be used in all cases where kbuild do preprocessing including | ||
building C files and assembler files. | ||
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KAFLAGS | ||
-------------------------------------------------- | ||
Additional options to the assembler. | ||
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KCFLAGS | ||
-------------------------------------------------- | ||
Additional options to the C compiler. | ||
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KBUILD_VERBOSE | ||
-------------------------------------------------- | ||
Set the kbuild verbosity. Can be assinged same values as "V=...". | ||
See make help for the full list. | ||
Setting "V=..." takes precedence over KBUILD_VERBOSE. | ||
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KBUILD_EXTMOD | ||
-------------------------------------------------- | ||
Set the directory to look for the kernel source when building external | ||
modules. | ||
The directory can be specified in several ways: | ||
1) Use "M=..." on the command line | ||
2) Environmnet variable KBUILD_EXTMOD | ||
3) Environmnet variable SUBDIRS | ||
The possibilities are listed in the order they take precedence. | ||
Using "M=..." will always override the others. | ||
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KBUILD_OUTPUT | ||
-------------------------------------------------- | ||
Specify the output directory when building the kernel. | ||
The output directory can also be specificed using "O=...". | ||
Setting "O=..." takes precedence over KBUILD_OUTPUT | ||
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ARCH | ||
-------------------------------------------------- | ||
Set ARCH to the architecture to be built. | ||
In most cases the name of the architecture is the same as the | ||
directory name found in the arch/ directory. | ||
But some architectures suach as x86 and sparc has aliases. | ||
x86: i386 for 32 bit, x86_64 for 64 bit | ||
sparc: sparc for 32 bit, sparc64 for 64 bit | ||
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CROSS_COMPILE | ||
-------------------------------------------------- | ||
Specify an optional fixed part of the binutils filename. | ||
CROSS_COMPILE can be a part of the filename or the full path. | ||
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CROSS_COMPILE is also used for ccache is some setups. | ||
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CF | ||
-------------------------------------------------- | ||
Additional options for sparse. | ||
CF is often used on the command-line like this: | ||
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make CF=-Wbitwise C=2 | ||
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INSTALL_PATH | ||
-------------------------------------------------- | ||
INSTALL_PATH specifies where to place the updated kernel and system map | ||
images. Default is /boot, but you can set it to other values | ||
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MODLIB | ||
-------------------------------------------------- | ||
Specify where to install modules. | ||
The default value is: | ||
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$(INSTALL_MOD_PATH)/lib/modules/$(KERNELRELEASE) | ||
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The value can be overridden in which case the default value is ignored. | ||
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INSTALL_MOD_PATH | ||
-------------------------------------------------- | ||
INSTALL_MOD_PATH specifies a prefix to MODLIB for module directory | ||
relocations required by build roots. This is not defined in the | ||
makefile but the argument can be passed to make if needed. | ||
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INSTALL_MOD_STRIP | ||
-------------------------------------------------- | ||
INSTALL_MOD_STRIP, if defined, will cause modules to be | ||
stripped after they are installed. If INSTALL_MOD_STRIP is '1', then | ||
the default option --strip-debug will be used. Otherwise, | ||
INSTALL_MOD_STRIP will used as the options to the strip command. | ||
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INSTALL_FW_PATH | ||
-------------------------------------------------- | ||
INSTALL_FW_PATH specify where to install the firmware blobs. | ||
The default value is: | ||
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$(INSTALL_MOD_PATH)/lib/firmware | ||
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The value can be overridden in which case the default value is ignored. | ||
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INSTALL_HDR_PATH | ||
-------------------------------------------------- | ||
INSTALL_HDR_PATH specify where to install user space headers when | ||
executing "make headers_*". | ||
The default value is: | ||
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$(objtree)/usr | ||
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$(objtree) is the directory where output files are saved. | ||
The output directory is often set using "O=..." on the commandline. | ||
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The value can be overridden in which case the default value is ignored. | ||
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KBUILD_MODPOST_WARN | ||
-------------------------------------------------- | ||
KBUILD_MODPOST_WARN can be set to avoid error out in case of undefined | ||
symbols in the final module linking stage. | ||
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KBUILD_MODPOST_FINAL | ||
-------------------------------------------------- | ||
KBUILD_MODPOST_NOFINAL can be set to skip the final link of modules. | ||
This is solely usefull to speed up test compiles. | ||
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KBUILD_EXTRA_SYMBOLS | ||
-------------------------------------------------- | ||
For modules use symbols from another modules. | ||
See more details in modules.txt. |
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