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yaml --- r: 125135 b: refs/heads/master c: 80618fa h: refs/heads/master i: 125133: a51d690 125131: 8ff74cd 125127: 327328e 125119: 437cb9f v: v3
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Linus Torvalds
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Jan 2, 2009
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| Original file line number | Diff line number | Diff line change |
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| --- | ||
| refs/heads/master: b21a207141d83a06abc5f492b80204602e02ca44 | ||
| refs/heads/master: 80618fa83a34a26199fa99cfd06476a81ddf57df |
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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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