[PATCH v2 1/2] mount.2: document MS_REC and shared subtree flags

[PATCH v2 1/2] mount.2: document MS_REC and shared subtree flags

[Omar Sandoval]

From: Omar Sandoval <osandov@fb.com>

This stuff is all documented in the kernel source tree under
Documentation/filesystems/sharedsubtree.txt, so let's document it
properly for users.

Signed-off-by: Omar Sandoval <osandov@fb.com>
---
 man2/mount.2 | 133 +++++++++++++++++++++++++++++++++++++++--------------------
 1 file changed, 88 insertions(+), 45 deletions(-)

diff --git a/man2/mount.2 b/man2/mount.2
index aa6558a87cd9..419e22da800f 100644
--- a/man2/mount.2
+++ b/man2/mount.2
@@ -83,44 +83,6 @@ and various mount flags
 .\" (as defined in \fI<linux/fs.h>\fP for libc4 and libc5
 .\" and in \fI<sys/mount.h>\fP for glibc2)
 in the low order 16 bits:
-.\" FIXME 2.6.15 added flags for "shared subtree" functionality:
-.\"
-.\"	MS_PRIVATE
-.\"		All mounts are private by default. Previously shared mounts
-.\"		can be re-marked PRIVATE.
-.\"	MS_SHARED
-.\"		Mount points that are marked SHARED propagate mount events
-.\"		to one another after being cloned.
-.\"			mount --make-rshared ==> MS_SHARED | MS_REC
-.\"	MS_SLAVE
-.\"		A previously shared mount point can be marked SLAVE, meaning
-.\"		it receives propagated events, but does not propagate events.
-.\"	MS_UNBINDABLE
-.\"		mounts cannot be bound into other places, and will not be
-.\"		propagated into new subtrees	
-.\"
-.\" These settings are visible in /proc/$$/mountinfo
-.\"
-.\" These need to be documented on this page.
-.\" See:
-.\"
-.\" * Documentation/filesystems/sharedsubtree.txt
-.\"
-.\" * http://lwn.net/Articles/159077/
-.\"
-.\" * https://www.kernel.org/doc/ols/2006/ols2006v2-pages-209-222.pdf
-.\"       Shared-Subtree Concept, Implementation, and Applications in Linux
-.\"       Al Viro viro@ftp.linux.org.uk
-.\"       Ram Pai linuxram@us.ibm.com
-.\"
-.\" * http://www.ibm.com/developerworks/linux/library/l-mount-namespaces/index.html
-.\"       Applying mount namespaces
-.\"
-.\"       Uncover practical applications for advanced Linux mounts features
-.\"       Serge E. Hallyn (sergeh@us.ibm.com), Software Engineer, IBM
-.\"       Ram Pai (linuxram@us.ibm.com), Software Engineer, IBM
-.\"       Date:  17 Sep 2007
-.\"
 .\" FIXME 2.6.25 Added MS_I_VERSION, which needs to be documented.
 .\"
 .TP
@@ -240,12 +202,34 @@ programs from this filesystem.
 .\" (This is a security feature to prevent users executing set-user-ID and
 .\" set-group-ID programs from removable disk devices.)
 .TP
+.BR MS_PRIVATE " (since Linux 2.6.15)"
+Set the mount propagation on this mountpoint to private. Mounts will not
+propagate to or from replicated subtrees.
+This flag can only be combined with
+.B MS_REC
+and
+.BR MS_SILENT .
+The
+.IR source ,
+.IR filesystemtype ,
+and
+.IR data
+arguments are ignored.
+.TP
 .B MS_RDONLY
 Mount filesystem read-only.
-.\"
-.\" FIXME Document MS_REC, available since 2.4.11.
-.\" This flag has meaning in conjunction with MS_BIND and
-.\" also with the shared subtree flags.
+.TP
+.BR MS_REC " (since Linux 2.4.11)"
+When passed together with
+.BR MS_BIND ,
+recursively bind mount all mountpoints underneath the source subtree.
+When passed together with a shared subtree flag (one of
+.BR MS_PRIVATE ,
+.BR MS_SHARED ,
+.BR MS_SLAVE ", or"
+.BR MS_UNBINDABLE ),
+recursively apply the shared subtree setting to all mountpoints
+underneath the target subtree.
 .TP
 .BR MS_RELATIME " (since Linux 2.6.20)"
 When a file on this filesystem is accessed,
@@ -306,6 +290,21 @@ and, additionally, before kernel 2.4.10, the following could also be changed:
 .BR MS_NODEV ,
 .BR MS_NOEXEC .
 .TP
+.BR MS_SHARED " (since Linux 2.6.15)"
+Set the mount propagation on this mountpoint to shared.
+Mounts will be propagated to and from replicated subtrees
+(i.e., all replicas will continue to be the same).
+This flag can only be combined with
+.B MS_REC
+and
+.BR MS_SILENT .
+The
+.IR source ,
+.IR filesystemtype ,
+and
+.IR data
+arguments are ignored.
+.TP
 .BR MS_SILENT " (since Linux 2.6.17)"
 Suppress the display of certain
 .RI ( printk ())
@@ -314,6 +313,28 @@ This flag supersedes the misnamed and obsolete
 .BR MS_VERBOSE
 flag (available since Linux 2.4.12), which has the same meaning.
 .TP
+.BR MS_SLAVE " (since Linux 2.6.15)"
+Make this mountpoint a slave of its source mountpoint, referred to as
+its master.
+Mounts made underneath the master subtree will be propagated to this
+subtree, but mounts made underneath this subtree will not be propagated
+back to the master.
+Note that a slave mountpoint can still have either private propagation
+(see
+.BR MS_PRIVATE )
+or shared propagation (see
+.BR MS_SHARED ).
+This flag can only be combined with
+.B MS_REC
+and
+.BR MS_SILENT .
+The
+.IR source ,
+.IR filesystemtype ,
+and
+.IR data
+arguments are ignored.
+.TP
 .BR MS_STRICTATIME " (since Linux 2.6.30)"
 Always update the last access time (atime) when files on this
 filesystem are accessed.
@@ -331,6 +352,21 @@ the
 flag to
 .BR open (2)
 was specified for all file opens to this filesystem).
+.TP
+.BR MS_UNBINDABLE " (since Linux 2.6.15)"
+Set the mount propagation on this mountpoint to private (see
+.BR MS_PRIVATE )
+and disallow bind mounts with this mountpoint as the source.
+This flag can only be combined with
+.B MS_REC
+and
+.BR MS_SILENT .
+The
+.IR source ,
+.IR filesystemtype ,
+and
+.IR data
+arguments are ignored.
 .PP
 From Linux 2.4 onward, the
 .BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
@@ -408,6 +444,11 @@ Or, a move
 was attempted, but
 .I source
 was not a mount point, or was \(aq/\(aq.
+Or, more than one shared subtree flag was given.
+Or, a shared subtree flag was combined with a flag other than
+.B MS_REC
+or
+.BR MS_SILENT .
 .TP
 .B ELOOP
 Too many links encountered during pathname resolution.
@@ -457,11 +498,13 @@ The definitions of
 .BR MS_MOVE ,
 .BR MS_REC ,
 .BR MS_RELATIME ,
+.BR MS_STRICTATIME ,
+.BR MS_PRIVATE ,
+.BR MS_SHARED ,
+.BR MS_SLAVE ,
 and
-.BR MS_STRICTATIME
+.BR MS_UNBINDABLE
 were added to glibc headers in version 2.12.
-.\" FIXME . Definitions of the so-far-undocumented MS_UNBINDABLE, MS_PRIVATE,
-.\" MS_SHARED, and MS_SLAVE were (also) only added to glibc headers in 2.12.
 .SH CONFORMING TO
 This function is Linux-specific and should not be used in
 programs intended to be portable.
-- 
2.8.2

[PATCH v2 2/2] mount.2: MS_BIND still ignores mountflags

[Omar Sandoval]

From: Omar Sandoval <osandov@fb.com>

This is clear from the do_mount() function in the kernel as of v4.6.

Signed-off-by: Omar Sandoval <osandov@fb.com>
---
 man2/mount.2 | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/man2/mount.2 b/man2/mount.2
index 419e22da800f..bd4c35bd15ff 100644
--- a/man2/mount.2
+++ b/man2/mount.2
@@ -98,10 +98,10 @@ The
 and
 .IR data
 arguments are ignored.
-Up until Linux 2.6.26,
+The
 .I mountflags
-was also ignored
-.\" with the exception of the "hidden" MS_REC mountflags bit
+argument is also ignored, with the exception of
+.B MS_REC
 (the bind mount has the same mount options as
 the underlying mount point).
 .TP
-- 
2.8.2

Re: [PATCH v2 1/2] mount.2: document MS_REC and shared subtree flags

[Michael Kerrisk (man-pages)]

Hello Omar,

Thanks for tackling this. Some comments below.

On 05/18/2016 08:57 PM, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> This stuff is all documented in the kernel source tree under
> Documentation/filesystems/sharedsubtree.txt, so let's document it
> properly for users.
> 
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
>  man2/mount.2 | 133 +++++++++++++++++++++++++++++++++++++++--------------------
>  1 file changed, 88 insertions(+), 45 deletions(-)
> 
> diff --git a/man2/mount.2 b/man2/mount.2
> index aa6558a87cd9..419e22da800f 100644
> --- a/man2/mount.2
> +++ b/man2/mount.2
> @@ -83,44 +83,6 @@ and various mount flags
>  .\" (as defined in \fI<linux/fs.h>\fP for libc4 and libc5
>  .\" and in \fI<sys/mount.h>\fP for glibc2)
>  in the low order 16 bits:
> -.\" FIXME 2.6.15 added flags for "shared subtree" functionality:
> -.\"
> -.\"	MS_PRIVATE
> -.\"		All mounts are private by default. Previously shared mounts
> -.\"		can be re-marked PRIVATE.
> -.\"	MS_SHARED
> -.\"		Mount points that are marked SHARED propagate mount events
> -.\"		to one another after being cloned.
> -.\"			mount --make-rshared ==> MS_SHARED | MS_REC
> -.\"	MS_SLAVE
> -.\"		A previously shared mount point can be marked SLAVE, meaning
> -.\"		it receives propagated events, but does not propagate events.
> -.\"	MS_UNBINDABLE
> -.\"		mounts cannot be bound into other places, and will not be
> -.\"		propagated into new subtrees	
> -.\"
> -.\" These settings are visible in /proc/$$/mountinfo
> -.\"
> -.\" These need to be documented on this page.
> -.\" See:
> -.\"
> -.\" * Documentation/filesystems/sharedsubtree.txt
> -.\"
> -.\" * http://lwn.net/Articles/159077/
> -.\"
> -.\" * https://www.kernel.org/doc/ols/2006/ols2006v2-pages-209-222.pdf
> -.\"       Shared-Subtree Concept, Implementation, and Applications in Linux
> -.\"       Al Viro viro@ftp.linux.org.uk
> -.\"       Ram Pai linuxram@us.ibm.com
> -.\"
> -.\" * http://www.ibm.com/developerworks/linux/library/l-mount-namespaces/index.html
> -.\"       Applying mount namespaces
> -.\"
> -.\"       Uncover practical applications for advanced Linux mounts features
> -.\"       Serge E. Hallyn (sergeh@us.ibm.com), Software Engineer, IBM
> -.\"       Ram Pai (linuxram@us.ibm.com), Software Engineer, IBM
> -.\"       Date:  17 Sep 2007
> -.\"
>  .\" FIXME 2.6.25 Added MS_I_VERSION, which needs to be documented.
>  .\"
>  .TP
> @@ -240,12 +202,34 @@ programs from this filesystem.
>  .\" (This is a security feature to prevent users executing set-user-ID and
>  .\" set-group-ID programs from removable disk devices.)
>  .TP
> +.BR MS_PRIVATE " (since Linux 2.6.15)"
> +Set the mount propagation on this mountpoint to private. Mounts will not

s/mountpoint/mount point/ (all occurrences)

Change "this mountpoint" to

the
.I target
mount point

(and other instances below).

> +propagate to or from replicated subtrees.

What is a "replicated subtree"? This isn't explained in the text (and, doesn't
feel quite right as a term, but perhaps that is just because you do not explain
what you mean).

> +This flag can only be combined with

s/can only be combined with/ can be combined with only/
(all occurrences)

> +.B MS_REC
> +and
> +.BR MS_SILENT .
> +The
> +.IR source ,
> +.IR filesystemtype ,
> +and
> +.IR data
> +arguments are ignored.
> +.TP
>  .B MS_RDONLY
>  Mount filesystem read-only.
> -.\"
> -.\" FIXME Document MS_REC, available since 2.4.11.
> -.\" This flag has meaning in conjunction with MS_BIND and
> -.\" also with the shared subtree flags.
> +.TP
> +.BR MS_REC " (since Linux 2.4.11)"
> +When passed together with
> +.BR MS_BIND ,
> +recursively bind mount all mountpoints underneath the source subtree.

Change "source subtree" to

.I source
subtree.

> +When passed together with a shared subtree flag (one of
> +.BR MS_PRIVATE ,
> +.BR MS_SHARED ,
> +.BR MS_SLAVE ", or"
> +.BR MS_UNBINDABLE ),
> +recursively apply the shared subtree setting to all mountpoints
> +underneath the target subtree.

Change "target subtree" to

.I target
subtree.


>  .TP
>  .BR MS_RELATIME " (since Linux 2.6.20)"
>  When a file on this filesystem is accessed,
> @@ -306,6 +290,21 @@ and, additionally, before kernel 2.4.10, the following could also be changed:
>  .BR MS_NODEV ,
>  .BR MS_NOEXEC .
>  .TP
> +.BR MS_SHARED " (since Linux 2.6.15)"
> +Set the mount propagation on this mountpoint to shared.
> +Mounts will be propagated to and from replicated subtrees

s/Mount/Mount and unmount events/

> +(i.e., all replicas will continue to be the same).
> +This flag can only be combined with
> +.B MS_REC
> +and
> +.BR MS_SILENT .
> +The
> +.IR source ,
> +.IR filesystemtype ,
> +and
> +.IR data
> +arguments are ignored.
> +.TP
>  .BR MS_SILENT " (since Linux 2.6.17)"
>  Suppress the display of certain
>  .RI ( printk ())
> @@ -314,6 +313,28 @@ This flag supersedes the misnamed and obsolete
>  .BR MS_VERBOSE
>  flag (available since Linux 2.4.12), which has the same meaning.
>  .TP
> +.BR MS_SLAVE " (since Linux 2.6.15)"
> +Make this mountpoint a slave of its source mountpoint, referred to as
> +its master.
> +Mounts made underneath the master subtree will be propagated to this

s/Mounts made/Mount and unmount events/

> +subtree, but mounts made underneath this subtree will not be propagated

s/mounts made/events/

> +back to the master.
> +Note that a slave mountpoint can still have either private propagation
> +(see
> +.BR MS_PRIVATE )
> +or shared propagation (see
> +.BR MS_SHARED ).
> +This flag can only be combined with
> +.B MS_REC
> +and
> +.BR MS_SILENT .
> +The
> +.IR source ,
> +.IR filesystemtype ,
> +and
> +.IR data
> +arguments are ignored.
> +.TP
>  .BR MS_STRICTATIME " (since Linux 2.6.30)"
>  Always update the last access time (atime) when files on this
>  filesystem are accessed.
> @@ -331,6 +352,21 @@ the
>  flag to
>  .BR open (2)
>  was specified for all file opens to this filesystem).
> +.TP
> +.BR MS_UNBINDABLE " (since Linux 2.6.15)"
> +Set the mount propagation on this mountpoint to private (see
> +.BR MS_PRIVATE )
> +and disallow bind mounts with this mountpoint as the source.
> +This flag can only be combined with
> +.B MS_REC
> +and
> +.BR MS_SILENT .
> +The
> +.IR source ,
> +.IR filesystemtype ,
> +and
> +.IR data
> +arguments are ignored.
>  .PP
>  From Linux 2.4 onward, the
>  .BR MS_NODEV ", " MS_NOEXEC ", and " MS_NOSUID
> @@ -408,6 +444,11 @@ Or, a move
>  was attempted, but
>  .I source
>  was not a mount point, or was \(aq/\(aq.
> +Or, more than one shared subtree flag was given.
> +Or, a shared subtree flag was combined with a flag other than
> +.B MS_REC
> +or
> +.BR MS_SILENT .
>  .TP
>  .B ELOOP
>  Too many links encountered during pathname resolution.
> @@ -457,11 +498,13 @@ The definitions of
>  .BR MS_MOVE ,
>  .BR MS_REC ,
>  .BR MS_RELATIME ,
> +.BR MS_STRICTATIME ,
> +.BR MS_PRIVATE ,
> +.BR MS_SHARED ,
> +.BR MS_SLAVE ,
>  and
> -.BR MS_STRICTATIME
> +.BR MS_UNBINDABLE
>  were added to glibc headers in version 2.12.
> -.\" FIXME . Definitions of the so-far-undocumented MS_UNBINDABLE, MS_PRIVATE,
> -.\" MS_SHARED, and MS_SLAVE were (also) only added to glibc headers in 2.12.
>  .SH CONFORMING TO
>  This function is Linux-specific and should not be used in
>  programs intended to be portable.

Cheers,

Michael


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

Re: [PATCH v2 2/2] mount.2: MS_BIND still ignores mountflags

[Michael Kerrisk (man-pages)]

On 05/18/2016 01:57 PM, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> This is clear from the do_mount() function in the kernel as of v4.6.
> 

Hello Omar

I've applied this patch, and await your input on the other patch.

Cheers,

Michael


> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
>  man2/mount.2 | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/man2/mount.2 b/man2/mount.2
> index 419e22da800f..bd4c35bd15ff 100644
> --- a/man2/mount.2
> +++ b/man2/mount.2
> @@ -98,10 +98,10 @@ The
>  and
>  .IR data
>  arguments are ignored.
> -Up until Linux 2.6.26,
> +The
>  .I mountflags
> -was also ignored
> -.\" with the exception of the "hidden" MS_REC mountflags bit
> +argument is also ignored, with the exception of
> +.B MS_REC
>  (the bind mount has the same mount options as
>  the underlying mount point).
>  .TP
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/

Re: [PATCH v2 1/2] mount.2: document MS_REC and shared subtree flags

[Omar Sandoval]

On Thu, May 19, 2016 at 01:22:26PM +0200, Michael Kerrisk (man-pages) wrote:
> Hello Omar,
> 
> Thanks for tackling this. Some comments below.
> 

Hi, Michael, I see you've revamped the mount(2) man page and documented
these flags. Thanks, it looks great!

-- 
Omar

Re: [PATCH v2 1/2] mount.2: document MS_REC and shared subtree flags

[Michael Kerrisk (man-pages)]

On 06/07/2016 08:15 PM, Omar Sandoval wrote:
> On Thu, May 19, 2016 at 01:22:26PM +0200, Michael Kerrisk (man-pages) wrote:
>> Hello Omar,
>>
>> Thanks for tackling this. Some comments below.
>>
> 
> Hi, Michael, I see you've revamped the mount(2) man page and documented
> these flags. Thanks, it looks great!

You're welcome. That page has long needed some attention, so I did 
some work. Thanks for your input, which in part motivated that work.

Cheers,

Michael



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/