diff mbox series

[v6,2/2] listmount.2: New page describing the listmount syscall

Message ID 2d72a44fa49f47bd7258d7efb931926b26de4004.1720549824.git.josef@toxicpanda.com (mailing list archive)
State New
Headers show
Series man-pages: add documentation for statmount/listmount | expand

Commit Message

Josef Bacik July 9, 2024, 6:31 p.m. UTC 
Add some documentation for the new listmount syscall.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
 man/man2/listmount.2 | 112 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 112 insertions(+)
 create mode 100644 man/man2/listmount.2

Comments

Alejandro Colomar July 22, 2024, 8:27 p.m. UTC | #1 
Hi Josef,

On Tue, Jul 09, 2024 at 02:31:23PM GMT, Josef Bacik wrote:
> Add some documentation for the new listmount syscall.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>

Thanks!  I've applied the patch with some minor tweaks:

	diff --git i/man/man2/listmount.2 w/man/man2/listmount.2
	index 212929fb6..8f7c7afaa 100644
	--- i/man/man2/listmount.2
	+++ w/man/man2/listmount.2
	@@ -4,7 +4,9 @@
	 .\"
	 .TH listmount 2 (date) "Linux man-pages (unreleased)"
	 .SH NAME
	-listmount \- get a list of mount ID's
	+listmount
	+\-
	+get a list of mount ID's
	 .SH LIBRARY
	 Standard C library
	 .RI ( libc ", " \-lc )
	@@ -14,15 +16,15 @@ .SH SYNOPSIS
	 .B #include <unistd.h>
	 .P
	 .BI "int syscall(SYS_listmount, struct mnt_id_req * " req ,
	-.BI "            u64 * " mnt_ids ", size_t " nr_mnt_ids ,
	+.BI "            uint64_t * " mnt_ids ", size_t " nr_mnt_ids ,
	 .BI "            unsigned long " flags );
	 .P
	 .B #include <linux/mount.h>
	 .P
	 .B struct mnt_id_req {
	-.BR "    __u32 size;" "    /* sizeof(struct mnt_id_req) */"
	-.BR "    __u64 mnt_id;" "  /* The parent mnt_id being searched */"
	-.BR "    __u64 param;" "   /* The next mnt_id we want to find */"
	+.BR "    __u32  size;" "    /* sizeof(struct mnt_id_req) */"
	+.BR "    __u64  mnt_id;" "  /* The parent mnt_id being searched */"
	+.BR "    __u64  param;" "   /* The next mnt_id we want to find */"
	 .B };
	 .fi
	 .P
	@@ -45,7 +47,8 @@ .SS The mnt_id_req structure
	 is used by the kernel to determine which struct
	 .I mnt_id_req
	 is being passed in,
	-it should always be set to sizeof(struct mnt_id req).
	+it should always be set to
	+.IR \%sizeof(struct\~mnt_id_req) .
	 .P
	 .I req.mnt_id
	 is the parent mnt_id that we will list from,
	@@ -69,7 +72,8 @@ .SS The mnt_id_req structure
	 .SH RETURN VALUE
	 On success, the number of entries filled into
	 .I mnt_ids
	-is returned, 0 if there are no more mounts left.
	+is returned;
	+0 if there are no more mounts left.
	 On error, \-1 is returned, and
	 .I errno
	 is set to indicate the error.

Would you mind adding an example program in a new patch?

Have a lovely night!
Alex

> ---
>  man/man2/listmount.2 | 112 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 112 insertions(+)
>  create mode 100644 man/man2/listmount.2
> 
> diff --git a/man/man2/listmount.2 b/man/man2/listmount.2
> new file mode 100644
> index 000000000..212929fb6
> --- /dev/null
> +++ b/man/man2/listmount.2
> @@ -0,0 +1,112 @@
> +.\" Copyright (c) 2024 Josef Bacik <josef@toxicpanda.com>
> +.\"
> +.\" SPDX-License-Identifier: Linux-man-pages-copyleft
> +.\"
> +.TH listmount 2 (date) "Linux man-pages (unreleased)"
> +.SH NAME
> +listmount \- get a list of mount ID's
> +.SH LIBRARY
> +Standard C library
> +.RI ( libc ", " \-lc )
> +.SH SYNOPSIS
> +.nf
> +.BR "#include <linux/mount.h>" "  /* Definition of struct mnt_id_req constants */"
> +.B #include <unistd.h>
> +.P
> +.BI "int syscall(SYS_listmount, struct mnt_id_req * " req ,
> +.BI "            u64 * " mnt_ids ", size_t " nr_mnt_ids ,
> +.BI "            unsigned long " flags );
> +.P
> +.B #include <linux/mount.h>
> +.P
> +.B struct mnt_id_req {
> +.BR "    __u32 size;" "    /* sizeof(struct mnt_id_req) */"
> +.BR "    __u64 mnt_id;" "  /* The parent mnt_id being searched */"
> +.BR "    __u64 param;" "   /* The next mnt_id we want to find */"
> +.B };
> +.fi
> +.P
> +.IR Note :
> +glibc provides no wrapper for
> +.BR listmount (),
> +necessitating the use of
> +.BR syscall (2).
> +.SH DESCRIPTION
> +To access the mounts in your namespace,
> +you must have CAP_SYS_ADMIN in the user namespace.
> +.P
> +This function returns a list of mount IDs under the
> +.BR req.mnt_id .
> +This is meant to be used in conjuction with
> +.BR statmount (2)
> +in order to provide a way to iterate and discover mounted file systems.
> +.SS The mnt_id_req structure
> +.I req.size
> +is used by the kernel to determine which struct
> +.I mnt_id_req
> +is being passed in,
> +it should always be set to sizeof(struct mnt_id req).
> +.P
> +.I req.mnt_id
> +is the parent mnt_id that we will list from,
> +which can either be
> +.B LSMT_ROOT
> +which means the root mount of the current mount namespace,
> +or a mount ID obtained from either
> +.BR statx (2)
> +using
> +.B STATX_MNT_ID_UNIQUE
> +or from
> +.BR listmount (2) .
> +.P
> +.I req.param
> +is used to tell the kernel what mount ID to start the list from.
> +This is useful if multiple calls to
> +.BR listmount (2)
> +are required.
> +This can be set to the last mount ID returned + 1 in order to
> +resume from a previous spot in the list.
> +.SH RETURN VALUE
> +On success, the number of entries filled into
> +.I mnt_ids
> +is returned, 0 if there are no more mounts left.
> +On error, \-1 is returned, and
> +.I errno
> +is set to indicate the error.
> +.SH ERRORS
> +.TP
> +.B EPERM
> +Permission is denied for accessing this mount.
> +.TP
> +.B EFAULT
> +.I req
> +or
> +.I mnt_ids
> +points to a location outside the process's accessible
> +address space.
> +.TP
> +.B EINVAL
> +Invalid flag specified in
> +.IR flags .
> +.TP
> +.B EINVAL
> +.I req
> +is of insufficient size to be utilized.
> +.TP
> +.B E2BIG
> +.I req
> +is too large,
> +the limit is the architectures page size.
> +.TP
> +.B ENOENT
> +The specified
> +.I req.mnt_id
> +doesn't exist.
> +.TP
> +.B ENOMEM
> +Out of memory (i.e., kernel memory).
> +.SH STANDARDS
> +Linux.
> +.SH SEE ALSO
> +.BR statmount (2),
> +.BR statx (2)
> -- 
> 2.43.0
> 
>
Josef Bacik July 22, 2024, 8:44 p.m. UTC | #2 
On Mon, Jul 22, 2024 at 10:27:23PM +0200, Alejandro Colomar wrote:
> Hi Josef,
> 
> On Tue, Jul 09, 2024 at 02:31:23PM GMT, Josef Bacik wrote:
> > Add some documentation for the new listmount syscall.
> > 
> > Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> 
> Thanks!  I've applied the patch with some minor tweaks:
> 
> 	diff --git i/man/man2/listmount.2 w/man/man2/listmount.2
> 	index 212929fb6..8f7c7afaa 100644
> 	--- i/man/man2/listmount.2
> 	+++ w/man/man2/listmount.2
> 	@@ -4,7 +4,9 @@
> 	 .\"
> 	 .TH listmount 2 (date) "Linux man-pages (unreleased)"
> 	 .SH NAME
> 	-listmount \- get a list of mount ID's
> 	+listmount
> 	+\-
> 	+get a list of mount ID's
> 	 .SH LIBRARY
> 	 Standard C library
> 	 .RI ( libc ", " \-lc )
> 	@@ -14,15 +16,15 @@ .SH SYNOPSIS
> 	 .B #include <unistd.h>
> 	 .P
> 	 .BI "int syscall(SYS_listmount, struct mnt_id_req * " req ,
> 	-.BI "            u64 * " mnt_ids ", size_t " nr_mnt_ids ,
> 	+.BI "            uint64_t * " mnt_ids ", size_t " nr_mnt_ids ,
> 	 .BI "            unsigned long " flags );
> 	 .P
> 	 .B #include <linux/mount.h>
> 	 .P
> 	 .B struct mnt_id_req {
> 	-.BR "    __u32 size;" "    /* sizeof(struct mnt_id_req) */"
> 	-.BR "    __u64 mnt_id;" "  /* The parent mnt_id being searched */"
> 	-.BR "    __u64 param;" "   /* The next mnt_id we want to find */"
> 	+.BR "    __u32  size;" "    /* sizeof(struct mnt_id_req) */"
> 	+.BR "    __u64  mnt_id;" "  /* The parent mnt_id being searched */"
> 	+.BR "    __u64  param;" "   /* The next mnt_id we want to find */"
> 	 .B };
> 	 .fi
> 	 .P
> 	@@ -45,7 +47,8 @@ .SS The mnt_id_req structure
> 	 is used by the kernel to determine which struct
> 	 .I mnt_id_req
> 	 is being passed in,
> 	-it should always be set to sizeof(struct mnt_id req).
> 	+it should always be set to
> 	+.IR \%sizeof(struct\~mnt_id_req) .
> 	 .P
> 	 .I req.mnt_id
> 	 is the parent mnt_id that we will list from,
> 	@@ -69,7 +72,8 @@ .SS The mnt_id_req structure
> 	 .SH RETURN VALUE
> 	 On success, the number of entries filled into
> 	 .I mnt_ids
> 	-is returned, 0 if there are no more mounts left.
> 	+is returned;
> 	+0 if there are no more mounts left.
> 	 On error, \-1 is returned, and
> 	 .I errno
> 	 is set to indicate the error.
> 
> Would you mind adding an example program in a new patch?

Yup I can do that, I was going to follow-up with a patch for the new extensions
that have landed in this merge window, after the final release has been cut.  Is
it cool if I wait until then, or would you like something sooner?  Thanks,

Josef
Alejandro Colomar July 22, 2024, 9:03 p.m. UTC | #3 
Hi Josef,

On Mon, Jul 22, 2024 at 04:44:16PM GMT, Josef Bacik wrote:
> > Would you mind adding an example program in a new patch?
> 
> Yup I can do that, I was going to follow-up with a patch for the new extensions
> that have landed in this merge window, after the final release has been cut.  Is
> it cool if I wait until then, or would you like something sooner?  Thanks,

Thanks!  And sure; you can wait as much as necessary.  :-)

Have a lovely night!
Alex

> 
> Josef
>
Miklos Szeredi Aug. 22, 2024, 5:33 p.m. UTC | #4 
Thanks, Josef for doing the man pages.

On Tue, 9 Jul 2024 at 20:32, Josef Bacik <josef@toxicpanda.com> wrote:
> +.I req.param
> +is used to tell the kernel what mount ID to start the list from.
> +This is useful if multiple calls to
> +.BR listmount (2)
> +are required.
> +This can be set to the last mount ID returned + 1 in order to

The "+ 1" is done by the kernel, so this should be just set to the
last returned mount ID.   Also maybe explicitly mention that a value
of zero is used to start from the beginning of the list (zero is a
reserved mount ID value).

Thanks,
Miklos
Jeff Layton Nov. 11, 2024, 9:17 p.m. UTC | #5 
On Tue, 2024-07-09 at 14:31 -0400, Josef Bacik wrote:
> Add some documentation for the new listmount syscall.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  man/man2/listmount.2 | 112 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 112 insertions(+)
>  create mode 100644 man/man2/listmount.2
> 
> diff --git a/man/man2/listmount.2 b/man/man2/listmount.2
> new file mode 100644
> index 000000000..212929fb6
> --- /dev/null
> +++ b/man/man2/listmount.2
> @@ -0,0 +1,112 @@
> +.\" Copyright (c) 2024 Josef Bacik <josef@toxicpanda.com>
> +.\"
> +.\" SPDX-License-Identifier: Linux-man-pages-copyleft
> +.\"
> +.TH listmount 2 (date) "Linux man-pages (unreleased)"
> +.SH NAME
> +listmount \- get a list of mount ID's
> +.SH LIBRARY
> +Standard C library
> +.RI ( libc ", " \-lc )
> +.SH SYNOPSIS
> +.nf
> +.BR "#include <linux/mount.h>" "  /* Definition of struct mnt_id_req constants */"
> +.B #include <unistd.h>
> +.P
> +.BI "int syscall(SYS_listmount, struct mnt_id_req * " req ,
> +.BI "            u64 * " mnt_ids ", size_t " nr_mnt_ids ,
> +.BI "            unsigned long " flags );
> +.P
> +.B #include <linux/mount.h>
> +.P
> +.B struct mnt_id_req {
> +.BR "    __u32 size;" "    /* sizeof(struct mnt_id_req) */"
> +.BR "    __u64 mnt_id;" "  /* The parent mnt_id being searched */"
> +.BR "    __u64 param;" "   /* The next mnt_id we want to find */"
> +.B };
> +.fi
> +.P
> +.IR Note :
> +glibc provides no wrapper for
> +.BR listmount (),
> +necessitating the use of
> +.BR syscall (2).
> +.SH DESCRIPTION
> +To access the mounts in your namespace,
> +you must have CAP_SYS_ADMIN in the user namespace.

The above does not seem to be correct. I can access the mounts in my
own namespace just fine. What I can't do is access mounts in other
namespaces without CAP_SYS_ADMIN, right?

Maybe this?

"To access mounts outside your own namespace, you must
have CAP_SYS_ADMIN in the user namespace."


> +.P
> +This function returns a list of mount IDs under the
> +.BR req.mnt_id .
> +This is meant to be used in conjuction with
> +.BR statmount (2)
> +in order to provide a way to iterate and discover mounted file systems.
> +.SS The mnt_id_req structure
> +.I req.size
> +is used by the kernel to determine which struct
> +.I mnt_id_req
> +is being passed in,
> +it should always be set to sizeof(struct mnt_id req).
> +.P
> +.I req.mnt_id
> +is the parent mnt_id that we will list from,
> +which can either be
> +.B LSMT_ROOT
> +which means the root mount of the current mount namespace,
> +or a mount ID obtained from either
> +.BR statx (2)
> +using
> +.B STATX_MNT_ID_UNIQUE
> +or from
> +.BR listmount (2) .

The above is a little vague. It's not clear whether the list is just
immediate child mounts or if you get all mounts below that point. The
implementation in the kernel is the latter. You get a list of any
descendent mount of the given mnt_id.

Should we make that more clear?

> +.P
> +.I req.param
> +is used to tell the kernel what mount ID to start the list from.
> +This is useful if multiple calls to
> +.BR listmount (2)
> +are required.
> +This can be set to the last mount ID returned + 1 in order to
> +resume from a previous spot in the list.
> +.SH RETURN VALUE
> +On success, the number of entries filled into
> +.I mnt_ids
> +is returned, 0 if there are no more mounts left.
> +On error, \-1 is returned, and
> +.I errno
> +is set to indicate the error.
> +.SH ERRORS
> +.TP
> +.B EPERM
> +Permission is denied for accessing this mount.
> +.TP
> +.B EFAULT
> +.I req
> +or
> +.I mnt_ids
> +points to a location outside the process's accessible
> +address space.
> +.TP
> +.B EINVAL
> +Invalid flag specified in
> +.IR flags .
> +.TP
> +.B EINVAL
> +.I req
> +is of insufficient size to be utilized.
> +.TP
> +.B E2BIG
> +.I req
> +is too large,
> +the limit is the architectures page size.
> +.TP
> +.B ENOENT
> +The specified
> +.I req.mnt_id
> +doesn't exist.
> +.TP
> +.B ENOMEM
> +Out of memory (i.e., kernel memory).
> +.SH STANDARDS
> +Linux.
> +.SH SEE ALSO
> +.BR statmount (2),
> +.BR statx (2)
diff mbox series

Patch

diff --git a/man/man2/listmount.2 b/man/man2/listmount.2
new file mode 100644
index 000000000..212929fb6
--- /dev/null
+++ b/man/man2/listmount.2
@@ -0,0 +1,112 @@ 
+.\" Copyright (c) 2024 Josef Bacik <josef@toxicpanda.com>
+.\"
+.\" SPDX-License-Identifier: Linux-man-pages-copyleft
+.\"
+.TH listmount 2 (date) "Linux man-pages (unreleased)"
+.SH NAME
+listmount \- get a list of mount ID's
+.SH LIBRARY
+Standard C library
+.RI ( libc ", " \-lc )
+.SH SYNOPSIS
+.nf
+.BR "#include <linux/mount.h>" "  /* Definition of struct mnt_id_req constants */"
+.B #include <unistd.h>
+.P
+.BI "int syscall(SYS_listmount, struct mnt_id_req * " req ,
+.BI "            u64 * " mnt_ids ", size_t " nr_mnt_ids ,
+.BI "            unsigned long " flags );
+.P
+.B #include <linux/mount.h>
+.P
+.B struct mnt_id_req {
+.BR "    __u32 size;" "    /* sizeof(struct mnt_id_req) */"
+.BR "    __u64 mnt_id;" "  /* The parent mnt_id being searched */"
+.BR "    __u64 param;" "   /* The next mnt_id we want to find */"
+.B };
+.fi
+.P
+.IR Note :
+glibc provides no wrapper for
+.BR listmount (),
+necessitating the use of
+.BR syscall (2).
+.SH DESCRIPTION
+To access the mounts in your namespace,
+you must have CAP_SYS_ADMIN in the user namespace.
+.P
+This function returns a list of mount IDs under the
+.BR req.mnt_id .
+This is meant to be used in conjuction with
+.BR statmount (2)
+in order to provide a way to iterate and discover mounted file systems.
+.SS The mnt_id_req structure
+.I req.size
+is used by the kernel to determine which struct
+.I mnt_id_req
+is being passed in,
+it should always be set to sizeof(struct mnt_id req).
+.P
+.I req.mnt_id
+is the parent mnt_id that we will list from,
+which can either be
+.B LSMT_ROOT
+which means the root mount of the current mount namespace,
+or a mount ID obtained from either
+.BR statx (2)
+using
+.B STATX_MNT_ID_UNIQUE
+or from
+.BR listmount (2) .
+.P
+.I req.param
+is used to tell the kernel what mount ID to start the list from.
+This is useful if multiple calls to
+.BR listmount (2)
+are required.
+This can be set to the last mount ID returned + 1 in order to
+resume from a previous spot in the list.
+.SH RETURN VALUE
+On success, the number of entries filled into
+.I mnt_ids
+is returned, 0 if there are no more mounts left.
+On error, \-1 is returned, and
+.I errno
+is set to indicate the error.
+.SH ERRORS
+.TP
+.B EPERM
+Permission is denied for accessing this mount.
+.TP
+.B EFAULT
+.I req
+or
+.I mnt_ids
+points to a location outside the process's accessible
+address space.
+.TP
+.B EINVAL
+Invalid flag specified in
+.IR flags .
+.TP
+.B EINVAL
+.I req
+is of insufficient size to be utilized.
+.TP
+.B E2BIG
+.I req
+is too large,
+the limit is the architectures page size.
+.TP
+.B ENOENT
+The specified
+.I req.mnt_id
+doesn't exist.
+.TP
+.B ENOMEM
+Out of memory (i.e., kernel memory).
+.SH STANDARDS
+Linux.
+.SH SEE ALSO
+.BR statmount (2),
+.BR statx (2)