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add listmount(2) syscall

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Add way to query the children of a particular mount.  This is a more
flexible way to iterate the mount tree than having to parse
/proc/self/mountinfo.

Lookup the mount by the new 64bit mount ID. If a mount needs to be
queried based on path, then statx(2) can be used to first query the
mount ID belonging to the path.

Return an array of new (64bit) mount ID's. Without privileges only
mounts are listed which are reachable from the task's root.

Folded into this patch are several later improvements. Keeping them
separate would make the history pointlessly confusing:

* Recursive listing of mounts is the default now (cf. [1]).
* Remove explicit LISTMOUNT_UNREACHABLE flag (cf. [1]) and fail if mount
  is unreachable from current root. This also makes permission checking
  consistent with statmount() (cf. [3]).
* Start listing mounts in unique mount ID order (cf. [2]) to allow
  continuing listmount() from a midpoint.
* Allow to continue listmount(). The @request_mask parameter is renamed
  and to @param to be usable by both statmount() and listmount().
  If @param is set to a mount id then listmount() will continue listing
  mounts from that id on. This allows listing mounts in multiple
  listmount invocations without having to resize the buffer. If @param
  is zero then the listing starts from the beginning (cf. [4]).
* Don't return EOVERFLOW, instead return the buffer size which allows to
  detect a full buffer as well (cf. [4]).

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Link: https://lore.kernel.org/r/20231025140205.3586473-6-mszeredi@redhat.com
Reviewed-by: Ian Kent <raven@themaw.net>
Link: https://lore.kernel.org/r/20231128160337.29094-2-mszeredi@redhat.com [1] (folded)
Link: https://lore.kernel.org/r/20231128160337.29094-3-mszeredi@redhat.com [2] (folded)
Link: https://lore.kernel.org/r/20231128160337.29094-4-mszeredi@redhat.com [3] (folded)
Link: https://lore.kernel.org/r/20231128160337.29094-5-mszeredi@redhat.com [4] (folded)
[Christian Brauner <brauner@kernel.org>: various smaller fixes]
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Miklos Szeredi
2023-10-25 16:02:03 +02:00
committed by Christian Brauner
parent 68385d77c0
commit b4c2bea8ce
3 changed files with 100 additions and 3 deletions
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14
include/uapi/linux/mount.h
View File

@@ -176,9 +176,16 @@ struct statmount {
char str[]; /* Variable size part containing strings */
};
/*
* Structure for passing mount ID and miscellaneous parameters to statmount(2)
* and listmount(2).
*
* For statmount(2) @param represents the request mask.
* For listmount(2) @param represents the last listed mount id (or zero).
*/
struct mnt_id_req {
__u64 mnt_id;
__u64 request_mask;
__u64 param;
};
/*
@@ -191,4 +198,9 @@ struct mnt_id_req {
#define STATMOUNT_MNT_POINT 0x00000010U /* Want/got mnt_point */
#define STATMOUNT_FS_TYPE 0x00000020U /* Want/got fs_type */
/*
* Special @mnt_id values that can be passed to listmount
*/
#define LSMT_ROOT 0xffffffffffffffff /* root mount */
#endif /* _UAPI_LINUX_MOUNT_H */