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    Subject[RFC PATCH 01/14] uapi: General notification queue definitions [ver #3]
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    Add UAPI definitions for the general notification queue, including the
    following pieces:

    (*) struct watch_notification.

    This is the metadata header for notification messages. It includes a
    type and subtype that indicate the source of the message
    (eg. WATCH_TYPE_MOUNT_NOTIFY) and the kind of the message
    (eg. NOTIFY_MOUNT_NEW_MOUNT).

    The header also contains an information field that conveys the
    following information:

    - WATCH_INFO_LENGTH. The size of the entry (entries are variable
    length).

    - WATCH_INFO_ID. The watch ID specified when the watchpoint was
    set.

    - WATCH_INFO_TYPE_INFO. (Sub)type-specific information.

    - WATCH_INFO_FLAG_*. Flag bits overlain on the type-specific
    information. For use by the type.

    All the information in the header can be used in filtering messages at
    the point of writing into the buffer.

    (*) struct watch_notification_removal

    This is an extended watch-removal notification record that includes an
    'id' field that can indicate the identifier of the object being
    removed if available (for instance, a keyring serial number).

    Signed-off-by: David Howells <dhowells@redhat.com>
    ---

    include/uapi/linux/watch_queue.h | 55 ++++++++++++++++++++++++++++++++++++++
    1 file changed, 55 insertions(+)
    create mode 100644 include/uapi/linux/watch_queue.h

    diff --git a/include/uapi/linux/watch_queue.h b/include/uapi/linux/watch_queue.h
    new file mode 100644
    index 000000000000..5f3d21e8a34b
    --- /dev/null
    +++ b/include/uapi/linux/watch_queue.h
    @@ -0,0 +1,55 @@
    +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
    +#ifndef _UAPI_LINUX_WATCH_QUEUE_H
    +#define _UAPI_LINUX_WATCH_QUEUE_H
    +
    +#include <linux/types.h>
    +
    +enum watch_notification_type {
    + WATCH_TYPE_META = 0, /* Special record */
    + WATCH_TYPE__NR = 1
    +};
    +
    +enum watch_meta_notification_subtype {
    + WATCH_META_REMOVAL_NOTIFICATION = 0, /* Watched object was removed */
    + WATCH_META_LOSS_NOTIFICATION = 1, /* Data loss occurred */
    +};
    +
    +/*
    + * Notification record header. This is aligned to 64-bits so that subclasses
    + * can contain __u64 fields.
    + */
    +struct watch_notification {
    + __u32 type:24; /* enum watch_notification_type */
    + __u32 subtype:8; /* Type-specific subtype (filterable) */
    + __u32 info;
    +#define WATCH_INFO_LENGTH 0x0000007f /* Length of record */
    +#define WATCH_INFO_LENGTH__SHIFT 0
    +#define WATCH_INFO_ID 0x0000ff00 /* ID of watchpoint */
    +#define WATCH_INFO_ID__SHIFT 8
    +#define WATCH_INFO_TYPE_INFO 0xffff0000 /* Type-specific info */
    +#define WATCH_INFO_TYPE_INFO__SHIFT 16
    +#define WATCH_INFO_FLAG_0 0x00010000 /* Type-specific info, flag bit 0 */
    +#define WATCH_INFO_FLAG_1 0x00020000 /* ... */
    +#define WATCH_INFO_FLAG_2 0x00040000
    +#define WATCH_INFO_FLAG_3 0x00080000
    +#define WATCH_INFO_FLAG_4 0x00100000
    +#define WATCH_INFO_FLAG_5 0x00200000
    +#define WATCH_INFO_FLAG_6 0x00400000
    +#define WATCH_INFO_FLAG_7 0x00800000
    +};
    +
    +
    +/*
    + * Extended watch removal notification. This is used optionally if the type
    + * wants to indicate an identifier for the object being watched, if there is
    + * such. This can be distinguished by the length.
    + *
    + * type -> WATCH_TYPE_META
    + * subtype -> WATCH_META_REMOVAL_NOTIFICATION
    + */
    +struct watch_notification_removal {
    + struct watch_notification watch;
    + __u64 id; /* Type-dependent identifier */
    +};
    +
    +#endif /* _UAPI_LINUX_WATCH_QUEUE_H */
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