It's been $MANY years since shutdown hooks have been replaced with
per-device ca_activate(DVACT_xxx) calls.

As a result:
- the second argument of hook_establish() always has the same value (1).
- hook_disestablish() is not used anymore.
- the second argument of dohooks() always has the same value
  (HOOK_REMOVE | HOOK_FREE).

The following diff:
- removes hook_disestablish().
- simplifies hook_establish() and dohooks() by removing the second
  argument and hardwiring the behaviour given the value this argument
  used to have.
- updates the manual pages accordingly.

Index: share/man/man9/dohooks.9
===================================================================
RCS file: /OpenBSD/src/share/man/man9/dohooks.9,v
diff -u -p -r1.8 dohooks.9
--- share/man/man9/dohooks.9	12 Dec 2015 11:26:32 -0000	1.8
+++ share/man/man9/dohooks.9	31 Oct 2026 13:06:13 -0000
@@ -30,12 +30,12 @@
 .Os
 .Sh NAME
 .Nm dohooks
-.Nd run all hooks in a list
+.Nd run and remove all hooks in a list
 .Sh SYNOPSIS
 .In sys/types.h
 .In sys/systm.h
 .Ft void
-.Fn dohooks "struct hook_desc_head *head" "int flags"
+.Fn dohooks "struct hook_desc_head *head"
 .Sh DESCRIPTION
 The
 .Fn dohooks
@@ -44,24 +44,8 @@ function invokes all hooks established u
 function.
 Hooks are called in the order of the TAILQ that
 .Fa head
-points to, however
-.Xr hook_establish 9
-can put the hooks either at the head or the tail of that queue,
-making it possible to call the hooks either in the order of
-establishment, or its reverse.
-.Pp
-The flags can specify
-.Dv HOOK_REMOVE
-to remove already processed hooks from the hook list and
-.Dv HOOK_FREE
-to also free them.
-In most cases either no flags should be used or
-.Dv HOOK_REMOVE
-and
-.Dv HOOK_FREE
-at the same time, since just HOOK_REMOVE will drop the only reference to
-allocated memory and should only be used in situations where freeing
-memory would be illegal and unnecessary.
+points to, which is the order of their establishment.
+Hooks are then removed from the hook list and freed.
 .Pp
 This function is used to implement the
 .Xr dostartuphooks 9
Index: share/man/man9/hook_establish.9
===================================================================
RCS file: /OpenBSD/src/share/man/man9/hook_establish.9,v
diff -u -p -r1.12 hook_establish.9
--- share/man/man9/hook_establish.9	12 Dec 2015 11:26:32 -0000	1.12
+++ share/man/man9/hook_establish.9	31 Oct 2026 13:06:13 -0000
@@ -29,16 +29,13 @@
 .Dt HOOK_ESTABLISH 9
 .Os
 .Sh NAME
-.Nm hook_establish ,
-.Nm hook_disestablish
-.Nd add or remove a hook from a specified list
+.Nm hook_establish
+.Nd add a hook to a specified list
 .Sh SYNOPSIS
 .In sys/types.h
 .In sys/systm.h
 .Ft void *
-.Fn hook_establish "struct hook_desc_head *head" "int tail" "void (*fn)(void *)" "void *arg"
-.Ft void
-.Fn hook_disestablish "struct hook_desc_head *head" "void *cookie"
+.Fn hook_establish "struct hook_desc_head *head" "void (*fn)(void *)" "void *arg"
 .Sh DESCRIPTION
 The
 .Fn hook_establish
@@ -46,12 +43,8 @@ function adds
 .Fa fn
 to the list of hooks invoked by
 .Xr dohooks 9 .
-If
-.Fa tail
-is non-zero, the hook is added to the tail of the list
-denoted by the TAILQ_HEAD pointer
-.Fa head ,
-otherwise to the front.
+The hook is added to the tail of the list denoted by the TAILQ_HEAD pointer
+.Fa head .
 The
 .Xr dohooks 9
 function will at its invocation call each hook from the front of this
@@ -62,25 +55,12 @@ will be passed
 .Fa arg
 as its only argument.
 .Pp
-The
-.Fn hook_disestablish
-function removes the hook described by the opaque pointer
-.Fa cookie
-from the list of hooks denoted by the TAILQ_HEAD pointer
-.Fa head .
-If
-.Fa cookie
-is invalid, the result of
-.Fn hook_disestablish
-is undefined.
-.Pp
 The startup and mountroot systems use this API for their
 implementation.
 .Sh RETURN VALUES
 If successful,
 .Fn hook_establish
-returns an opaque pointer describing the newly established
-hook.
+returns an opaque pointer describing the newly established hook.
 Otherwise, it returns
 .Dv NULL .
 .Sh SEE ALSO
Index: share/man/man9/startuphook_establish.9
===================================================================
RCS file: /OpenBSD/src/share/man/man9/startuphook_establish.9,v
diff -u -p -r1.8 startuphook_establish.9
--- share/man/man9/startuphook_establish.9	10 Dec 2014 15:29:52 -0000	1.8
+++ share/man/man9/startuphook_establish.9	31 Oct 2026 13:06:13 -0000
@@ -29,16 +29,13 @@
 .Dt STARTUPHOOK_ESTABLISH 9
 .Os
 .Sh NAME
-.Nm startuphook_establish ,
-.Nm startuphook_disestablish
-.Nd add or remove a startup hook
+.Nm startuphook_establish
+.Nd add a startup hook
 .Sh SYNOPSIS
 .In sys/types.h
 .In sys/systm.h
 .Ft void *
 .Fn startuphook_establish "void (*fn)(void *)" "void *arg"
-.Ft void
-.Fn startuphook_disestablish "void *cookie"
 .Sh DESCRIPTION
 The
 .Fn startuphook_establish
@@ -52,17 +49,6 @@ When invoked, the hook function
 will be passed
 .Fa arg
 as its only argument.
-.Pp
-The
-.Fn startuphook_disestablish
-function removes the hook described by the opaque pointer
-.Fa cookie
-from the list of hooks to be invoked at startup.
-If
-.Fa cookie
-is invalid, the result of
-.Fn startuphook_disestablish
-is undefined.
 .Pp
 Startup hooks should be used to perform one-time activities
 that must happen immediately before the root and swap devices
Index: sys/kern/kern_subr.c
===================================================================
RCS file: /OpenBSD/src/sys/kern/kern_subr.c,v
diff -u -p -r1.52 kern_subr.c
--- sys/kern/kern_subr.c	31 Jan 2023 15:18:56 -0000	1.52
+++ sys/kern/kern_subr.c	31 Oct 2026 13:06:15 -0000
@@ -262,7 +262,7 @@ struct hook_desc_head startuphook_list =
     TAILQ_HEAD_INITIALIZER(startuphook_list);
 
 void *
-hook_establish(struct hook_desc_head *head, int tail, void (*fn)(void *),
+hook_establish(struct hook_desc_head *head, void (*fn)(void *),
     void *arg)
 {
 	struct hook_desc *hdp;
@@ -273,53 +273,23 @@ hook_establish(struct hook_desc_head *he
 
 	hdp->hd_fn = fn;
 	hdp->hd_arg = arg;
-	if (tail)
-		TAILQ_INSERT_TAIL(head, hdp, hd_list);
-	else
-		TAILQ_INSERT_HEAD(head, hdp, hd_list);
+	TAILQ_INSERT_TAIL(head, hdp, hd_list);
 
 	return (hdp);
 }
 
-void
-hook_disestablish(struct hook_desc_head *head, void *vhook)
-{
-	struct hook_desc *hdp;
-
-#ifdef DIAGNOSTIC
-	for (hdp = TAILQ_FIRST(head); hdp != NULL;
-	    hdp = TAILQ_NEXT(hdp, hd_list))
-                if (hdp == vhook)
-			break;
-	if (hdp == NULL)
-		return;
-#endif
-	hdp = vhook;
-	TAILQ_REMOVE(head, hdp, hd_list);
-	free(hdp, M_DEVBUF, sizeof(*hdp));
-}
-
 /*
  * Run hooks.  Startup hooks are invoked right after scheduler_start but
- * before root is mounted.  Shutdown hooks are invoked immediately before the
- * system is halted or rebooted, i.e. after file systems unmounted,
- * after crash dump done, etc.
+ * before root is mounted.
  */
 void
-dohooks(struct hook_desc_head *head, int flags)
+dohooks(struct hook_desc_head *head)
 {
-	struct hook_desc *hdp, *hdp_temp;
+	struct hook_desc *hdp;
 
-	if ((flags & HOOK_REMOVE) == 0) {
-		TAILQ_FOREACH_SAFE(hdp, head, hd_list, hdp_temp) {
-			(*hdp->hd_fn)(hdp->hd_arg);
-		}
-	} else {
-		while ((hdp = TAILQ_FIRST(head)) != NULL) {
-			TAILQ_REMOVE(head, hdp, hd_list);
-			(*hdp->hd_fn)(hdp->hd_arg);
-			if ((flags & HOOK_FREE) != 0)
-				free(hdp, M_DEVBUF, sizeof(*hdp));
-		}
+	while ((hdp = TAILQ_FIRST(head)) != NULL) {
+		TAILQ_REMOVE(head, hdp, hd_list);
+		(*hdp->hd_fn)(hdp->hd_arg);
+		free(hdp, M_DEVBUF, sizeof(*hdp));
 	}
 }
Index: sys/sys/systm.h
===================================================================
RCS file: /OpenBSD/src/sys/sys/systm.h,v
diff -u -p -r1.179 systm.h
--- sys/sys/systm.h	22 Apr 2026 01:51:37 -0000	1.179
+++ sys/sys/systm.h	31 Oct 2026 13:06:15 -0000
@@ -310,18 +310,12 @@ TAILQ_HEAD(hook_desc_head, hook_desc);
 
 extern struct hook_desc_head startuphook_list;
 
-void	*hook_establish(struct hook_desc_head *, int, void (*)(void *), void *);
-void	hook_disestablish(struct hook_desc_head *, void *);
-void	dohooks(struct hook_desc_head *, int);
-
-#define HOOK_REMOVE	0x01
-#define HOOK_FREE	0x02
+void	*hook_establish(struct hook_desc_head *, void (*)(void *), void *);
+void	dohooks(struct hook_desc_head *);
 
 #define startuphook_establish(fn, arg) \
-	hook_establish(&startuphook_list, 1, (fn), (arg))
-#define startuphook_disestablish(vhook) \
-	hook_disestablish(&startuphook_list, (vhook))
-#define dostartuphooks() dohooks(&startuphook_list, HOOK_REMOVE|HOOK_FREE)
+	hook_establish(&startuphook_list, (fn), (arg))
+#define dostartuphooks() dohooks(&startuphook_list)
 
 struct uio;
 int	uiomove(void *, size_t, struct uio *);