[v23,08/22] richacl: Compute maximum file masks from an acl
diff mbox series

Message ID 1467294433-3222-9-git-send-email-agruenba@redhat.com
State New, archived
Headers show
Series
  • Richacls (Core and Ext4)
Related show

Commit Message

Andreas Gruenbacher June 30, 2016, 1:46 p.m. UTC
Compute upper bound owner, group, and other file masks with as few
permissions as possible without denying any permissions that the NFSv4
acl in a richacl grants.

This algorithm is used when a file inherits an acl at create time and
when an acl is set via a mechanism that does not provide file masks
(such as setting an acl via nfsd).  When user-space sets an acl via
setxattr, the extended attribute already includes the file masks.

Setting an acl also sets the file mode permission bits: they are
determined by the file masks; see richacl_masks_to_mode().

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: J. Bruce Fields <bfields@redhat.com>
---
 fs/richacl.c            | 157 ++++++++++++++++++++++++++++++++++++++++++++++++
 include/linux/richacl.h |   1 +
 2 files changed, 158 insertions(+)

Comments

Jeff Layton July 5, 2016, 2:22 p.m. UTC | #1
On Thu, 2016-06-30 at 15:46 +0200, Andreas Gruenbacher wrote:
> Compute upper bound owner, group, and other file masks with as few
> permissions as possible without denying any permissions that the NFSv4
> acl in a richacl grants.
> 
> This algorithm is used when a file inherits an acl at create time and
> when an acl is set via a mechanism that does not provide file masks
> (such as setting an acl via nfsd).  When user-space sets an acl via
> setxattr, the extended attribute already includes the file masks.
> 
> Setting an acl also sets the file mode permission bits: they are
> determined by the file masks; see richacl_masks_to_mode().
> 
> Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
> Reviewed-by: J. Bruce Fields <bfields@redhat.com>
> ---
>  fs/richacl.c            | 157 ++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/richacl.h |   1 +
>  2 files changed, 158 insertions(+)
> 
> diff --git a/fs/richacl.c b/fs/richacl.c
> index d0a4135..056228f 100644
> --- a/fs/richacl.c
> +++ b/fs/richacl.c
> @@ -181,3 +181,160 @@ richacl_want_to_mask(unsigned int want)
>  	return mask;
>  }
>  EXPORT_SYMBOL_GPL(richacl_want_to_mask);
> +
> +/*
> + * Note: functions like richacl_allowed_to_who(), richacl_group_class_allowed(),
> + * and richacl_compute_max_masks() iterate through the entire acl in reverse
> + * order as an optimization.
> + *
> + * In the standard algorithm, aces are considered in forward order.  When a
> + * process matches an ace, the permissions in the ace are either allowed or
> + * denied depending on the ace type.  Once a permission has been allowed or
> + * denied, it is no longer considered in further aces.
> + *
> + * By iterating through the acl in reverse order, we can compute the same
> + * result without having to keep track of which permissions have been allowed
> + * and denied already.
> + */
> 

Clever!


> +
> +/**
> + * richacl_allowed_to_who  -  permissions allowed to a specific who value
> + *
> + * Compute the maximum mask values allowed to a specific who value, taking
> + * everyone@ aces into account.
> + */
> +static unsigned int richacl_allowed_to_who(struct richacl *acl,
> +					   struct richace *who)
> +{
> +	struct richace *ace;
> +	unsigned int allowed = 0;
> +
> +	richacl_for_each_entry_reverse(ace, acl) {
> +		if (richace_is_inherit_only(ace))
> +			continue;
> +		if (richace_is_same_identifier(ace, who) ||
> +		    richace_is_everyone(ace)) {
> +			if (richace_is_allow(ace))
> +				allowed |= ace->e_mask;
> +			else if (richace_is_deny(ace))
> +				allowed &= ~ace->e_mask;
> +		}
> +	}
> +	return allowed;
> +}
> +
> +/**
> + * richacl_group_class_allowed  -  maximum permissions of the group class
> + *
> + * Compute the maximum mask values allowed to a process in the group class
> + * (i.e., a process which is not the owner but is in the owning group or
> + * matches a user or group acl entry).  This includes permissions granted or
> + * denied by everyone@ aces.
> + *
> + * See richacl_compute_max_masks().
> + */
> +static unsigned int richacl_group_class_allowed(struct richacl *acl)
> +{
> +	struct richace *ace;
> +	unsigned int everyone_allowed = 0, group_class_allowed = 0;
> +	int had_group_ace = 0;
> +
> +	richacl_for_each_entry_reverse(ace, acl) {
> +		if (richace_is_inherit_only(ace) ||
> +		    richace_is_owner(ace))
> +			continue;
> +
> +		if (richace_is_everyone(ace)) {
> +			if (richace_is_allow(ace))
> +				everyone_allowed |= ace->e_mask;
> +			else if (richace_is_deny(ace))
> +				everyone_allowed &= ~ace->e_mask;
> +		} else {
> +			group_class_allowed |=
> +				richacl_allowed_to_who(acl, ace);
> +
> +			if (richace_is_group(ace))
> +				had_group_ace = 1;
> +		}
> +	}
> +	/*
> +	 * If the acl doesn't contain any group@ aces, richacl_allowed_to_who()
> +	 * wasn't called for the owning group.  We could make that call now, but
> +	 * we already know the result (everyone_allowed).
> +	 */
> +	if (!had_group_ace)
> +		group_class_allowed |= everyone_allowed;
> +	return group_class_allowed;
> +}
> +
> +/**
> + * richacl_compute_max_masks  -  compute upper bound masks
> + *
> + * Computes upper bound owner, group, and other masks so that none of the
> + * permissions allowed by the acl are disabled.
> + *
> + * We don't make assumptions about who the owner is so that the owner can
> + * change with no effect on the file masks or file mode permission bits; this
> + * means that we must assume that all entries can match the owner.
> + */
> +void richacl_compute_max_masks(struct richacl *acl)
> +{
> +	unsigned int gmask = ~0;
> +	struct richace *ace;
> +
> +	/*
> +	 * @gmask contains all permissions which the group class is ever
> +	 * allowed.  We use it to avoid adding permissions to the group mask
> +	 * from everyone@ allow aces which the group class is always denied
> +	 * through other aces.  For example, the following acl would otherwise
> +	 * result in a group mask of rw:
> +	 *
> +	 *	group@:w::deny
> +	 *	everyone@:rw::allow
> +	 *
> +	 * Avoid computing @gmask for acls which do not include any group class
> +	 * deny aces: in such acls, the group class is never denied any
> +	 * permissions from everyone@ allow aces, and the group class cannot
> +	 * have fewer permissions than the other class.
> +	 */
> +
> +restart:
> +	acl->a_owner_mask = 0;
> +	acl->a_group_mask = 0;
> +	acl->a_other_mask = 0;
> +
> +	richacl_for_each_entry_reverse(ace, acl) {
> +		if (richace_is_inherit_only(ace))
> +			continue;
> +
> +		if (richace_is_owner(ace)) {
> +			if (richace_is_allow(ace))
> +				acl->a_owner_mask |= ace->e_mask;
> +			else if (richace_is_deny(ace))
> +				acl->a_owner_mask &= ~ace->e_mask;
> +		} else if (richace_is_everyone(ace)) {
> +			if (richace_is_allow(ace)) {
> +				acl->a_owner_mask |= ace->e_mask;
> +				acl->a_group_mask |= ace->e_mask & gmask;
> +				acl->a_other_mask |= ace->e_mask;
> +			} else if (richace_is_deny(ace)) {
> +				acl->a_owner_mask &= ~ace->e_mask;
> +				acl->a_group_mask &= ~ace->e_mask;
> +				acl->a_other_mask &= ~ace->e_mask;
> +			}
> +		} else {
> +			if (richace_is_allow(ace)) {
> +				acl->a_owner_mask |= ace->e_mask & gmask;
> +				acl->a_group_mask |= ace->e_mask & gmask;
> +			} else if (richace_is_deny(ace) && gmask == ~0) {
> +				gmask = richacl_group_class_allowed(acl);
> +				if (likely(gmask != ~0))
> +					/* should always be true */
> +					goto restart;
> +			}
> +		}
> +	}
> +
> +	acl->a_flags &= ~(RICHACL_WRITE_THROUGH | RICHACL_MASKED);
> +}
> +EXPORT_SYMBOL_GPL(richacl_compute_max_masks);
> diff --git a/include/linux/richacl.h b/include/linux/richacl.h
> index 9102ef0..3559b2c 100644
> --- a/include/linux/richacl.h
> +++ b/include/linux/richacl.h
> @@ -178,5 +178,6 @@ extern void richace_copy(struct richace *, const struct richace *);
>  extern int richacl_masks_to_mode(const struct richacl *);
>  extern unsigned int richacl_mode_to_mask(umode_t);
>  extern unsigned int richacl_want_to_mask(unsigned int);
> +extern void richacl_compute_max_masks(struct richacl *);
>  
>  #endif /* __RICHACL_H */

Reviewed-by: Jeff Layton <jlayton@redhat.com>
Frank Filz July 5, 2016, 5:08 p.m. UTC | #2
> > + * Note: functions like richacl_allowed_to_who(),
> > +richacl_group_class_allowed(),
> > + * and richacl_compute_max_masks() iterate through the entire acl in
> > +reverse
> > + * order as an optimization.
> > + *
> > + * In the standard algorithm, aces are considered in forward order.
> > +When a
> > + * process matches an ace, the permissions in the ace are either
> > +allowed or
> > + * denied depending on the ace type.  Once a permission has been
> > +allowed or
> > + * denied, it is no longer considered in further aces.
> > + *
> > + * By iterating through the acl in reverse order, we can compute the
> > +same
> > + * result without having to keep track of which permissions have been
> > +allowed
> > + * and denied already.
> > + */
> >
> 
> Clever!

Hmm, but does that result in examining the whole ACL for most access checks, at least for files where most of the accesses are by the owner, or a member of a specific group (with perhaps a ton of special case users added on the end)?

Frank

> > +
> > +/**
> > + * richacl_allowed_to_who  -  permissions allowed to a specific who
> > +value
> > + *
> > + * Compute the maximum mask values allowed to a specific who value,
> > +taking
> > + * everyone@ aces into account.
> > + */
> > +static unsigned int richacl_allowed_to_who(struct richacl *acl,
> > +					   struct richace *who)
> > +{
> > +	struct richace *ace;
> > +	unsigned int allowed = 0;
> > +
> > +	richacl_for_each_entry_reverse(ace, acl) {
> > +		if (richace_is_inherit_only(ace))
> > +			continue;
> > +		if (richace_is_same_identifier(ace, who) ||
> > +		    richace_is_everyone(ace)) {
> > +			if (richace_is_allow(ace))
> > +				allowed |= ace->e_mask;
> > +			else if (richace_is_deny(ace))
> > +				allowed &= ~ace->e_mask;
> > +		}
> > +	}
> > +	return allowed;
> > +}
> > +
> > +/**
> > + * richacl_group_class_allowed  -  maximum permissions of the group
> > +class
> > + *
> > + * Compute the maximum mask values allowed to a process in the group
> > +class
> > + * (i.e., a process which is not the owner but is in the owning group
> > +or
> > + * matches a user or group acl entry).  This includes permissions
> > +granted or
> > + * denied by everyone@ aces.
> > + *
> > + * See richacl_compute_max_masks().
> > + */
> > +static unsigned int richacl_group_class_allowed(struct richacl *acl)
> > +{
> > +	struct richace *ace;
> > +	unsigned int everyone_allowed = 0, group_class_allowed = 0;
> > +	int had_group_ace = 0;
> > +
> > +	richacl_for_each_entry_reverse(ace, acl) {
> > +		if (richace_is_inherit_only(ace) ||
> > +		    richace_is_owner(ace))
> > +			continue;
> > +
> > +		if (richace_is_everyone(ace)) {
> > +			if (richace_is_allow(ace))
> > +				everyone_allowed |= ace->e_mask;
> > +			else if (richace_is_deny(ace))
> > +				everyone_allowed &= ~ace->e_mask;
> > +		} else {
> > +			group_class_allowed |=
> > +				richacl_allowed_to_who(acl, ace);
> > +
> > +			if (richace_is_group(ace))
> > +				had_group_ace = 1;
> > +		}
> > +	}
> > +	/*
> > +	 * If the acl doesn't contain any group@ aces,
> richacl_allowed_to_who()
> > +	 * wasn't called for the owning group.  We could make that call now,
> but
> > +	 * we already know the result (everyone_allowed).
> > +	 */
> > +	if (!had_group_ace)
> > +		group_class_allowed |= everyone_allowed;
> > +	return group_class_allowed;
> > +}
> > +
> > +/**
> > + * richacl_compute_max_masks  -  compute upper bound masks
> > + *
> > + * Computes upper bound owner, group, and other masks so that none of
> > +the
> > + * permissions allowed by the acl are disabled.
> > + *
> > + * We don't make assumptions about who the owner is so that the owner
> > +can
> > + * change with no effect on the file masks or file mode permission
> > +bits; this
> > + * means that we must assume that all entries can match the owner.
> > + */
> > +void richacl_compute_max_masks(struct richacl *acl) {
> > +	unsigned int gmask = ~0;
> > +	struct richace *ace;
> > +
> > +	/*
> > +	 * @gmask contains all permissions which the group class is ever
> > +	 * allowed.  We use it to avoid adding permissions to the group mask
> > +	 * from everyone@ allow aces which the group class is always denied
> > +	 * through other aces.  For example, the following acl would
> otherwise
> > +	 * result in a group mask of rw:
> > +	 *
> > +	 *	group@:w::deny
> > +	 *	everyone@:rw::allow
> > +	 *
> > +	 * Avoid computing @gmask for acls which do not include any group
> class
> > +	 * deny aces: in such acls, the group class is never denied any
> > +	 * permissions from everyone@ allow aces, and the group class
> cannot
> > +	 * have fewer permissions than the other class.
> > +	 */
> > +
> > +restart:
> > +	acl->a_owner_mask = 0;
> > +	acl->a_group_mask = 0;
> > +	acl->a_other_mask = 0;
> > +
> > +	richacl_for_each_entry_reverse(ace, acl) {
> > +		if (richace_is_inherit_only(ace))
> > +			continue;
> > +
> > +		if (richace_is_owner(ace)) {
> > +			if (richace_is_allow(ace))
> > +				acl->a_owner_mask |= ace->e_mask;
> > +			else if (richace_is_deny(ace))
> > +				acl->a_owner_mask &= ~ace->e_mask;
> > +		} else if (richace_is_everyone(ace)) {
> > +			if (richace_is_allow(ace)) {
> > +				acl->a_owner_mask |= ace->e_mask;
> > +				acl->a_group_mask |= ace->e_mask &
> gmask;
> > +				acl->a_other_mask |= ace->e_mask;
> > +			} else if (richace_is_deny(ace)) {
> > +				acl->a_owner_mask &= ~ace->e_mask;
> > +				acl->a_group_mask &= ~ace->e_mask;
> > +				acl->a_other_mask &= ~ace->e_mask;
> > +			}
> > +		} else {
> > +			if (richace_is_allow(ace)) {
> > +				acl->a_owner_mask |= ace->e_mask &
> gmask;
> > +				acl->a_group_mask |= ace->e_mask &
> gmask;
> > +			} else if (richace_is_deny(ace) && gmask == ~0) {
> > +				gmask = richacl_group_class_allowed(acl);
> > +				if (likely(gmask != ~0))
> > +					/* should always be true */
> > +					goto restart;
> > +			}
> > +		}
> > +	}
> > +
> > +	acl->a_flags &= ~(RICHACL_WRITE_THROUGH | RICHACL_MASKED); }
> > +EXPORT_SYMBOL_GPL(richacl_compute_max_masks);
> > diff --git a/include/linux/richacl.h b/include/linux/richacl.h index
> > 9102ef0..3559b2c 100644
> > --- a/include/linux/richacl.h
> > +++ b/include/linux/richacl.h
> > @@ -178,5 +178,6 @@ extern void richace_copy(struct richace *, const
> > struct richace *);
> >  extern int richacl_masks_to_mode(const struct richacl *);
> >  extern unsigned int richacl_mode_to_mask(umode_t);
> >  extern unsigned int richacl_want_to_mask(unsigned int);
> > +extern void richacl_compute_max_masks(struct richacl *);
> >
> >  #endif /* __RICHACL_H */
> 
> Reviewed-by: Jeff Layton <jlayton@redhat.com>
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Andreas Gruenbacher July 13, 2016, 12:34 p.m. UTC | #3
Frank,

On Tue, Jul 5, 2016 at 7:08 PM, Frank Filz <ffilzlnx@mindspring.com> wrote:
>> > + * Note: functions like richacl_allowed_to_who(),
>> > +richacl_group_class_allowed(),
>> > + * and richacl_compute_max_masks() iterate through the entire acl in
>> > +reverse
>> > + * order as an optimization.
>> > + *
>> > + * In the standard algorithm, aces are considered in forward order.
>> > +When a
>> > + * process matches an ace, the permissions in the ace are either
>> > +allowed or
>> > + * denied depending on the ace type.  Once a permission has been
>> > +allowed or
>> > + * denied, it is no longer considered in further aces.
>> > + *
>> > + * By iterating through the acl in reverse order, we can compute the
>> > +same
>> > + * result without having to keep track of which permissions have been
>> > +allowed
>> > + * and denied already.
>> > + */
>> >
>>
>> Clever!
>
> Hmm, but does that result in examining the whole ACL for most access checks, at least for files where most of the accesses are by the owner, or a member of a specific group (with perhaps a ton of special case users added on the end)?

I don't understand -- what does this algorithm have to do with access checks?

Thanks,
Andreas
Frank Filz July 13, 2016, 7:38 p.m. UTC | #4
> > Hmm, but does that result in examining the whole ACL for most access
> checks, at least for files where most of the accesses are by the owner, or a
> member of a specific group (with perhaps a ton of special case users added
> on the end)?
> 
> I don't understand -- what does this algorithm have to do with access checks?

Oh, sorry, misread the patch... got caught up looking at a tree and not seeing the forest...

Frank



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Patch
diff mbox series

diff --git a/fs/richacl.c b/fs/richacl.c
index d0a4135..056228f 100644
--- a/fs/richacl.c
+++ b/fs/richacl.c
@@ -181,3 +181,160 @@  richacl_want_to_mask(unsigned int want)
 	return mask;
 }
 EXPORT_SYMBOL_GPL(richacl_want_to_mask);
+
+/*
+ * Note: functions like richacl_allowed_to_who(), richacl_group_class_allowed(),
+ * and richacl_compute_max_masks() iterate through the entire acl in reverse
+ * order as an optimization.
+ *
+ * In the standard algorithm, aces are considered in forward order.  When a
+ * process matches an ace, the permissions in the ace are either allowed or
+ * denied depending on the ace type.  Once a permission has been allowed or
+ * denied, it is no longer considered in further aces.
+ *
+ * By iterating through the acl in reverse order, we can compute the same
+ * result without having to keep track of which permissions have been allowed
+ * and denied already.
+ */
+
+/**
+ * richacl_allowed_to_who  -  permissions allowed to a specific who value
+ *
+ * Compute the maximum mask values allowed to a specific who value, taking
+ * everyone@ aces into account.
+ */
+static unsigned int richacl_allowed_to_who(struct richacl *acl,
+					   struct richace *who)
+{
+	struct richace *ace;
+	unsigned int allowed = 0;
+
+	richacl_for_each_entry_reverse(ace, acl) {
+		if (richace_is_inherit_only(ace))
+			continue;
+		if (richace_is_same_identifier(ace, who) ||
+		    richace_is_everyone(ace)) {
+			if (richace_is_allow(ace))
+				allowed |= ace->e_mask;
+			else if (richace_is_deny(ace))
+				allowed &= ~ace->e_mask;
+		}
+	}
+	return allowed;
+}
+
+/**
+ * richacl_group_class_allowed  -  maximum permissions of the group class
+ *
+ * Compute the maximum mask values allowed to a process in the group class
+ * (i.e., a process which is not the owner but is in the owning group or
+ * matches a user or group acl entry).  This includes permissions granted or
+ * denied by everyone@ aces.
+ *
+ * See richacl_compute_max_masks().
+ */
+static unsigned int richacl_group_class_allowed(struct richacl *acl)
+{
+	struct richace *ace;
+	unsigned int everyone_allowed = 0, group_class_allowed = 0;
+	int had_group_ace = 0;
+
+	richacl_for_each_entry_reverse(ace, acl) {
+		if (richace_is_inherit_only(ace) ||
+		    richace_is_owner(ace))
+			continue;
+
+		if (richace_is_everyone(ace)) {
+			if (richace_is_allow(ace))
+				everyone_allowed |= ace->e_mask;
+			else if (richace_is_deny(ace))
+				everyone_allowed &= ~ace->e_mask;
+		} else {
+			group_class_allowed |=
+				richacl_allowed_to_who(acl, ace);
+
+			if (richace_is_group(ace))
+				had_group_ace = 1;
+		}
+	}
+	/*
+	 * If the acl doesn't contain any group@ aces, richacl_allowed_to_who()
+	 * wasn't called for the owning group.  We could make that call now, but
+	 * we already know the result (everyone_allowed).
+	 */
+	if (!had_group_ace)
+		group_class_allowed |= everyone_allowed;
+	return group_class_allowed;
+}
+
+/**
+ * richacl_compute_max_masks  -  compute upper bound masks
+ *
+ * Computes upper bound owner, group, and other masks so that none of the
+ * permissions allowed by the acl are disabled.
+ *
+ * We don't make assumptions about who the owner is so that the owner can
+ * change with no effect on the file masks or file mode permission bits; this
+ * means that we must assume that all entries can match the owner.
+ */
+void richacl_compute_max_masks(struct richacl *acl)
+{
+	unsigned int gmask = ~0;
+	struct richace *ace;
+
+	/*
+	 * @gmask contains all permissions which the group class is ever
+	 * allowed.  We use it to avoid adding permissions to the group mask
+	 * from everyone@ allow aces which the group class is always denied
+	 * through other aces.  For example, the following acl would otherwise
+	 * result in a group mask of rw:
+	 *
+	 *	group@:w::deny
+	 *	everyone@:rw::allow
+	 *
+	 * Avoid computing @gmask for acls which do not include any group class
+	 * deny aces: in such acls, the group class is never denied any
+	 * permissions from everyone@ allow aces, and the group class cannot
+	 * have fewer permissions than the other class.
+	 */
+
+restart:
+	acl->a_owner_mask = 0;
+	acl->a_group_mask = 0;
+	acl->a_other_mask = 0;
+
+	richacl_for_each_entry_reverse(ace, acl) {
+		if (richace_is_inherit_only(ace))
+			continue;
+
+		if (richace_is_owner(ace)) {
+			if (richace_is_allow(ace))
+				acl->a_owner_mask |= ace->e_mask;
+			else if (richace_is_deny(ace))
+				acl->a_owner_mask &= ~ace->e_mask;
+		} else if (richace_is_everyone(ace)) {
+			if (richace_is_allow(ace)) {
+				acl->a_owner_mask |= ace->e_mask;
+				acl->a_group_mask |= ace->e_mask & gmask;
+				acl->a_other_mask |= ace->e_mask;
+			} else if (richace_is_deny(ace)) {
+				acl->a_owner_mask &= ~ace->e_mask;
+				acl->a_group_mask &= ~ace->e_mask;
+				acl->a_other_mask &= ~ace->e_mask;
+			}
+		} else {
+			if (richace_is_allow(ace)) {
+				acl->a_owner_mask |= ace->e_mask & gmask;
+				acl->a_group_mask |= ace->e_mask & gmask;
+			} else if (richace_is_deny(ace) && gmask == ~0) {
+				gmask = richacl_group_class_allowed(acl);
+				if (likely(gmask != ~0))
+					/* should always be true */
+					goto restart;
+			}
+		}
+	}
+
+	acl->a_flags &= ~(RICHACL_WRITE_THROUGH | RICHACL_MASKED);
+}
+EXPORT_SYMBOL_GPL(richacl_compute_max_masks);
diff --git a/include/linux/richacl.h b/include/linux/richacl.h
index 9102ef0..3559b2c 100644
--- a/include/linux/richacl.h
+++ b/include/linux/richacl.h
@@ -178,5 +178,6 @@  extern void richace_copy(struct richace *, const struct richace *);
 extern int richacl_masks_to_mode(const struct richacl *);
 extern unsigned int richacl_mode_to_mask(umode_t);
 extern unsigned int richacl_want_to_mask(unsigned int);
+extern void richacl_compute_max_masks(struct richacl *);
 
 #endif /* __RICHACL_H */