Re: [RFC v7 13/41] richacl: Check if an acl is equivalent to a file mode

[J. Bruce Fields]

On Mon, Sep 21, 2015 at 09:59:07AM -0400, Austin S Hemmelgarn wrote:
> Mode equivalence get's even trickier when you throw in permissions
> just beyond rwx

Note by the way that the major technical achievement here is the
reconciliation of two completely different evaluation algorithms.
(Windows/v4-like ACLs are evaluated top-to-bottom and stop as soon as
the requested permissions are allowed, or a remaining requested
permission is denied. Mode bits match the requester first and then check
the permissions associated with that requester.)

(You're correct that reconciling the permission bits is tricky (and
requires some judgement calls), but that's conceptually simpler.)

And then the handling of ID's looks like the major remaining problem.

--b.

> (for example, by Windows
> standards, the usage of the execute bit on directories is weird
> (they have a separate permission in their ACE's for directory
> listing), or by VMS standards, write permission on a directory
> doesn't mean that you can delete things in it (VMS actually had a
> separate bit for the delete permission, and even had separate
> permissions for system access)).
> >
> >I think the choices you've made probably make the most sense, they just
> >wouldn't have been obvious to me.  Anyway, so, OK by me:
> >
> >	Reviewed-by: J. Bruce Fields <bfields@xxxxxxxxxx>
> >
> >--b.
> >
> >>
> >>--b.
> >>
> >>>
> >>>Signed-off-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
> >>>---
> >>>  fs/richacl_base.c       | 104 ++++++++++++++++++++++++++++++++++++++++++++++++
> >>>  include/linux/richacl.h |   1 +
> >>>  2 files changed, 105 insertions(+)
> >>>
> >>>diff --git a/fs/richacl_base.c b/fs/richacl_base.c
> >>>index 3163152..106e988 100644
> >>>--- a/fs/richacl_base.c
> >>>+++ b/fs/richacl_base.c
> >>>@@ -379,3 +379,107 @@ richacl_chmod(struct richacl *acl, mode_t mode)
> >>>  	return clone;
> >>>  }
> >>>  EXPORT_SYMBOL_GPL(richacl_chmod);
> >>>+
> >>>+/**
> >>>+ * richacl_equiv_mode  -  compute the mode equivalent of @acl
> >>>+ *
> >>>+ * An acl is considered equivalent to a file mode if it only consists of
> >>>+ * owner@, group@, and everyone@ entries and the owner@ permissions do not
> >>>+ * depend on whether the owner is a member in the owning group.
> >>>+ */
> >>>+int
> >>>+richacl_equiv_mode(const struct richacl *acl, mode_t *mode_p)
> >>>+{
> >>>+	mode_t mode = *mode_p;
> >>>+
> >>>+	/*
> >>>+	 * The RICHACE_DELETE_CHILD flag is meaningless for non-directories, so
> >>>+	 * we ignore it.
> >>>+	 */
> >>>+	unsigned int x = S_ISDIR(mode) ? 0 : RICHACE_DELETE_CHILD;
> >>>+	struct {
> >>>+		unsigned int allowed;
> >>>+		unsigned int defined;  /* allowed or denied */
> >>>+	} owner = {
> >>>+		.defined = RICHACE_POSIX_ALWAYS_ALLOWED |
> >>>+			   RICHACE_POSIX_OWNER_ALLOWED  | x,
> >>>+	}, group = {
> >>>+		.defined = RICHACE_POSIX_ALWAYS_ALLOWED | x,
> >>>+	}, everyone = {
> >>>+		.defined = RICHACE_POSIX_ALWAYS_ALLOWED | x,
> >>>+	};
> >>>+	const struct richace *ace;
> >>>+
> >>>+	if (acl->a_flags & ~(RICHACL_WRITE_THROUGH | RICHACL_MASKED))
> >>>+		return -1;
> >>>+
> >>>+	richacl_for_each_entry(ace, acl) {
> >>>+		if (ace->e_flags & ~RICHACE_SPECIAL_WHO)
> >>>+			return -1;
> >>>+
> >>>+		if (richace_is_owner(ace) || richace_is_everyone(ace)) {
> >>>+			x = ace->e_mask & ~owner.defined;
> >>>+			if (richace_is_allow(ace)) {
> >>>+				unsigned int group_denied =
> >>>+					group.defined & ~group.allowed;
> >>>+
> >>>+				if (x & group_denied)
> >>>+					return -1;
> >>>+				owner.allowed |= x;
> >>>+			} else /* if (richace_is_deny(ace)) */ {
> >>>+				if (x & group.allowed)
> >>>+					return -1;
> >>>+			}
> >>>+			owner.defined |= x;
> >>>+
> >>>+			if (richace_is_everyone(ace)) {
> >>>+				x = ace->e_mask;
> >>>+				if (richace_is_allow(ace)) {
> >>>+					group.allowed |=
> >>>+						x & ~group.defined;
> >>>+					everyone.allowed |=
> >>>+						x & ~everyone.defined;
> >>>+				}
> >>>+				group.defined |= x;
> >>>+				everyone.defined |= x;
> >>>+			}
> >>>+		} else if (richace_is_group(ace)) {
> >>>+			x = ace->e_mask & ~group.defined;
> >>>+			if (richace_is_allow(ace))
> >>>+				group.allowed |= x;
> >>>+			group.defined |= x;
> >>>+		} else
> >>>+			return -1;
> >>>+	}
> >>>+
> >>>+	if (group.allowed & ~owner.defined)
> >>>+		return -1;
> >>>+
> >>>+	if (acl->a_flags & RICHACL_MASKED) {
> >>>+		if (acl->a_flags & RICHACL_WRITE_THROUGH) {
> >>>+			owner.allowed = acl->a_owner_mask;
> >>>+			everyone.allowed = acl->a_other_mask;
> >>>+		} else {
> >>>+			owner.allowed &= acl->a_owner_mask;
> >>>+			everyone.allowed &= acl->a_other_mask;
> >>>+		}
> >>>+		group.allowed &= acl->a_group_mask;
> >>>+	}
> >>>+
> >>>+	mode = (mode & ~S_IRWXUGO) |
> >>>+	       (richacl_mask_to_mode(owner.allowed) << 6) |
> >>>+	       (richacl_mask_to_mode(group.allowed) << 3) |
> >>>+		richacl_mask_to_mode(everyone.allowed);
> >>>+
> >>>+	/* Mask flags we can ignore */
> >>>+	x = S_ISDIR(mode) ? 0 : RICHACE_DELETE_CHILD;
> >>>+
> >>>+	if (((richacl_mode_to_mask(mode >> 6) ^ owner.allowed)    & ~x) ||
> >>>+	    ((richacl_mode_to_mask(mode >> 3) ^ group.allowed)    & ~x) ||
> >>>+	    ((richacl_mode_to_mask(mode)      ^ everyone.allowed) & ~x))
> >>>+		return -1;
> >>>+
> >>>+	*mode_p = mode;
> >>>+	return 0;
> >>>+}
> >>>+EXPORT_SYMBOL_GPL(richacl_equiv_mode);
> >>>diff --git a/include/linux/richacl.h b/include/linux/richacl.h
> >>>index d4a576c..6535ce5 100644
> >>>--- a/include/linux/richacl.h
> >>>+++ b/include/linux/richacl.h
> >>>@@ -304,6 +304,7 @@ extern unsigned int richacl_mode_to_mask(mode_t);
> >>>  extern unsigned int richacl_want_to_mask(unsigned int);
> >>>  extern void richacl_compute_max_masks(struct richacl *);
> >>>  extern struct richacl *richacl_chmod(struct richacl *, mode_t);
> >>>+extern int richacl_equiv_mode(const struct richacl *, mode_t *);
> >>>
> >>>  /* richacl_inode.c */
> >>>  extern int richacl_permission(struct inode *, const struct richacl *, int);
> >>>--
> >>>2.4.3
> >>>
> >>>--
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> 
> 


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