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

On Mon, Sep 21, 2015 at 09:59:07AM -0400, Austin S Hemmelgarn wrote:

On 2015-09-17 20:56, J. Bruce Fields wrote:

quoted

On Thu, Sep 17, 2015 at 02:22:19PM -0400, bfields wrote:

quoted

On Sat, Sep 05, 2015 at 12:27:08PM +0200, Andreas Gruenbacher wrote:

quoted

ACLs are considered equivalent to file modes if they only consist of
owner@, group@, and everyone@ entries, the owner@ permissions do not
depend on whether the owner is a member in the owning group, and no
inheritance flags are set.  This test is used to avoid storing richacls
if the acl can be computed from the file permission bits.

We're assuming here that it's OK for us to silently rearrange an ACL as
long as the result is still equivalent (in the sense that the permission
algorithm would always produce the same result).

I guess that's OK by me, but it might violate user expectations in some
simple common cases, so may be worth mentioning in documentation
someplace if we don't already.

Also your notion of mode-equivalence here is interesting, it's actually
a strict subset of the ACLs that produce the same permission results as
a mode.  (For example, everyone:rwx,bfields:rwx is equivalent to 0777
but won't be considered mode-equivalent by this algorithm.)

Although it could also be equivalent to 0707, or (if bfields is the
group name also) 0077, or even (if bfields isn't the group or owner
of the file) 0007.

I disagree.  I think you've misread my example ACL (may be my sloppy
notation, sorry) or misunderstood the ACL evalutation algorithm.

Mode equivalence get's even trickier when you
throw in permissions just beyond rwx (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 believe these patches handle all of those details correctly; if you
see anything to the contrary, please do speak up.

Note that Windows also has a DELETE bit, though it is ORed with the
directory permission not ANDed (so it is sufficient for the directory to
allow MAY_DELETE_CHILD *or* for the file to allow DELETE).

(But I believe you're correct that VMS required both permissions, if
e.g. http://www.djesys.com/vms/freevms/mentor/vms_prot.html is correct).

--b.

quoted

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 <redacted>

--b.

quoted

--b.

quoted

Signed-off-by: Andreas Gruenbacher <redacted>
---
 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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