linux-kernel - [PATCH 1/4] cred: add cred_fscmp() for comparing creds.

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Message-ID: <151737573947.14845.5248732383783116263.stgit@noble>
Date:   Wed, 31 Jan 2018 16:15:39 +1100
From:   NeilBrown <neilb@...e.com>
To:     David Howells <dhowells@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Anna Schumaker <Anna.Schumaker@...app.com>
Cc:     NFS <linux-nfs@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: [PATCH 1/4] cred: add cred_fscmp() for comparing creds.

NFS needs to compare to credentials, to see if they can
be treated the same w.r.t. filesystem access.  Sometimes
an ordering is needed when credentials are used as a key
to an rbtree.
NFS current has its own private credential management from
before 'struct cred' existed.  To move it over to more consistent
use of 'struct cred' we need a comparison function.
This patch adds that function.

Signed-off-by: NeilBrown <neilb@...e.com>
---
 include/linux/cred.h |    1 +
 kernel/cred.c        |   55 ++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+)

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 631286535d0f..6dd51e503f23 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -164,6 +164,7 @@ extern int change_create_files_as(struct cred *, struct inode *);
 extern int set_security_override(struct cred *, u32);
 extern int set_security_override_from_ctx(struct cred *, const char *);
 extern int set_create_files_as(struct cred *, struct inode *);
+extern int cred_fscmp(const struct cred *, const struct cred *);
 extern void __init cred_init(void);
 
 /*
diff --git a/kernel/cred.c b/kernel/cred.c
index ecf03657e71c..4ce75c6fb752 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -19,6 +19,7 @@
 #include <linux/security.h>
 #include <linux/binfmts.h>
 #include <linux/cn_proc.h>
+#include <linux/uidgid.h>
 
 #if 0
 #define kdebug(FMT, ...)						\
@@ -564,6 +565,60 @@ void revert_creds(const struct cred *old)
 }
 EXPORT_SYMBOL(revert_creds);
 
+/**
+ * cred_fscmp - Compare to credentials with respect to filesystem access.
+ * @a: The first credential
+ * @b: The second credential
+ *
+ * cred_cmp() will return zero if both credentials have the same
+ * fsuid, fsgid, and supplementary groups.  That is, if they will both
+ * provide the same access to files based on mode/uid/gid.
+ * If the credentials are different, then either -1 or 1 will
+ * be returned depending on whether @a comes before or after @b
+ * respectively in an arbitrary, but stable, ordering of credentials.
+ *
+ * Return: -1, 0, or 1 depending on comparison
+ */
+int cred_fscmp(const struct cred *a, const struct cred *b)
+{
+	struct group_info *ga, *gb;
+	int g;
+
+	if (a == b)
+		return 0;
+	if (uid_lt(a->fsuid, b->fsuid))
+		return -1;
+	if (uid_gt(a->fsuid, b->fsuid))
+		return 1;
+
+	if (gid_lt(a->fsgid, b->fsgid))
+		return -1;
+	if (gid_gt(a->fsgid, b->fsgid))
+		return 1;
+
+	ga = a->group_info;
+	gb = b->group_info;
+	if (ga == gb)
+		return 0;
+	if (ga == NULL)
+		return -1;
+	if (gb == NULL)
+		return 1;
+	if (ga->ngroups < gb->ngroups)
+		return -1;
+	if (ga->ngroups > gb->ngroups)
+		return 1;
+
+	for (g = 0; g < ga->ngroups; g++) {
+		if (gid_lt(ga->gid[g], gb->gid[g]))
+			return -1;
+		if (gid_gt(ga->gid[g], gb->gid[g]))
+			return 1;
+	}
+	return 0;
+}
+EXPORT_SYMBOL(cred_fscmp);
+
 /*
  * initialise the credentials stuff
  */