[PATCH 2/2] fs: avoid unnecessary prepare_creds in faccessat

[Mateusz Guzik <mguzik@xxxxxxxxxx>]

Sometimes faccessat needs to modify current thread's credentials, but
calls prepare_creds unconditionally.

Take advantage of the fact that we can detect whether any modification
to credentials is needed and in turn avoid unnecessary allocations.

Signed-off-by: Mateusz Guzik <mguzik@xxxxxxxxxx>
---
 fs/open.c | 53 +++++++++++++++++++++++++++++++++++------------------
 1 file changed, 35 insertions(+), 18 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index 33f9cbf..166eb45 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -330,8 +330,10 @@ SYSCALL_DEFINE4(fallocate, int, fd, int, mode, loff_t, offset, loff_t, len)
  */
 SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)
 {
-	const struct cred *old_cred;
-	struct cred *override_cred;
+	const struct cred *old_cred = current_cred();
+	struct cred *override_cred = NULL;
+	kernel_cap_t cap_effective;
+	int modify_cap_effective = 0;
 	struct path path;
 	struct inode *inode;
 	int res;
@@ -340,24 +342,37 @@ SYSCALL_DEFINE3(faccessat, int, dfd, const char __user *, filename, int, mode)
 	if (mode & ~S_IRWXO)	/* where's F_OK, X_OK, W_OK, R_OK? */
 		return -EINVAL;
 
-	override_cred = prepare_creds();
-	if (!override_cred)
-		return -ENOMEM;
-
-	override_cred->fsuid = override_cred->uid;
-	override_cred->fsgid = override_cred->gid;
-
 	if (!issecure(SECURE_NO_SETUID_FIXUP)) {
 		/* Clear the capabilities if we switch to a non-root user */
-		kuid_t root_uid = make_kuid(override_cred->user_ns, 0);
-		if (!uid_eq(override_cred->uid, root_uid))
-			cap_clear(override_cred->cap_effective);
-		else
-			override_cred->cap_effective =
-				override_cred->cap_permitted;
+		kuid_t root_uid = make_kuid(old_cred->user_ns, 0);
+		if (!uid_eq(old_cred->uid, root_uid)) {
+			if (!cap_isclear(old_cred->cap_effective)) {
+				cap_clear(cap_effective);
+				modify_cap_effective = 1;
+			}
+		} else {
+			if (!cap_isequal(old_cred->cap_effective,
+				old_cred->cap_permitted)) {
+				cap_effective = old_cred->cap_permitted;
+				modify_cap_effective = 1;
+			}
+		}
 	}
 
-	old_cred = override_creds(override_cred);
+	if (!uid_eq(old_cred->fsuid, old_cred->uid) ||
+	    !gid_eq(old_cred->fsgid, old_cred->gid) ||
+	    modify_cap_effective) {
+		override_cred = prepare_creds();
+		if (!override_cred)
+			return -ENOMEM;
+
+		override_cred->fsuid = override_cred->uid;
+		override_cred->fsgid = override_cred->gid;
+		if (modify_cap_effective)
+			override_cred->cap_effective = cap_effective;
+
+		override_creds(override_cred);
+	}
 retry:
 	res = user_path_at(dfd, filename, lookup_flags, &path);
 	if (res)
@@ -399,8 +414,10 @@ out_path_release:
 		goto retry;
 	}
 out:
-	revert_creds(old_cred);
-	put_cred(override_cred);
+	if (override_cred) {
+		revert_creds(old_cred);
+		put_cred(override_cred);
+	}
 	return res;
 }
 
-- 
1.8.3.1

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