Merge patch series "creds: add {scoped_}with_kernel_creds()"

Christian Brauner <brauner@kernel.org> says:

A few months ago I did work to make override_creds()/revert_creds()
completely reference count free - mostly for the sake of
overlayfs but it has been beneficial to everyone using this.

In a recent pull request from Jens that introduced another round of
override_creds()/revert_creds() for nbd Linus asked whether we could
avoide the prepare_kernel_creds() calls that duplicate the kernel
credentials and then drop them again later.

Yes, we can actually. We can use the guard infrastructure to completely
avoid the allocation and then also to never expose the temporary
variable to hold the kernel credentials anywhere in the callers.

So add with_kernel_creds() and scoped_with_kernel_creds() for this
purpose. Also take the opportunity to fixup the scoped_class() macro I
introduced two cycles ago.

* patches from https://patch.msgid.link/20251103-work-creds-init_cred-v1-0-cb3ec8711a6a@kernel.org:
  unix: don't copy creds
  target: don't copy kernel creds
  nbd: don't copy kernel creds
  firmware: don't copy kernel creds
  cred: add {scoped_}with_kernel_creds
  cred: make init_cred static
  cred: add kernel_cred() helper
  cleanup: fix scoped_class()

Link: https://patch.msgid.link/20251103-work-creds-init_cred-v1-0-cb3ec8711a6a@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

3 files changed, 23 insertions, 8 deletions

diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
index 2573585b7f06..19c7e475d3a4 100644
--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@ -290,15 +290,16 @@ static inline class_##_name##_t class_##_name##ext##_constructor(_init_args) \
 	class_##_name##_t var __cleanup(class_##_name##_destructor) =	\
 		class_##_name##_constructor
 
-#define scoped_class(_name, var, args)                          \
-	for (CLASS(_name, var)(args);                           \
-	     __guard_ptr(_name)(&var) || !__is_cond_ptr(_name); \
-	     ({ goto _label; }))                                \
-		if (0) {                                        \
-_label:                                                         \
-			break;                                  \
+#define __scoped_class(_name, var, _label, args...)        \
+	for (CLASS(_name, var)(args); ; ({ goto _label; })) \
+		if (0) {                                   \
+_label:                                                    \
+			break;                             \
 		} else
 
+#define scoped_class(_name, var, args...) \
+	__scoped_class(_name, var, __UNIQUE_ID(label), args)
+
 /*
  * DEFINE_GUARD(name, type, lock, unlock):
  *	trivial wrapper around DEFINE_CLASS() above specifically
diff --git a/include/linux/cred.h b/include/linux/cred.h
index 89ae50ad2ace..be2cd07b174c 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -20,6 +20,8 @@
 struct cred;
 struct inode;
 
+extern struct task_struct init_task;
+
 /*
  * COW Supplementary groups list
  */
@@ -156,6 +158,11 @@ extern struct cred *prepare_exec_creds(void);
 extern int commit_creds(struct cred *);
 extern void abort_creds(struct cred *);
 extern struct cred *prepare_kernel_cred(struct task_struct *);
+static inline const struct cred *kernel_cred(void)
+{
+	/* shut up sparse */
+	return rcu_dereference_raw(init_task.cred);
+}
 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 *);
@@ -180,6 +187,14 @@ static inline const struct cred *revert_creds(const struct cred *revert_cred)
 	return rcu_replace_pointer(current->cred, revert_cred, 1);
 }
 
+DEFINE_CLASS(override_creds,
+	     const struct cred *,
+	     revert_creds(_T),
+	     override_creds(override_cred), const struct cred *override_cred)
+
+#define scoped_with_kernel_creds() \
+	scoped_class(override_creds, __UNIQUE_ID(cred), kernel_cred())
+
 /**
  * get_cred_many - Get references on a set of credentials
  * @cred: The credentials to reference
diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index bccb3f1f6262..a6cb241ea00c 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -25,7 +25,6 @@
 extern struct files_struct init_files;
 extern struct fs_struct init_fs;
 extern struct nsproxy init_nsproxy;
-extern struct cred init_cred;
 
 #ifndef CONFIG_VIRT_CPU_ACCOUNTING_NATIVE
 #define INIT_PREV_CPUTIME(x)	.prev_cputime = {			\