sched_getattr: port to copy_struct_to_user

sched_getattr(2) doesn't care about trailing non-zero bytes in the
(ksize > usize) case, so just use copy_struct_to_user() without checking
ignored_trailing.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Link: https://lore.kernel.org/r/20241010-extensible-structs-check_fields-v3-2-d2833dfe6edd@cyphar.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: cyphar

kernel/sched/syscalls.c

@@ -1076,45 +1076,6 @@ SYSCALL_DEFINE2(sched_getparam, pid_t, pid, struct sched_param __user *, param)
 	return copy_to_user(param, &lp, sizeof(*param)) ? -EFAULT : 0;
 }
 
-/*
- * Copy the kernel size attribute structure (which might be larger
- * than what user-space knows about) to user-space.
- *
- * Note that all cases are valid: user-space buffer can be larger or
- * smaller than the kernel-space buffer. The usual case is that both
- * have the same size.
- */
-static int
-sched_attr_copy_to_user(struct sched_attr __user *uattr,
-			struct sched_attr *kattr,
-			unsigned int usize)
-{
-	unsigned int ksize = sizeof(*kattr);
-
-	if (!access_ok(uattr, usize))
-		return -EFAULT;
-
-	/*
-	 * sched_getattr() ABI forwards and backwards compatibility:
-	 *
-	 * If usize == ksize then we just copy everything to user-space and all is good.
-	 *
-	 * If usize < ksize then we only copy as much as user-space has space for,
-	 * this keeps ABI compatibility as well. We skip the rest.
-	 *
-	 * If usize > ksize then user-space is using a newer version of the ABI,
-	 * which part the kernel doesn't know about. Just ignore it - tooling can
-	 * detect the kernel's knowledge of attributes from the attr->size value
-	 * which is set to ksize in this case.
-	 */
-	kattr->size = min(usize, ksize);
-
-	if (copy_to_user(uattr, kattr, kattr->size))
-		return -EFAULT;
-
-	return 0;
-}
-
 /**
  * sys_sched_getattr - similar to sched_getparam, but with sched_attr
  * @pid: the pid in question.
@@ -1159,7 +1120,8 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
 #endif
 	}
 
-	return sched_attr_copy_to_user(uattr, &kattr, usize);
+	kattr.size = min(usize, sizeof(kattr));
+	return copy_struct_to_user(uattr, usize, &kattr, sizeof(kattr), NULL);
 }
 
 #ifdef CONFIG_SMP