userswap: fix VM_BUG_ON() in handle_userfault()

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I6CAIM

--------------------------------

When CONFIG_VM_BUG_ON=y and userswap feature is used, there is a kernel
BUG in handle_userfault(). VM_BUG_ON() didn't allow more than one reason
flag.
Fix this by skipping VM_BUG_ON() if reason is VM_UFFD_MISSING|VM_USWAP.
Signed-off-by: NZhangPeng <zhangpeng362@huawei.com>

fs/userfaultfd.c

@@ -406,8 +406,12 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 
 	BUG_ON(ctx->mm != mm);
 
+#ifdef CONFIG_USERSWAP
+	VM_BUG_ON(uswap_vm_flag_bug_on(reason));
+#else
 	VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP));
 	VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP));
+#endif
 
 	if (ctx->features & UFFD_FEATURE_SIGBUS)
 		goto out;

include/linux/userswap.h

@@ -47,6 +47,18 @@ static inline bool uswap_validate_mremap_flags(unsigned long flags)
 	return true;
 }
 
+/* When CONFIG_USERSWAP=y, VM_UFFD_MISSING|VM_USWAP is right;
+ * 0 or > 1 flags set is a bug; we expect exactly 1.
+ */
+static inline bool uswap_vm_flag_bug_on(unsigned long reason)
+{
+	if (reason & ~(VM_UFFD_MISSING | VM_UFFD_WP | VM_USWAP))
+		return true;
+	if (reason & VM_USWAP)
+		return !(reason & VM_UFFD_MISSING) || reason & ~(VM_USWAP|VM_UFFD_MISSING);
+	return !(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP);
+}
+
 #endif /* CONFIG_USERSWAP */
 
 #endif /* _LINUX_USERSWAP_H */