x86: introduce copy_user_handle_tail() routine

Introduce generic C routine for handling necessary tail operations after
protection fault in copy_*_user on x86.

Signed-off-by: Vitaly Mayatskikh <v.mayatskih@gmail.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 26 insertions, 0 deletions

diff --git a/arch/x86/lib/usercopy_64.c b/arch/x86/lib/usercopy_64.c
index 0c89d1bb0287..f4df6e7c718b 100644
--- a/arch/x86/lib/usercopy_64.c
+++ b/arch/x86/lib/usercopy_64.c
@@ -158,3 +158,26 @@ unsigned long copy_in_user(void __user *to, const void __user *from, unsigned le
 }
 EXPORT_SYMBOL(copy_in_user);
 
+/*
+ * Try to copy last bytes and clear the rest if needed.
+ * Since protection fault in copy_from/to_user is not a normal situation,
+ * it is not necessary to optimize tail handling.
+ */
+unsigned long
+copy_user_handle_tail(char *to, char *from, unsigned len, unsigned zerorest)
+{
+        char c;
+        unsigned zero_len;
+
+        for (; len; --len) {
+                if (__get_user_nocheck(c, from++, sizeof(char)))
+                        break;
+                if (__put_user_nocheck(c, to++, sizeof(char)))
+                        break;
+        }
+
+        for (c = 0, zero_len = len; zerorest && zero_len; --zero_len)
+                if (__put_user_nocheck(c, to++, sizeof(char)))
+                        break;
+        return len;
+}
diff --git a/include/asm-x86/uaccess_64.h b/include/asm-x86/uaccess_64.h
index 4e3ec004e14a..515d4dce96b5 100644
--- a/include/asm-x86/uaccess_64.h
+++ b/include/asm-x86/uaccess_64.h
@@ -195,4 +195,7 @@ static inline int __copy_from_user_inatomic_nocache(void *dst,
         return __copy_user_nocache(dst, src, size, 0);
 }
 
+unsigned long
+copy_user_handle_tail(char *to, char *from, unsigned len, unsigned zerorest);
+
 #endif /* __X86_64_UACCESS_H */