Commit 2865baf54077a added 'user address masking' to avoid the
serialising instructions associated with access_ok() when using
unsafe_get_user().
However the code pattern required is non-trivial.
Add a new wrapper masked_user_read_access_begin() to simplify things.
Code can then be changed:
- if (!user_read_access_begin(from, sizeof(*from)))
+ if (!masked_user_read_access_begin(&from, sizeof(*from)))
return -EFAULT;
unsafe_get_user(xxx, &from->xxx, Efault);
If address masking is supported the 'return -EFAULT' will never happen.
Add the matching masked_user_write_access_begin().
Although speculative accesses aren't an issue, it saves the conditional
branch.
Signed-off-by: David Laight <david.laight.linux@xxxxxxxxx>
---
include/linux/uaccess.h | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index e9c702c1908d..5a55152c0010 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -33,6 +33,15 @@
})
#endif
+/*
+ * Architectures can reduce the cost of validating user addresses by
+ * defining masked_user_access_begin().
+ * It should convert any kernel address to the base of an unmapped
+ * page (eg that of a guard page between user and kernel addresses)
+ * and enable accesses to user memory.
+ * To avoid speculative accesses it should use ALU instructions
+ * (eg a compare and conditional move).
+ */
#ifdef masked_user_access_begin
#define can_do_masked_user_access() 1
#else
@@ -41,6 +50,18 @@
#define mask_user_address(src) (src)
#endif
+#ifdef masked_user_access_begin
+#define masked_user_read_access_begin(from, size) \
+ ((*(from) = masked_user_access_begin(*(from))), 1)
+#define masked_user_write_access_begin(from, size) \
+ ((*(from) = masked_user_access_begin(*(from))), 1)
+#else
+#define masked_user_read_access_begin(from, size) \
+ user_read_access_begin(*(from), size)
+#define masked_user_write_access_begin(from, size) \
+ user_write_access_begin(*(from), size)
+#endif
+
/*
* Architectures should provide two primitives (raw_copy_{to,from}_user())
* and get rid of their private instances of copy_{to,from}_user() and
--
2.39.5
