lib/strn*,uaccess: Use masked_user_{read/write}_access_begin when required

Properly use masked_user_read_access_begin() and
masked_user_write_access_begin() instead of masked_user_access_begin() in
order to match user_read_access_end() and user_write_access_end().  This is
important for architectures like PowerPC that enable separately user reads
and user writes.

That means masked_user_read_access_begin() is used when user memory is
exclusively read during the window and masked_user_write_access_begin()
is used when user memory is exclusively writen during the window.
masked_user_access_begin() remains and is used when both reads and
writes are performed during the open window. Each of them is expected
to be terminated by the matching user_read_access_end(),
user_write_access_end() and user_access_end().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/cb5e4b0fa49ea9c740570949d5e3544423389757.1763396724.git.christophe.leroy@csgroup.eu

Author: chleroy

lib/strncpy_from_user.c

@@ -126,7 +126,7 @@ long strncpy_from_user(char *dst, const char __user *src, long count)
 	if (can_do_masked_user_access()) {
 		long retval;
 
-		src = masked_user_access_begin(src);
+		src = masked_user_read_access_begin(src);
 		retval = do_strncpy_from_user(dst, src, count, count);
 		user_read_access_end();
 		return retval;

lib/strnlen_user.c

@@ -99,7 +99,7 @@ long strnlen_user(const char __user *str, long count)
 	if (can_do_masked_user_access()) {
 		long retval;
 
-		str = masked_user_access_begin(str);
+		str = masked_user_read_access_begin(str);
 		retval = do_strnlen_user(str, count, count);
 		user_read_access_end();
 		return retval;