[PATCH net-next] scm: optimize put_cmsg()

[Eric Dumazet]

From: Eric Dumazet <eduma...@google.com>

Calling two copy_to_user() for very small regions has very high overhead.

Switch to inlined unsafe_put_user() to save one stac/clac sequence,
and avoid copy_to_user().

Signed-off-by: Eric Dumazet <eduma...@google.com>
Cc: Soheil Hassas Yeganeh <soh...@google.com>
---
 net/core/scm.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/net/core/scm.c b/net/core/scm.c
index 
8156d4fb8a3966122fdfcfd0ebc9e5520aa7b67c..bd96c922041d22a2f3b7ee73e4b3183316f9b616
 100644
--- a/net/core/scm.c
+++ b/net/core/scm.c
@@ -228,14 +228,16 @@ int put_cmsg(struct msghdr * msg, int level, int type, 
int len, void *data)
 
        if (msg->msg_control_is_user) {
                struct cmsghdr __user *cm = msg->msg_control_user;
-               struct cmsghdr cmhdr;
 
-               cmhdr.cmsg_level = level;
-               cmhdr.cmsg_type = type;
-               cmhdr.cmsg_len = cmlen;
-               if (copy_to_user(cm, &cmhdr, sizeof cmhdr) ||
-                   copy_to_user(CMSG_USER_DATA(cm), data, cmlen - sizeof(*cm)))
-                       return -EFAULT;
+               if (!user_write_access_begin(cm, cmlen))
+                       goto efault;
+
+               unsafe_put_user(len, &cm->cmsg_len, efault_end);
+               unsafe_put_user(level, &cm->cmsg_level, efault_end);
+               unsafe_put_user(type, &cm->cmsg_type, efault_end);
+               unsafe_copy_to_user(CMSG_USER_DATA(cm), data,
+                                   cmlen - sizeof(*cm), efault_end);
+               user_write_access_end();
        } else {
                struct cmsghdr *cm = msg->msg_control;
 
@@ -249,6 +251,11 @@ int put_cmsg(struct msghdr * msg, int level, int type, int 
len, void *data)
        msg->msg_control += cmlen;
        msg->msg_controllen -= cmlen;
        return 0;
+
+efault_end:
+       user_write_access_end();
+efault:
+       return -EFAULT;
 }
 EXPORT_SYMBOL(put_cmsg);
 
-- 
2.31.1.368.gbe11c130af-goog