linux-kernel - uaccess.h: implement unsafe_copy_{to,from}

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Message-ID: <11edd44d-71c5-cf06-65df-1bf67c93f13d@godking.net>
Date:   Sat, 30 Dec 2017 13:40:40 -0800
From:   Alexander Kappner <agk@...king.net>
To:     linux-kernel@...r.kernel.org
Cc:     viro@...iv.linux.org.uk, torvalds@...uxfoundation.org
Subject: uaccess.h: implement unsafe_copy_{to,from}_user()?

Commit 5b24a7a2aa2040c8c50c3b71122901d01661ff78 introduced the 
unsafe_get_user and unsafe_put_user replacement functions for batched calls 
to put_user and get_user. I'm trying to make the kernel smaller and reduce 
stac/clac overhead on x86 by substituting the new functions for such 
batched calls. But there's no corresponding unsafe_copy_to_user() 
or unsafe_copy_from_user() functions to copy an arbitrary-sized buffer to 
and from userspace without calling access_ok and __uaccess_begin/end.

I know that the matter of replacing these uaccess functions has been 
discussed at length (see https://lkml.org/lkml/2017/5/13/134), so before I 
started hacking away implementing new unsafe_copy_{to,from}_user functions, 
I wanted to ask if a solution to this is already being worked on or if
there's some way of accomplishing this goal without new functions.

To illustrate, here's a batched function call (from fs/fat/dir.c):

if (put_user(0, d2->d_name)                     ||       
                    put_user(0, &d2->d_reclen)  ||         
                    copy_to_user(d1->d_name, name, name_len) ||         
		    // etc...
                        goto efault;              

This should read:

if (!access_ok(VERIFY_WRITE, d1, 2*sizeof(*infop))
    goto efault;                      
user_access_begin();
unsafe_put_user(0, d2->d_name, efault)
unsafe_put_user(0, &d2->d_reclen, efault)
unsafe_copy_to_user(d1->d_name, name, name_len, efault); // we don't have 
// this function

// etc...
user_access_end();

Thanks.