CopyUser - Linux Kernel Newbies

Intention of this page

This page was created by [mailto:per.persson@gnosjo.pp.se Per Persson] for describing the functions and macros copy_{to,from}_user() and __copy_{to,from}_user. Several people have asked about them at kernelnewbies@nl.linux.org.

Hopefully, I will not just start creating this page, but also complete it (if it can be completed ).

In the meantime, take a look at http://www.kernelnewbies.org/documents/copy_user/

/Per

Introduction

During the time I've been on the KernelNewbies mailing list, I've seen questions about the "copy_user" functions several times, and also tried to explain some of them. Although I'm just a kernel newbie, the response to my explanations has been very positive. John Levon even has published my explanation of __copy_user at KernelNewbies: http://www.kernelnewbies.org/documents/copy_user/.

Even if you're not especially interested in the "copy_user" functions/macros themselves, their implementations contain some techniques that can be useful in other situations:

macros and inline functions
inline assembly (gcc style)
sections
removal of unneeded branches

Most of the code considered in this text can be found in the file [http://lxr.linux.no/source/include/asm-i386/uaccess.h?v=2.4.16 include/asm-i386/uaccess.h].

There are four functions/macros that I will cover here:

copy_to_user
copy_from_user
__copy_to_user
__copy_from_user

	Direction
Accesschecking?	Kernel-->User	User-->Kernel
Yes	`copy_to_user`	`copy_from_user`
No	`__copy_to_user`	`__copy_from_user`

Perhaps the __copy_{to,from}_user macros would better be called copy_{to,from}_user_nocheck because they don't do the access-checking that copy_{to,from}_user do.

Constant or Generic?

At the top level, the four "functions" are implemented as macros:

#define copy_to_user(to,from,n)                         \
        (__builtin_constant_p(n) ?                      \
         __constant_copy_to_user((to),(from),(n)) :     \
         __generic_copy_to_user((to),(from),(n)))

This code tells the compiler to check if n is a constant (known at compile time), and if so the optimised (inline) function __constant_copy_to_user should be used, in other cases the function __generic_copy_to_user should be used.