On 19 Feb 2003, Alan Cox wrote:
>
> copy_from_user sorts out the whole thing itself. In general __copy_from_user and
>verify_area
> isnt a win, except if you do lots of small copies to/from the same area, which is
>often
> a bad idea anyway.
It _can_ be a good idea, though.
In particular, it's a _really_ good idea if the loop is basically a
"glorified memcpy", at which point it can be very much worthwhile to move
the limit checking outside the loop.
An example of this is doing things like "copy-and-checksum", or, in the
case of DRM "copy-and-check-validity", where the code should sanely look
something like
u32 *addr; /* Command list */
if (!access_ok(VERIFY_READ, addr, size))
return -EFAULT;
while (size > 0) {
unsigned int cmd;
if (__get_user(cmd, addr))
goto efault;
addr++;
switch (cmd) {
... verify it ...
}
*dst++ = cmd;
}
where the difference between a "get_user()" and a "__get_user()" can be
quite noticeable (the unchecked version is usually a single instruction in
size and has no nasty branch behaviour or anything like that, while the
checking version usually has at least one conditional branch and uses up
one or two registers etc).
Of course, this is really only worth doing for tight loops that really
matter. If the code in the loop is crap, the optimization just isn't worth
the complexity.
(And the extreme case of this is when the code is _so_ performance
critical that you end up writing it all in assembly language. The example
of this is the TCP copy-and-checksum stuff. The above example is somewhere
between the "normal usage" and the "TCP copy-and-checksum" extremes).
Linus
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