Re: Optimization of strcpy with an offset outside the string

[Daniel Jacobowitz <drow at mvista dot com>]

On Mon, Dec 03, 2001 at 08:45:26PM -0600, Corey Minyard wrote:
> In the PowerPC Linux kernel, at some points in time the reference to 
> some constant data is at an offset (because the MMU is not set up yet). 
> So you see expressions like:
> 
>   print("test" + offset);
> 
> where offset is far outside the bounds of the string.  This causes some 
> very strange behaviour.  Basically, gcc emits code to subtract the 
> offset varaible from the length of the string  to calculate the length 
> to memcpy.  In builtins.c in the function c_strlen(), I find the comment:
> 
>      /* We don't know the starting offset, but we do know that the string
>         has no internal zero bytes.  We can assume that the offset falls
>         within the bounds of the string; otherwise, the programmer deserves
>         what he gets.  Subtract the offset from the length of the string,
>         and return that.  This would perhaps not be valid if we were 
> dealing
>         with named arrays in addition to literal string constants.  */
> 
> Should the programmer really lose in this case?  I can work around it in 
> the kernel, but I thought I would point this out.
> 
> -Corey
> 
> BTW, My cable modem is now operational again, so you can mail me at acm.org.

Is this still in the current PPC kernels?

Franz raised this issue several months ago, and I believe the verdict
was that it was illegal.

I personally wouldn't mind a warning being generated, though!

-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer