Re: negative seek offsets in VFS

[Andi Kleen <ak@muc.de>]

On Thu, May 26, 2005 at 10:49:57AM -0700, Bryan Henderson wrote:
> >The addresses returned from /proc/kallsyms on the x86_64 are negative and 
> when
> >I print the address of a kernel variable with "%p" it comes out negative.
> 
> Wow.  So can someone explain this?  Does the x86_64 architecture actually 
> have a concept of negative addresses?

Yes, it has. It has a 48bit address space, and the 48th bit 
is sign extended to 64bits. So you have a big hole in the middle
and positive and negative address spaces. The kernel uses the negative
half, user space the positive half.

> 
> Ordinarily, I'd think this was a bug if I saw it, but I don't see how %p 
> could format a minus sign without it being deliberate.

It doesnt, but they are still negative. 
 
> In any case, I know POSIX doesn't have a concept of a negative absolute 
> file offset, so a kmem device cannot represent a negative address as a 
> negative file offset.  A negative loff_t means something else.

I dont think POSIX has anything to say about /dev/kmem.

-Andi
>