Re: Brainy: bug in x86/cpu_ucode_intel.c

[Joerg Sonnenberger <joerg%britannica.bec.de@localhost>]

On Sat, Oct 03, 2015 at 04:29:23PM +0200, Jean-Yves Migeon wrote:
> Le 03/10/2015 15:45, Joerg Sonnenberger a écrit :
> > On Sat, Oct 03, 2015 at 02:46:26PM +0200, Maxime Villard wrote:
> >> You can see that the pointer given to kmem_free() is not the one kmem_alloc
> >> returned. Actually, I don't understand at all what this code is supposed to
> >> do. Found by Brainy.
> > 
> > It tries to make sure the pointer has a certain alignment. ...which
> > likely is something we do by default, so this is not really visible.
> 
> Regarding kmem_alloc(9) this is not necessarily true, the allocator
> "steals" the first few bytes of the allocation to store the requested
> size (see the KMEM_SIZE option in [1] which is enabled under DIAGNOSTIC).

That's not really relevant. For 64bit architectures for example, all allocations must
be 64bit aligned otherwise you can run into issues down the line. I
think we effectively guarantee 128bit alignment for all allocations at
least that size, but I don't know for sure.

That still means that the original pointer should be remembered, but the
way to require alignment is sane.

Joerg