[Bug libc/12495] Malloc fails if MALLOC_ALIGNMENT > 2 * SIZE_SZ

["hjl.tools at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=12495

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dan at codesourcery dot com

--- Comment #3 from H.J. Lu <hjl.tools at gmail dot com> 2011-03-02 02:14:47 UTC ---
(In reply to comment #1)
> On Thu, 17 Feb 2011, hjl.tools at gmail dot com wrote:
> 
> > If I define  MALLOC_ALIGNMENT > 2 * SIZE_SZ, malloc will
> > fail in mysterious ways.
> 
> Yes, this is known.  There's a patch at 
> <http://sourceware.org/ml/libc-alpha/2007-11/msg00062.html> that fixes the 
> problem in the context of PowerPC malloc not allocating memory 
> sufficiently aligned for long double.

Unfortunately, this patch is incorrect.  This code

          front_misalign = (INTERNAL_SIZE_T)chunk2mem(brk) & MALLOC_ALIGN_MASK;
          if (front_misalign > 0) { 

            /*   
              Skip over some bytes to arrive at an aligned position.
              We don't need to specially mark these wasted front bytes.
              They will never be accessed anyway because
              prev_inuse of av->top (and any chunk created from its start)
              is always true after initialization.
            */

            correction = MALLOC_ALIGNMENT - front_misalign;
            aligned_brk += correction;
          }    

misaligns memory when

1. MALLOC_ALIGNMENT == 16
2. front_misalign == 8
3. aligned_brk == brk == 0xf7b61000

-- 
Configure bugmail: http://sourceware.org/bugzilla/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are on the CC list for the bug.