Re: [SOLVED + PATCH]: documented Oops running big-endian reiserfs on parisc architecture

[Ulrich Weigand <weigand@immd1.informatik.uni-erlangen.de>]

Jeff Mahoney wrote:

> Are the S/390 asm/unaligned.h versions broken, or is the ReiserFS code doing 
> something not planned for? It's a 16-bit member, at a 16-bit alignment 
> in the structure. The structure itself need not be aligned in any 
> particular manner as it is read directly from disk, and is a packed structure.

One other point I overlooked before: using set_bit etc on a *16-bit* 
member is fundamentally broken on S/390 anyway (and I far as I know all 
other bigendian architectures as well).  set_bit assumes to operate on
a long (or an array of longs); if you use set_bit to set bit number 0
in the bitfield starting at address X, it will actually modify the byte 
located at address X+3 (or X+7 on 64-bit bigendian machines), because
this is where the bit with value 2^0 is located in a long.

If your bitfield is only 2 bytes long, this will obviously clobber
random memory after the field ...  (Note that the _unaligned variants
do not fix this problem, they will just cause it to clobber memory
*before* the field instead, if I interpret them correctly.)

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  weigand@informatik.uni-erlangen.de