Guest image and symtable alignement

Guest image and symtable alignement

[Mathieu Ropert]

Hi,

I found a difference between Xen's domain kernel symbol table alignement 
and alignement found in some other OS, like BSD.

On Xen (looking at loadelfsymtab() in common/elf.c), symtable length is 
stored on an ELFROUND rounded address (4 bytes on 32bits, 8 bytes on 64 
bits) followed by the ELF header. As length is an int, header isn't 8 
bytes aligned on 64 bits (no problem on 32 bit as sizeof (int) == 
ELFROUND), whereas OS like BSD expects the header to be aligned on a 
long boundary.

I'd like to know whichever is right (if there is any standard about 
that), because this may cause some incompatibilty problems with future 
ports attempts.
Was also wondering if we could just fix it be moving all this 4 bytes 
forward in the guest OS code (may break pointer references, if any?).

Thanks,
Mathieu

Re: Guest image and symtable alignement

[Keir Fraser]

On 11 Apr 2006, at 14:14, Mathieu Ropert wrote:

> I found a difference between Xen's domain kernel symbol table 
> alignement and alignement found in some other OS, like BSD.
>
> On Xen (looking at loadelfsymtab() in common/elf.c), symtable length 
> is stored on an ELFROUND rounded address (4 bytes on 32bits, 8 bytes 
> on 64 bits) followed by the ELF header. As length is an int, header 
> isn't 8 bytes aligned on 64 bits (no problem on 32 bit as sizeof (int) 
> == ELFROUND), whereas OS like BSD expects the header to be aligned on 
> a long boundary.
>
> I'd like to know whichever is right (if there is any standard about 
> that), because this may cause some incompatibilty problems with future 
> ports attempts.
> Was also wondering if we could just fix it be moving all this 4 bytes 
> forward in the guest OS code (may break pointer references, if any?).

That code is used *only* by BSD ports. So I guess it does work for them 
and, since the build of the OS will be targetted at Xen they can always 
do whatever is needed to get their symtab to be accepted by the 
loadelfsymtab() code. If there is a problem, it'll be up to them to 
submit a patch to fix it.

  -- Keir