On Thu, 06 Jul 2006 08:49:14 +0200, Magnus Damm wrote:
> On 7/6/06, Eric W. Biederman <[EMAIL PROTECTED]> wrote:
> > "Magnus Damm" <[EMAIL PROTECTED]> writes:
...
> > > I understand that it is possible to create a symbol that points to a
> > > fixed address say mapping some hardware resource, but is that a
> > > strategy currently used by Linux?
> >
> > Nope absolute symbols should not be changed in particular you will
> > mess up the linux-gate.so and the like.
>
> I did some more research and it looks like I'm wrong. So absolute
> symbols should be handled in a different way - I'll modify patches to
> get that working.
It is true some addresses must not be relocated. In 2.6.9 days it were the
addresses detected by SAFE_FIXED_ADDRESS() in:
http://mkdump.cvs.sourceforge.net/mkdump/linux/scripts/reltab.c?view=markup&pathrev=linux-2_6-minik
> The absolute symbols we are talking about are the ones that could be
> extracted from vmlinux with the following line, right?
>
> readelf -s vmlinux | grep ABS | grep -v FILE
It would be the right way. Unfortunately in those 2.6.9 days this "flag" was
broken in "vmlinux.lds.S" so Eric W. Biederman fixed it in the patch noted
below so that your "grep ABS" would work properly:
> > > > Eric had posted a patch in the past which made some the absolute symbols
> > > > section relative.
> > > >
> > > > http://marc.theaimsgroup.com/?l=linux-kernel&m=112266670613361&w=2
I did not fix it and workarounded it instead in my
http://mkdump.cvs.sourceforge.net/mkdump/linux/scripts/reltab.c?view=markup&pathrev=linux-2_6-minik
just due to the project management issues - I just checked the referenced
address range not looking at whether such reference was marked as being
pointing to fixed or relocatable address. I agree that technically speaking
the right way is fixing "vmlinux.lds.S" as Eric W. Biederman did.
(did not check which patches did / did-not get imported into which
version/maintainer/vendor of Linux kernel)
Extracted some old mail explaining the fixed vs. relative addresses and the
type ("PC-relative?") of relocation (according to the used instruction type):
On Thu, 14 Jul 2005 03:12:10 +0200, lace wrote:
...
> source destination PC-relative? resolving
> kernel-image kernel-image no ADDITION
> kernel-image kernel-image yes none
> kernel-image fixed no none
> kernel-image fixed yes SUBTRACTION
> fixed kernel-image no ADDITION
> fixed kernel-image yes *** ADDITION *** (NOT
> SUBTRACTION!)
> fixed fixed no none
> fixed fixed yes none
>
> always: "kernel-image" address < "fixed" address
>
> fixed kernel-image yes *** ADDITION *** (NOT
> SUBTRACTION!)
> Although the referenced distance decreases, the negative relative address in
> code is in binary complement and thus the relocation must be ADDITION to
> decrease the distance (as in absolute value).
>
> Fortunately this type of relocation is not being used anywhere now.
...
Regards,
Lace
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