Re: User defined sections

[Nick Clifton <nickc at cambridge dot redhat dot com>]

Hi Daniel,

> > Thirdly - which versions of the tools are you using ?  In particular
> > which version of ld are you using ?
> 
> Nasm 0.98
> GNU ld 2.9.5
> ld.so 2.1.3

Ok, so you are using an old version of ld.  It would probably be a
good idea to upgrade to the most recent release (2.11.2) although
this will not actually fix your problem :-)

> Does it also crash on your system?

Indeed it does.  I note that if I do not use your linker script
however, the executable runs just fine.  Why are you using a custom
linker script anyway ?  The default one appears to work OK: 

   % gcc problem.o image.o -o prob
   % readelf -S prob | grep custom
   [16] .custom           PROGBITS        08049544 000544 000014 00 WAX   0   0 1

Do you need the .custom section to be located at a specific address ?

Anyway I tried adding your .custom modifications to the current
default linker script and using that, and the linker complains with:

  ld: prob: Not enough room for program headers (allocated 6, need 7)

Which is not surprising really since your new section needs a segment
all to itself.  It is possible to work around this by tweaking the
linker script, but it still produces an executable that seg faults.

The problem is that your load address of 0x0 causes the section to be
written into protected memory, which generates the seg fault.  Since
you are running this program on a virtual memory system you are never
going to be able to just choose an arbitrary address for your section.
It would be much better to allow the linker to do its job, and link
the section in at an address that it deems fit, and then set up the
program headers appropriately.

Cheers
        Nick