Continuing kallsyms failures - large kernels, XIP kernels, and large XIP kernels

[Nicolas Pitre]

On Wed, 4 Feb 2015, Russell King - ARM Linux wrote:

> On Tue, Feb 03, 2015 at 08:59:15PM -0500, Nicolas Pitre wrote:
> > On Wed, 4 Feb 2015, Russell King - ARM Linux wrote:
> > 
> > > It looks like we have cases where this falsely triggers.  Consider EFM32:
> > > 
> > > CONFIG_DRAM_BASE=0x88000000
> > > CONFIG_DRAM_SIZE=0x00400000
> > > CONFIG_FLASH_MEM_BASE=0x8c000000
> > > CONFIG_FLASH_SIZE=0x01000000
> > > 
> > > This means that we quite legally end up with the .data section below the
> > > .text section, which makes:
> > > 
> > > ASSERT((_data >= __data_loc), "Text section oversize") 
> > > 
> > > falsely trigger.
> > > 
> > > The linker has the capacity to specify regions of ROM and RAM in the
> > > linker file, I wonder if we should be using those for XIP.  Merely
> > > adding the MEMORY table to the linker file is not good enough - we
> > > also need to explicitly tell the linker which memory regions to place
> > > the output sections, otherwise the linker ends up making assumptions.
> > > 
> > > What that means is... asm-generic/vmlinux.lds.h breaks for us.
> > > 
> > > Any ideas?  I think using the MEMORY table would be the best approach,
> > > because that should allow us to properly verify that the resulting
> > > binary should fit in the memory regions.
> > 
> > Maybe simply having an assert() on the size of the .text section could 
> > be all that is needed.  We already know the maximum size in advance.
> 
> That doesn't work, it's not just the .text section but also .rodata,
> __bug_table, __ksymtab, __ksymtab_gpl, __kcrctab, __kcrctab_gpl,
> __ksymtab_strings, __param, __modver, __ex_table, .notes, .vectors,
> .stubs, .init.text, maybe .exit.text, .init.arch.info, .init.tagtable,
> .init.smpalt, .init.pv_table, 

My suggestion would be to put a symbol at the end of it all to size the 
lot.

> and apparently .init.data (which is
> surely wrong?)  The following is from Arnd's failing configuration:
> 
>  18 .init.tagtable 00000040  80073a9c  80073a9c  0100ba9c  2**2
>                   CONTENTS, ALLOC, LOAD, READONLY, DATA
>  19 .init.data    000010e8  80073adc  80073adc  0100badc  2**2
>                   CONTENTS, ALLOC, LOAD, DATA
>  20 .data         003552c4  80008000  80074bc4  01010000  2**8
>                   CONTENTS, ALLOC, LOAD, DATA
> 
> Hmm, if .init.data is contained in the flash section (which it seemingly
> is), it seems that XIP support is currently broken - that section is
> definitely a read/write section.  No one has seemingly noticed that it's
> broken and it's been broken for a long time, so maybe the simple answer
> then is just to rip XIP support out?

That's what I proposed a couple years ago but some people were 
apparently still using it.

Now with all the buzz around IOT it is well possible that XIP will gain 
more traction again.


Nicolas