[PATCH] ppc64: re-enable kexec to allow module loads with CONFIG_MODVERSIONS and CONFIG_RELOCATABLE turned on

[Paul Mackerras]

Neil Horman writes:

> 	Before anyone flames me for what a oddball solution this is, let me just
> say I'm trying to get the ball rolling here.  I think there may be better
> solutions that can be impemented in reloc_64.S, but I've yet to make any of the
> ones I've tried work successfully.  I'm open to suggestions, but this solution
> is the only one so far that I've been able to get to work. thanks :)

I had thought that the CRCs would be the only things with reloc
addends less than 4G, but it turns out that the toolchain folds
expressions like __pa(&sym) into just a load from a doubleword (in the
TOC) containing the physical address of sym.  That needs to be
relocated and its reloc addend will be less than 4GB.  Hence the
crashes early in boot that you were seeing with my proposed patch to
reloc_64.S.

Given that, your solution seems as reasonable as any.  Should this go
via the modules maintainer, Rusty Russell, perhaps?

In any case,

Acked-by: Paul Mackerras <paulus at samba.org>

> Adjust crcs in __kcrctab_* sections if relocs are used with CONFIG_RELOCATABLE
> 
> When CONFIG_MODVERSIONS and CONFIG_RELOCATABLE are enabled on powerpc platforms,
> kdump has been failing in a rather odd way.  specifically modules will not
> install.  This is because when validating the crcs of symbols that the module
> needs, the crc of the module never matches the crc that is stored in the kernel.
> 
> The underlying cause of this is how CONFIG_MODVERSIONS is implemented, and how
> CONFIG_RELOCATABLE are implemented.  with CONFIG_MODVERSIONS enabled, for every
> exported symbol in the kernel we emit 2 symbols, __crc_#sym which is declared
> extern and __kcrctab_##sym, which is placed in the __kcrctab section of the
> binary.  The latter has its value set equal to the address of the former
> (recalling it is declared extern).  After the object file is built, genksyms is
> run on the processed source, and crcs are computed for each exported symbol.
> genksyms then emits a linker script which defines each of the needed __crc_##sym
> symbols, and sets their addresses euqal to their respective crcs.  This script
> is then used in a secondary link to the previously build object file, so that
> the crcs of the missing symbol can be validated on module insert.
> 
> The problem here is that because __kcrctab_sym is set equal to &__crc_##sym, a
> relocation entry is emitted by the compiler for the __kcrctab__##sym.  Normally
> this is not a problem, since relocation on other arches is done without the aid
> of .rel.dyn sections.  PPC however uses these relocations when
> CONFIG_RELOCATABLE is enabled.  nominally, since addressing starts at 0 for ppc,
> its irrelevant, but if we start at a non-zero address (like we do when booting
> via kexec from reserved crashkernel memory), the ppc boot code iterates over the
> relocation entries, and winds up adding that relocation offset to all symbols,
> including the symbols that are actually the aforementioned crc values in the
> __kcrctab_* sections.  This effectively corrupts the crcs and prevents any
> module loads from happening during a kdump.
> 
> My solution is to 'undo' these relocations prior to boot up.  If
> ARCH_USES_RELOC_ENTRIES is defined, we add a symbol at address zero to the
> linker script for that arch (I call it reloc_start, so that &reloc_start = 0).
> This symbol will then indicate the relocation offset for any given boot.  We
> also add an initcall to the module code that, during boot, scans the __kcrctab_*
> sections and subtracts &reloc_start from every entry in those sections,
> restoring the appropriate crc value.
> 
> I've verified that this allows kexec to work properly on ppc64 systems myself.
> 
> Signed-off-by: Neil Horman <nhorman at tuxdriver.com>
> 
> 
>  arch/powerpc/include/asm/local.h  |    6 ++++++
>  arch/powerpc/kernel/vmlinux.lds.S |    4 ++++
>  kernel/module.c                   |   30 ++++++++++++++++++++++++++++++
>  3 files changed, 40 insertions(+)
> 
> diff --git a/arch/powerpc/include/asm/local.h b/arch/powerpc/include/asm/local.h
> index 84b457a..9cc49e5 100644
> --- a/arch/powerpc/include/asm/local.h
> +++ b/arch/powerpc/include/asm/local.h
> @@ -4,6 +4,12 @@
>  #include <linux/percpu.h>
>  #include <asm/atomic.h>
>  
> +#ifdef CONFIG_MODVERSIONS
> +#define ARCH_USES_RELOC_ENTRIES
> +
> +extern unsigned long reloc_start;
> +#endif
> +
>  typedef struct
>  {
>  	atomic_long_t a;
> diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
> index 27735a7..2b9fb2e 100644
> --- a/arch/powerpc/kernel/vmlinux.lds.S
> +++ b/arch/powerpc/kernel/vmlinux.lds.S
> @@ -38,6 +38,10 @@ jiffies = jiffies_64 + 4;
>  #endif
>  SECTIONS
>  {
> +	. = 0;
> +	reloc_start = .;
> +	. = 0;
> +
>  	. = KERNELBASE;
>  
>  /*
> diff --git a/kernel/module.c b/kernel/module.c
> index 8b7d880..87a4928 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -181,8 +181,11 @@ extern const struct kernel_symbol __stop___ksymtab_gpl_future[];
>  extern const struct kernel_symbol __start___ksymtab_gpl_future[];
>  extern const struct kernel_symbol __stop___ksymtab_gpl_future[];
>  extern const unsigned long __start___kcrctab[];
> +extern const unsigned long __stop___kcrctab[];
>  extern const unsigned long __start___kcrctab_gpl[];
> +extern const unsigned long __stop___kcrctab_gpl[];
>  extern const unsigned long __start___kcrctab_gpl_future[];
> +extern const unsigned long __stop___kcrctab_gpl_future[];
>  #ifdef CONFIG_UNUSED_SYMBOLS
>  extern const struct kernel_symbol __start___ksymtab_unused[];
>  extern const struct kernel_symbol __stop___ksymtab_unused[];
> @@ -3144,3 +3147,30 @@ int module_get_iter_tracepoints(struct tracepoint_iter *iter)
>  	return found;
>  }
>  #endif
> +
> +#ifdef ARCH_USES_RELOC_ENTRIES
> +static __init int adjust_kcrctab(void)
> +{
> +	int i;
> +	int count;
> +	unsigned long  *crc ;
> +
> +	count = __stop___kcrctab - __start___kcrctab;
> +	crc = (unsigned long *)__start___kcrctab;
> +	for (i = 0; i < count; i++) {
> +		crc[i] -= (unsigned long)&reloc_start;
> +	}
> +	count = __stop___kcrctab_gpl - __start___kcrctab_gpl;
> +	crc = (unsigned long *)__start___kcrctab_gpl;
> +	for (i = 0; i < count; i++) {
> +		crc[i] -= (unsigned long)&reloc_start;
> +	}
> +	count = __stop___kcrctab_gpl_future - __start___kcrctab_gpl_future;
> +	crc = (unsigned long *)__start___kcrctab_gpl_future;
> +	for (i = 0; i< count; i++) {
> +                crc[i] -= (unsigned long)&reloc_start;
> +	}
> +	return 0;
> +}
> +early_initcall(adjust_kcrctab);
> +#endif