On Wed, Jan 6, 2016 at 4:33 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> On Wed, Dec 30, 2015 at 09:59:29AM -0800, Tony Luck wrote:
>> Starting with a patch from Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>> that used linker relocation trickery to free up a couple of bits
>> in the "fixup" field of the exception table (and generalized the
>> uaccess_err hack to use one of the classes).
>
> So I still think that the other idea Andy gave with putting the handler
> in the exception table is much cleaner and straightforward.
>
> Here's a totally untested patch which at least builds here. I think this
> approach is much more extensible and simpler for the price of a couple
> of KBs of __ex_table size.
>
> ---
> diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
> index 189679aba703..43b509c88b13 100644
> --- a/arch/x86/include/asm/asm.h
> +++ b/arch/x86/include/asm/asm.h
> @@ -44,18 +44,20 @@
>
> /* Exception table entry */
> #ifdef __ASSEMBLY__
> -# define _ASM_EXTABLE(from,to) \
> +# define _ASM_EXTABLE(from,to) \
> .pushsection "__ex_table","a" ; \
> .balign 8 ; \
> .long (from) - . ; \
> .long (to) - . ; \
> + .long 0 - .; \
I assume that this zero is to save the couple of bytes for the
relocation entry on relocatable kernels?
If so, ...
> +inline ex_handler_t ex_fixup_handler(const struct exception_table_entry *x)
> +{
> + return (ex_handler_t)&x->handler + x->handler;
I would check for zero here, because...
> + new_ip = ex_fixup_addr(e);
> + handler = ex_fixup_handler(e);
> +
> + if (!handler)
> + handler = ex_handler_default;
the !handler condition here will never trigger because the offset was
already applied.
Otherwise this looks generally sane.
--Andy
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