Re: [PATCH RFC 1/5] scripts: Add sortextable to sort the kernel's exception table.

[Mike Frysinger]

On Monday 21 November 2011 14:16:04 David Daney wrote:
> On 11/21/2011 10:50 AM, Mike Frysinger wrote:
> > On Monday 21 November 2011 13:25:36 David Daney wrote:
> >> On 11/20/2011 03:22 PM, Mike Frysinger wrote:
> >>> On Friday 18 November 2011 14:37:44 David Daney wrote:
> >>>> +	switch (w2(ehdr->e_machine)) {
> >>>> +	default:
> >>>> +		fprintf(stderr, "unrecognized e_machine %d %s\n",
> >>>> +			w2(ehdr->e_machine), fname);
> >>>> +		fail_file();
> >>>> +		break;
> >>>> +	case EM_386:
> >>>> +	case EM_MIPS:
> >>>> +	case EM_X86_64:
> >>>> +		break;
> >>>> +	}  /* end switch */
> >>> 
> >>> unlike recordmcount, this file doesn't do anything arch specific.  so
> >>> let's just delete this and be done.
> >> 
> >> Not really true at this point.  We don't know the size or layout of the
> >> architecture specific exception table entries, likewise for
> >> CONFIG_ARCH_HAS_SORT_EXTABLE, we don't even know how to do the
> >> comparison.
> > 
> > all of your code that i could see is based on "is it 32bit or is it
> > 64bit". there is no code that says "if it's x86, we need to do XXX".
> 
> At this point there is no need.  MIPS, i386 and x86_64 all store the key
> in the first word of a two word structure.
> 
> If there were some architecture that didn't fit this model, we would
> have to create a special sorting function and select it (and perhaps
> other parameters as well) in that switch statement.

that's trivial to check:
	sed -n '/^struct exception_table_entry/,/};/p'\
		arch/*/include/asm/uaccess* include/asm-generic/uaccess.h 

and indeed, the only arches that don't follow this model are the ones that 
define ARCH_HAS_SORT_EXTABLE.

> > when i look in the kernel, we have common code behind
> > ARCH_HAS_SORT_EXTABLE. so you could easily do the same thing:
> > 
> > scripts/sortextable.c:
> > 	#ifdef ARCH_HAS_SORT_EXTABLE
> > 	
> > 		switch (w2(ehdr->e_machine)) {
> > 		
> > 		default:
> > 			fprintf(stderr, "unrecognized e_machine %d %s\n",
> > 			
> > 				w2(ehdr->e_machine), fname);
> > 			
> > 			... return a unique exit code like 77 ...
> > 			break;
> > 		
> > 		/* add arch sorting info here */
> > 		}  /* end switch */
> > 	
> > 	#endif
> > 
> > kernel/extable.c:
> > 	#if defined(ARCH_HAS_SORT_EXTABLE)&&  !defined(ARCH_HAS_SORTED_EXTABLE)
> > 	void __init sort_main_extable(void)
> > 	{
> > 	
> > 		sort_extable(__start___ex_table, __stop___ex_table);
> > 	
> > 	}
> > 	#endif
> 
> Yes, I am familiar with that code.  One thing to keep in mind is that
> the compiler has access to struct exception_table_entry, and can easily
> figure out both how big the structure is *and* where the insn field is
> within the structure.
> 
> This is not the case for the author of sortextable.  Except for MIPS,
> MIPS64, i386 and x86_64, I know neither the size of struct
> exception_table_entry, nor the offset of its insn field.

a trivial sed/grep gets you the answer: they're all the same

> > this way all the people not doing unique stuff work out of the box.  only
> > the people who are doing funky stuff need to extend things.
> 
> I didn't want to include something like this that I cannot test.  An
> unsorted (or improperly sorted) exception table is not necessarily
> something that will be noticeable by simply booting the kernel.  Your
> only indication may be a panic or OOPS under rarely encountered conditions.

this is what linux-next is for :)
-mike

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