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


>
> 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.

For those with knowledge of the inner working of other architectures, it
may be as simple as a two line patch to add support, but it isn't
something that I want to take responsibility for at this point

> 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.

David Daney


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