Re: [PATCHV5 3/3] x86, ras: Add __mcsafe_copy() function to recover from machine checks

Andy Lutomirski
2015-12-27T12:19:19Z

On Sat, Dec 26, 2015 at 10:57 PM, Tony Luck <tony.luck@gmail.com> wrote:
> On Sat, Dec 26, 2015 at 6:16 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>>> We could make one of them 31-bits (since even an "allyesconfig" kernel
>>>> is still much smaller than a gigabyte) to free a bit for a flag. But there
>>>> are those external tools to pre-sort exception tables that would all
>>>> need to be fixed too.
>>
>> Wait, why? The external tools sort by source address, and we'd
>> squeeze the flag into the target address, no?
>
> I was thinking that we'd need to recompute the fixup when we move
> the entry to its new sorted location. So that:
>
> ex_fixup_addr(const struct exception_table_entry *x)
> {
> return (unsigned long)&x->fixup + x->fixup;
> }
>
> will get the right value. Maybe this would still work out
> if the fixup is a 31-bit value plus a flag, but the external
> tool thinks it is a 32-bit value? I'd have to ponder that.

I think I can save you some pondering. This old patch gives two flag
bits. Feel free to borrow the patch, but you'll probably want to
change the _EXTABLE_CLASS_XYZ macros:

https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/commit/?h=strict_uaccess_fixups/patch_v1&id=16644d9460fc6531456cf510d5efc57f89e5cd34

--Andy