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

[Andy Lutomirski]

On Sat, Dec 26, 2015 at 10:57 PM, Tony Luck <tony.luck@xxxxxxxxx> wrote:
> On Sat, Dec 26, 2015 at 6:16 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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
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