Re: [PATCH v8 1/3] x86: Expand exception table to allow new handling options

From: Brian Gerst
Date: Mon Jan 11 2016 - 18:09:51 EST

On Sat, Jan 9, 2016 at 1:36 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Jan 8, 2016 8:31 PM, "Brian Gerst" <brgerst@xxxxxxxxx> wrote:
>> On Fri, Jan 8, 2016 at 10:39 PM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>> > On Fri, Jan 8, 2016 at 8:52 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> >> On Fri, Jan 8, 2016 at 12:49 PM, Tony Luck <tony.luck@xxxxxxxxx> wrote:
>> >>> Huge amounts of help from Andy Lutomirski and Borislav Petkov to
>> >>> produce this. Andy provided the inspiration to add classes to the
>> >>> exception table with a clever bit-squeezing trick, Boris pointed
>> >>> out how much cleaner it would all be if we just had a new field.
>> >>>
>> >>> Linus Torvalds blessed the expansion with:
>> >>> I'd rather not be clever in order to save just a tiny amount of space
>> >>> in the exception table, which isn't really criticial for anybody.
>> >>>
>> >>> The third field is a simple integer indexing into an array of handler
>> >>> functions (I thought it couldn't be a relative pointer like the other
>> >>> fields because a module may have its ex_table loaded more than 2GB away
>> >>> from the handler function - but that may not be actually true. But the
>> >>> integer is pretty flexible, we are only really using low two bits now).
>> >>>
>> >>> We start out with three handlers:
>> >>>
>> >>> 0: Legacy - just jumps the to fixup IP
>> >>> 1: Fault - provide the trap number in %ax to the fixup code
>> >>> 2: Cleaned up legacy for the uaccess error hack
>> >>
>> >> I think I preferred the relative function pointer approach.
>> >>
>> >> Also, I think it would be nicer if the machine check code would invoke
>> >> the handler regardless of which handler (or class) is selected. Then
>> >> the handlers that don't want to handle #MC can just reject them.
>> >>
>> >> Also, can you make the handlers return bool instead of int?
>> >
>> > I'm hashing up an idea that could eliminate alot of text in the .fixup
>> > section, but it needs the integer handler method to work. We have
>> > alot of fixup code that does "mov $-EFAULT, reg; jmp xxxx". If we
>> > encode the register in the third word, the handler can be generic and
>> > no fixup code for each user access would be needed. That would
>> > recover alot of the memory used by expanding the exception table.
>> On second thought, this could still be implemented with a relative
>> function pointer. We'd just need a separate function for each
>> register.
> If we could get gcc to play along (which, IIRC, it already can for
> __put_user), we can do much better with jump labels -- the fixup
> target would be a jump label.
> Even without that, how about using @cc? Do:
> clc
> mov whatever, wherever
> The fixup sets the carry flag and skips the faulting instruction
> (either by knowing the length or by decoding it), and the inline asm
> causes gcc to emit jc to the error logic.
> --Andy

I agree that for at least put_user() using asm goto would be an even
better option. get_user() on the other hand, will be much messier to
deal with, since asm goto statements can't have outputs, plus it
zeroes the output register on fault.

Brian Gerst