Re: [PATCH] x86/mpx: Correctly report do_mpx_bt_fault() failures to user-space

[Joerg Roedel <jroedel@suse.de>]

Hi Dave,

On Mon, Apr 17, 2017 at 08:38:03AM -0700, Dave Hansen wrote:
> Just to be clear, the thing you're calling "correct" is this do_trap(),
> right?
> 
>         do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code, NULL);

Yes, because it signals the right trap_nr and error_code to user-space.

> do_mpx_bt_fault() can fail for a bunch of reasons:
>  * unexpected or invalid value in BNDCSR
>  * out of memory (physical or virtual)
>  * unresolvable fault walking/filling bounds tables
>  * !valid and non-empty bad entry in the bounds tables
> 
> This will end up sending a signal that *looks* like a X86_TRAP_BR for
> all of those, including those that are not really bounds-related, like
> unresolvable faults.  We also don't populate enough information in the
> siginfo that gets delivered for userspace to resolve the fault.
> 
> I'm not sure this patch is the right thing.

The problem is, without this patch the trap_nr reported to user-space is
0, which maps to divide-by-zero. I think this is wrong, and since all
failure cases from do_mpx_bt_fault() can only happen in the #BR
exception handler, I think that reporting X86_TRAP_BR for all failure
cases is the right thing to do.

I don't know whether user-space (with this patch) already gets enough
information from do_trap() to handle all of the above cases, but it is a
step in the right direction.


	Joerg