Re: [Xen-devel] [PATCH] x86/PV: properly populate descriptor tables

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 26/10/15 14:43, David Vrabel wrote:
> On 23/09/15 16:34, Jan Beulich wrote:
>> Us extending the GDT limit past the Xen descriptors so far meant that
>> guests (including user mode programs) accessing any descriptor table
>> slot above the original OS'es limit but below the first Xen descriptor
>> caused a #PF, converted to a #GP in our #PF handler. Which is quite
>> different from the native behavior, where some of such accesses (LAR
>> and LSL) don't fault. Mimic that behavior by mapping a blank page into
>> unused slots.
>>
>> While not strictly required, treat the LDT the same for consistency.
> This change causes a 32-bit userspace process running in a 32-bit PV
> guest to segfault.
>
> The process is a Go program and it is using the modify_ldt() system call
> (which is successful) but loading %gs with the new descriptor causes a
> fault.  Even a minimal (empty main()) go program faults.

D'uh - its obvious now you point it out.

By filling the shadow ldt slots as present, zero entries, we break their
demand-faulting.

We can't be safe to incorrect faults from LAR/LSL, *and* perform demand
faulting of the LDT.

Reverting hunk 2 for now is the best course of action.

~Andrew



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