Re: [v6 PATCH 00/21] x86: Enable User-Mode Instruction Prevention

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08.03.2017 19:06, Andy Lutomirski пишет:
On Wed, Mar 8, 2017 at 6:08 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
08.03.2017 03:32, Ricardo Neri пишет:
These are the instructions covered by UMIP:
* SGDT - Store Global Descriptor Table
* SIDT - Store Interrupt Descriptor Table
* SLDT - Store Local Descriptor Table
* SMSW - Store Machine Status Word
* STR - Store Task Register

This patchset initially treated tasks running in virtual-8086 mode as a
special case. However, I received clarification that DOSEMU[8] does not
support applications that use these instructions.
Can you remind me what was special about it?  It looks like you still
emulate them in v8086 mode.
Indeed, sorry, I meant prot mode here. :)
So I wonder what was cited to be special about v86.

Yes, this is the case.
But at least in the past there was an attempt to
support SLDT as it is used by an ancient pharlap
DOS extender (currently unsupported by dosemu1/2).
So how difficult would it be to add an optional
possibility of delivering such SIGSEGV to userspace
so that the kernel's dummy emulation can be overridden?
It doesn't need to be a matter of this particular
patch set, i.e. this proposal should not trigger a
v7 resend of all 21 patches. :) But it would be useful
for the future development of dosemu2.
What I'd actually like to see is a totally separate patchset that adds
an inheritable (but reset on exec) per-task mask of legacy
compatibility features to disable.  Maybe:

sys_adjust_compat_mask(int op, int word, u32 mask);
No no, since I meant prot mode, this is not what I need.
I would never need to disable UMIP as to allow the
prot mode apps to do SLDT. Instead it would be good
to have an ability to provide a replacement for the dummy
emulation that is currently being proposed for kernel.
All is needed for this, is just to deliver a SIGSEGV.
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