10.03.2017 05:41, Andy Lutomirski пишет:
On Wed, Mar 8, 2017 at 5:11 PM, Ricardo Neri
<ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:
On Wed, 2017-03-08 at 19:53 +0300, Stas Sergeev wrote:
08.03.2017 19:46, Andy Lutomirski пишет:
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.
That's what I meant. Turning off FIXUP_UMIP would leave UMIP on but
turn off the fixup, so you'd get a SIGSEGV indicating #GP (or a vm86
GP exit).
But then I am confused with the word "compat" in
your "COMPAT_MASK0_X86_UMIP_FIXUP" and
"sys_adjust_compat_mask(int op, int word, u32 mask);"
Leaving UMIP on and only disabling a fixup doesn't
sound like a compat option to me. I would expect
compat to disable it completely.
I guess that the _UMIP_FIXUP part makes it clear that emulation, not
UMIP is disabled, allowing the SIGSEGV be delivered to the user space
program.
Would having a COMPAT_MASK0_X86_UMIP_FIXUP to disable emulation and a
COMPAT_MASK0_X86_UMIP to disable UMIP make sense?
Also, wouldn't having a COMPAT_MASK0_X86_UMIP to disable UMIP defeat its
purpose? Applications could simply use this compat mask to bypass UMIP
and gain access to the instructions it protects.
I was obviously extremely unclear. The point of the proposed syscall
is to let programs opt out of legacy features.
I guess both "compat" and "legacy" are misleading
here. Maybe these are "x86-specific" or "hypervisor-specific",
but a mere enabling of UMIP doesn't immediately make
the use of SLDT instruction a legacy IMHO.
I'll ponder this a bit more.
So if we are to invent something new, it would be nice to
also think up a clear terminology for it. Maybe something
like "X86_FEATURE_xxx_MASK" or alike.
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