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SubjectRe: objtool clac/stac handling change..
On Thu, Jul 02, 2020 at 12:52:27PM -0700, Linus Torvalds wrote:
> On Thu, Jul 2, 2020 at 6:32 AM Michael Ellerman <> wrote:
> >
> > Probably the simplest option for us is to just handle it in our
> > unsafe_op_wrap(). I'll try and come up with something tomorrow.
> IMy suggestion was to basically just always handle it in all exception cases.
> And note that IU don't mean the fault handler: obviously page faults
> (or unaligned faults or whatever) can happen while in a user access
> region.
> But I mean any time fixup_exception() triggers.
> For x86, this is in fact particularly natural: it involves just always
> clearing the AC bit in the "struct pt_regs" that fixup_exception()
> gets anyway. We can do it without even bothering with checking for
> CLAC/STAC support, since without it, AC is meaningless in kernel mode
> anyway, but also because doing "user_access_end()" in the exception
> would be pointless: AC is restored by the exception routine, so on x86
> you *have* to do it by just modifying the return state.

What about

static inline int copy_xregs_to_user(struct xregs_state __user *buf)
XSTATE_OP(XSAVE, buf, -1, -1, err);
where XSTATE_OP() is
#define XSTATE_OP(op, st, lmask, hmask, err) \
asm volatile("1:" op "\n\t" \
"xor %[err], %[err]\n" \
"2:\n\t" \
".pushsection .fixup,\"ax\"\n\t" \
"3: movl $-2,%[err]\n\t" \
"jmp 2b\n\t" \
".popsection\n\t" \
_ASM_EXTABLE(1b, 3b) \
: [err] "=r" (err) \
: "D" (st), "m" (*st), "a" (lmask), "d" (hmask) \
: "memory")

Rely upon objtool not noticing that we have, in effect, clac() in a state
where AC is already cleared? We could massage that thing to take a label,
but it wouldn't be pretty...

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