[llvm-dev] retpoline mitigation and 6.0

David Woodhouse via llvm-dev llvm-dev at lists.llvm.org
Fri Feb 9 11:38:49 PST 2018

On Fri, 2018-02-09 at 11:24 -0800, Reid Kleckner wrote:
> I haven't read the all the emails in full detail, but it seems pretty
> clear that  __x86_indirect_thunk and __llvm_retpoline_push do not do
> the same things. It sounds like __llvm_retpoline_push is equivalent
> to __x86_indirect_thunk except first it swaps the two words on the
> top of the stack.
> I arranged it this way because the x86 call instruction puts the
> intended return address on the top of the stack, and there's no easy
> way to put it anywhere else. We use this thunk when we want to make
> an indirect call and there are no available scratch registers, i.e.
> 32-bit -mregparm=3 and the call has three or more arguments, which
> happens in Linux. One way to avoid this would be to compile with
> -mregparm=2, but that would pessimize direct calls unnecessarily.
> It sounds like Linux is already providing its own thunks, so it might
> be better to for us to go back to the __llvm_retpoline_push external
> thunk name and then get Linux to provide that thunk ifdef __clang__.

Yes, the thunks are different in this case. That one should indeed be
renamed, and I'll provide the correct one in the kernel.

However, even if I let LLVM emit the thunk for itself, code generation
seems hosed. It looks like after a call to __llvm_retpoline_push (or
perhaps more to the point after pushing the target address in
preparation for the call?) we're losing track of where %esp points, and
thus everything else that was spilled to the stack.

Please let me know if you need any more detail on that, than I provided
in my last email. You should have everything you need there to follow
along with my analysis and reproduce it for yourself. Except perhaps
the Grandma-suck-eggs command lines like

$ qemu-system-i386  -display none -serial stdio -kernel arch/x86/boot/bzImage  -append earlyprintk=ttyS0,keep -s -S

$ gdb vmlinux
(gdb) tar rem localhost:1234
(gdb) break mp_register_ioapic
(gdb) c
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