Re: Build warnings in Xen 5.15.y and 5.10.y with retbleed backports

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 7/12/22 3:31 PM, Greg KH wrote:
> On Tue, Jul 12, 2022 at 03:19:39PM -0400, Boris Ostrovsky wrote:
> > On 7/12/22 12:38 PM, Greg KH wrote:
> > > Hi all,
> > >
> > > I'm seeing the following build warning:
> > >         arch/x86/kernel/head_64.o: warning: objtool: 
> > > xen_hypercall_mmu_update(): can't find starting instruction
> > > in the 5.15.y and 5.10.y retbleed backports.
> > >
> > > I don't know why just this one hypercall is being called out by objtool,
> > > and this warning isn't in 5.18 and Linus's tree due to I think commit
> > > 5b2fc51576ef ("x86/ibt,xen: Sprinkle the ENDBR") being there.
> > >
> > > But, is this a ret call that we "forgot" here?  It's a "real" ret in
> > > Linus's branch:
> > >
> > > .pushsection .noinstr.text, "ax"
> > >         .balign PAGE_SIZE
> > > SYM_CODE_START(hypercall_page)
> > >         .rept (PAGE_SIZE / 32)
> > >                 UNWIND_HINT_FUNC
> > >                 ANNOTATE_NOENDBR
> > >                 ANNOTATE_UNRET_SAFE
> > >                 ret
> > >                 /*
> > >                  * Xen will write the hypercall page, and sort out ENDBR.
> > >                  */
> > >                 .skip 31, 0xcc
> > >         .endr
> > >
> > > while 5.15.y and older has:
> > > .pushsection .text
> > >         .balign PAGE_SIZE
> > > SYM_CODE_START(hypercall_page)
> > >         .rept (PAGE_SIZE / 32)
> > >                 UNWIND_HINT_FUNC
> > >                 .skip 31, 0x90
> > >                 ANNOTATE_UNRET_SAFE
> > >                 RET
> > >         .endr
> > >
> > > So should the "ret" remain or be turned into "RET" in mainline right
> > > now?
> >
> > It doesn't matter --- this is overwritten by the hypervisor during 
> > initialization when Xen fills in actual hypercall code.
> >
> >
> > So f4b4bc10b0b85ec66f1a9bf5dddf475e6695b6d2 added 'ret' to make objtool happy 
> > and then 14b476e07fab6 replaced 'ret' with RET as part of SLS fixes. The latter 
> > was not really necessary but harmless.
> >
> >
> > So it can be 'ret', RET, or anything else that tools don't complain about. It 
> > will not be executed.
> Cool, thanks.
>
> But what about the objtool warning that I now see?  Is that "real"?


It's not real in the sense that the code there is not real, it will be 
overwritten. (Originally the whole page was 'nop's)


I am getting a different error BTW:

        arch/x86/kernel/head_64.o: warning: objtool: .text+0x5: unreachable 
instruction




> I don't run any Xen systems, so I can't test any of this myself.

You can't test any changes to that code --- it is rewritten when Xen guest is 
running.


We probably do want to shut up objtool. Josh, any suggestions?


-boris