linux-kernel - Re: [PATCH 1/1] x86/vdso: fix a reference to a non-existing instruction

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Date:	Fri, 6 Mar 2015 09:36:38 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Jiri Slaby <jslaby@...e.cz>, Jan Beulich <jbeulich@...e.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	rguenther@...e.de, stable <stable@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 1/1] x86/vdso: fix a reference to a non-existing
 instruction


* Andy Lutomirski <luto@...capital.net> wrote:

> On Thu, Mar 5, 2015 at 12:13 AM, Jiri Slaby <jslaby@...e.cz> wrote:
> > .LSTARTFDEDLSI1 says:
> >         /* HACK: The dwarf2 unwind routines will subtract 1 from the
> >            return address to get an address in the middle of the
> >            presumed call instruction.  Since we didn't get here via
> >            a call, we need to include the nop before the real start
> >            to make up for it.  */
> >         .long .LSTART_sigreturn-1-.     /* PC-relative start address */
> >
> > But commit 69d0627a7f6e891189124d784d2fa90cae7c449a (x86 vDSO: reorder
> > vdso32 code) from 2.6.25 replaced
> >   .org __kernel_vsyscall+32,0x90
> > by ALIGN right before __kernel_sigreturn. Of course, ALIGN need not
> > generate any nop in there. Esp. gcc5 collapses vclock_gettime.o and
> > int80.o together with no generated nops as "ALIGN". In that case
> > kernel does not even link:
> > ld: .eh_frame_hdr table[4] FDE at 0000000000000648 overlaps table[5] FDE at 0000000000000670.
> >
> > So fix this by adding there at least a single nop and make the
> > function ALIGN possibly with more nops then.
> >
> > Kudos for reporting and diagnosing should go to Richard.
> >
> > Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> > Reported-by: Richard Biener <rguenther@...e.de>
> > Cc: stable@...r.kernel.org
> > Cc: Andy Lutomirski <luto@...capital.net>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Cc: x86@...nel.org
> > ---
> >  arch/x86/vdso/vdso32/sigreturn.S | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/arch/x86/vdso/vdso32/sigreturn.S b/arch/x86/vdso/vdso32/sigreturn.S
> > index 31776d0efc8c..d7ec4e251c0a 100644
> > --- a/arch/x86/vdso/vdso32/sigreturn.S
> > +++ b/arch/x86/vdso/vdso32/sigreturn.S
> > @@ -17,6 +17,7 @@
> >         .text
> >         .globl __kernel_sigreturn
> >         .type __kernel_sigreturn,@function
> > +       nop /* this guy is needed for .LSTARTFDEDLSI1 below (watch for HACK) */
> >         ALIGN
> >  __kernel_sigreturn:
> >  .LSTART_sigreturn:
> 
> Acked-by: Andy Lutomirski <luto@...capital.net>
> 
> Ingo or Thomas, can you apply this to tip:x86/urgent for this weekend?

Sure, I've applied it.

> Are we willing to depend on cfi-capable assemblers for 32-bit?  If 
> so, could we rewrite this garbage with .cfi directives?  The current 
> code is incomprehensible.

We could try, that code is certainly a mess. How much does that 
restrict our build compatibility?

Thanks,

	Ingo
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