[musl] Re: [PATCH v2] powerpc/64/signal: balance return predictor stack in signal trampoline

[Rich Felker]

On Fri, Jan 22, 2021 at 12:27:14PM +0100, Florian Weimer wrote:
> * Nicholas Piggin:
> 
> > diff --git a/arch/powerpc/kernel/vdso64/sigtramp.S b/arch/powerpc/kernel/vdso64/sigtramp.S
> > index a8cc0409d7d2..bbf68cd01088 100644
> > --- a/arch/powerpc/kernel/vdso64/sigtramp.S
> > +++ b/arch/powerpc/kernel/vdso64/sigtramp.S
> > @@ -6,6 +6,7 @@
> >   * Copyright (C) 2004 Benjamin Herrenschmuidt (benh at kernel.crashing.org), IBM Corp.
> >   * Copyright (C) 2004 Alan Modra (amodra at au.ibm.com)), IBM Corp.
> >   */
> > +#include <asm/cache.h>		/* IFETCH_ALIGN_BYTES */
> >  #include <asm/processor.h>
> >  #include <asm/ppc_asm.h>
> >  #include <asm/unistd.h>
> > @@ -14,21 +15,17 @@
> >  
> >  	.text
> >  
> > -/* The nop here is a hack.  The dwarf2 unwind routines subtract 1 from
> > -   the return address to get an address in the middle of the presumed
> > -   call instruction.  Since we don't have a call here, we artificially
> > -   extend the range covered by the unwind info by padding before the
> > -   real start.  */
> > -	nop
> >  	.balign 8
> > +	.balign IFETCH_ALIGN_BYTES
> >  V_FUNCTION_BEGIN(__kernel_sigtramp_rt64)
> > -.Lsigrt_start = . - 4
> > +.Lsigrt_start:
> > +	bctrl	/* call the handler */
> >  	addi	r1, r1, __SIGNAL_FRAMESIZE
> >  	li	r0,__NR_rt_sigreturn
> >  	sc
> >  .Lsigrt_end:
> >  V_FUNCTION_END(__kernel_sigtramp_rt64)
> > -/* The ".balign 8" above and the following zeros mimic the old stack
> > +/* The .balign 8 above and the following zeros mimic the old stack
> >     trampoline layout.  The last magic value is the ucontext pointer,
> >     chosen in such a way that older libgcc unwind code returns a zero
> >     for a sigcontext pointer.  */
> 
> As far as I understand it, this breaks cancellation handling on musl and
> future glibc because it is necessary to look at the signal delivery
> location to see if a system call sequence has result in an action, and
> that location is no longer in user code after this change.
> 
> We have a glibc test in preparation of our change, and it started
> failing:
> 
>   Linux 5.10 breaks sigcontext_get_pc on powerpc64
>   <https://sourceware.org/bugzilla/show_bug.cgi?id=27223>
> 
> Isn't it possible to avoid the return predictor desynchronization by
> adding the appropriate hint?

Maybe I'm missing something but I don't see how this would break musl;
we just inspect the PC in the mcontext, which I don't see any changes
to and which should still point to the next instruction of the
interrupted context. I don't have a test environment though so I'll
have to wait for feedback from ppc users to be sure. Are there any
further details on how it's breaking glibc?

Rich