Date: Fri, 22 Jan 2021 09:44:05 -0500 From: Rich Felker <dalias@...c.org> To: Florian Weimer <fweimer@...hat.com> Cc: Nicholas Piggin <npiggin@...il.com>, linuxppc-dev@...ts.ozlabs.org, Alan Modra <amodra@...il.com>, musl@...ts.openwall.com, libc-alpha@...rceware.org Subject: Re: Re: [PATCH v2] powerpc/64/signal: balance return predictor stack in signal trampoline 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@...nel.crashing.org), IBM Corp. > > * Copyright (C) 2004 Alan Modra (amodra@...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
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