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Re: [PATCH] ppc64: Add minimal fallback unwinder.


On 2017/05/24 10:05PM, Mark Wielaard wrote:
> This adds a minimal fallback unwinder for ppc64[le] in case we cannot find
> CFI for a particular address. It simply always sets the program counter to
> the link register, picks the previous stack pointer from the backchain,
> and the previous link register from the LR save area.
> 
> This isn't enough for some more complex situations like signal frames,
> but it is only used when we don't have CFI and seems to work nicely
> in the case of perf with libdw powerpc support:
> https://lkml.org/lkml/2017/5/18/998
> 
> Signed-off-by: Mark Wielaard <mark@klomp.org>

Mark,
Thanks for adding this! I am not entirely familiar with how this works, 
but...

> ---
>  backends/Makefile.am    |  2 +-
>  backends/ppc64_init.c   |  1 +
>  backends/ppc64_unwind.c | 76 +++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 78 insertions(+), 1 deletion(-)
>  create mode 100644 backends/ppc64_unwind.c
> 
> diff --git a/backends/Makefile.am b/backends/Makefile.am
> index ff80a82..ac45a45 100644
> --- a/backends/Makefile.am
> +++ b/backends/Makefile.am
> @@ -98,7 +98,7 @@ am_libebl_ppc_pic_a_OBJECTS = $(ppc_SRCS:.c=.os)
> 
>  ppc64_SRCS = ppc64_init.c ppc64_symbol.c ppc64_retval.c \
>  	     ppc64_corenote.c ppc_regs.c ppc_auxv.c ppc_attrs.c ppc_syscall.c \
> -	     ppc_cfi.c ppc_initreg.c ppc64_resolve_sym.c
> +	     ppc_cfi.c ppc_initreg.c ppc64_unwind.c ppc64_resolve_sym.c
>  libebl_ppc64_pic_a_SOURCES = $(ppc64_SRCS)
>  am_libebl_ppc64_pic_a_OBJECTS = $(ppc64_SRCS:.c=.os)
> 
> diff --git a/backends/ppc64_init.c b/backends/ppc64_init.c
> index 11d3a77..e567033 100644
> --- a/backends/ppc64_init.c
> +++ b/backends/ppc64_init.c
> @@ -73,6 +73,7 @@ ppc64_init (Elf *elf __attribute__ ((unused)),
>    eh->frame_nregs = (114 - 1) + 32;
>    HOOK (eh, set_initial_registers_tid);
>    HOOK (eh, dwarf_to_regno);
> +  HOOK (eh, unwind);
>    HOOK (eh, resolve_sym_value);
> 
>    /* Find the function descriptor .opd table for resolve_sym_value.  */
> diff --git a/backends/ppc64_unwind.c b/backends/ppc64_unwind.c
> new file mode 100644
> index 0000000..f7dffd6
> --- /dev/null
> +++ b/backends/ppc64_unwind.c
> @@ -0,0 +1,76 @@
> +/* Get previous frame state for an existing frame state.
> +   Copyright (C) 2017 Red Hat, Inc.
> +   This file is part of elfutils.
> +
> +   This file is free software; you can redistribute it and/or modify
> +   it under the terms of either
> +
> +     * the GNU Lesser General Public License as published by the Free
> +       Software Foundation; either version 3 of the License, or (at
> +       your option) any later version
> +
> +   or
> +
> +     * the GNU General Public License as published by the Free
> +       Software Foundation; either version 2 of the License, or (at
> +       your option) any later version
> +
> +   or both in parallel, as here.
> +
> +   elfutils is distributed in the hope that it will be useful, but
> +   WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   General Public License for more details.
> +
> +   You should have received copies of the GNU General Public License and
> +   the GNU Lesser General Public License along with this program.  If
> +   not, see <http://www.gnu.org/licenses/>.  */
> +
> +#ifdef HAVE_CONFIG_H
> +# include <config.h>
> +#endif
> +
> +#define BACKEND ppc64_
> +
> +#define LR_REG 65 /* Not 108, see ppc_dwarf_to_regno.  */
> +#define SP_REG  1
> +
> +#define LR_OFFSET 16
> +
> +#include "libebl_CPU.h"
> +
> +/* Simplistic fallback frame unwinder. SP points to the backchain (contains
> +   address of previous stack pointer). At SP offset 16 is the LR save area
> +   (contains the value of the previous LR).  */
> +
> +bool
> +EBLHOOK(unwind) (Ebl *ebl __attribute__ ((unused)),
> +		 Dwarf_Addr pc __attribute__ ((unused)),
> +                 ebl_tid_registers_t *setfunc, ebl_tid_registers_get_t *getfunc,
> +                 ebl_pid_memory_read_t *readfunc, void *arg,
> +                 bool *signal_framep __attribute__ ((unused)))
> +{
> +  Dwarf_Word sp, newSp, lr, newLr;
> +
> +  /* Stack pointer points to the backchain which contains the previous sp.  */
> +  if (! getfunc (SP_REG, 1, &sp, arg))
> +    sp = 0;
> +
> +  /* Link register contains previous program counter.  */
> +  if (! getfunc (LR_REG, 1, &lr, arg)
> +      || lr == 0
> +      || ! setfunc (-1, 1, &lr, arg))
> +    return false;
> +
> +  if (! readfunc(sp, &newSp, arg))
> +    newSp = 0;
> +
> +  if (! readfunc(sp + LR_OFFSET, &newLr, arg))

should this be newSp + LR_OFFSET, since the LR save area is in the 
caller's stack frame?

- Naveen

> +    newLr = 0;
> +
> +  setfunc(SP_REG, 1, &newSp, arg);
> +  setfunc(LR_REG, 1, &newLr, arg);
> +
> +  /* Sanity check the stack grows down.  */
> +  return newSp > sp;
> +}
> -- 
> 1.8.3.1
> 


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