linux-kernel - Re: [RFC PATCH 6/8] perf tools powerpc: Fix PPC64 ELF ABIv2 symbol decoding

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Message-ID: <20141210112111.GD16045@naverao1-tp.in.ibm.com>
Date:	Wed, 10 Dec 2014 16:51:11 +0530
From:	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	acme@...nel.org, ananth@...ibm.com
Subject: Re: [RFC PATCH 6/8] perf tools powerpc: Fix PPC64 ELF ABIv2 symbol
 decoding

On 2014/12/10 09:13PM, Michael Ellerman wrote:
> On Tue, 2014-12-09 at 23:04 +0530, Naveen N. Rao wrote:
> > PPC64 ELF ABIv2 has a Global Entry Point (GEP) and a Local Entry Point
> > (LEP). For purposes of probing, we need the LEP. Offset to the LEP is
> > encoded in st_other.
> > 
> > diff --git a/tools/perf/arch/powerpc/util/elf-sym-decode.c b/tools/perf/arch/powerpc/util/elf-sym-decode.c
> > new file mode 100644
> > index 0000000..7434656
> > --- /dev/null
> > +++ b/tools/perf/arch/powerpc/util/elf-sym-decode.c
> > @@ -0,0 +1,27 @@
> > +/*
> > + * Decode offset from Global Entry Point to Local Entry Point on PPC64
> > + * ELF ABIv2.
> > + *
> > + * Derived from definitions in arch/powerpc/kernel/module_64.c
> > + *
> > + * Copyright (C) 2014 Ananth N Mavinakayanahalli, IBM Corporation.
> > + *
> > + * This program is free software; you can redistribute it and/or
> > + * modify it under the terms of 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.
> > + */
> > +
> > +#include <gelf.h>
> > +#include "elf_sym.h"
> > +
> > +/* PowerPC64 ABIv2 specific values for the ELF64_Sym st_other field. */
> > +#define STO_PPC64_LOCAL_BIT	5
> > +#define STO_PPC64_LOCAL_MASK	(7 << STO_PPC64_LOCAL_BIT)
> > +#define PPC64_LOCAL_ENTRY_OFFSET(other)					\
> > + (((1 << (((other) & STO_PPC64_LOCAL_MASK) >> STO_PPC64_LOCAL_BIT)) >> 2) << 2)
> 
> You're in userspace, you should be able to get these from elf.h

Ah, ok.

> 
> > +unsigned int arch_elf_sym_decode_offset(GElf_Sym *sym)
> > +{
> > +	return PPC64_LOCAL_ENTRY_OFFSET(sym->st_other);
> 
> What happens on ABIv1 ? We hope st_other is zero?

Yes, st_other is zero in ABIv1.

> 
> > diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
> > index 67e4392..92a424e 100644
> > --- a/tools/perf/util/symbol-elf.c
> > +++ b/tools/perf/util/symbol-elf.c
> > @@ -10,6 +10,7 @@
> >  #include "vdso.h"
> >  #include <symbol/kallsyms.h>
> >  #include "debug.h"
> > +#include "elf_sym.h"
> >  
> >  #ifndef HAVE_ELF_GETPHDRNUM_SUPPORT
> >  static int elf_getphdrnum(Elf *elf, size_t *dst)
> > @@ -848,6 +849,13 @@ int dso__load_sym(struct dso *dso, struct map *map,
> >  		    (sym.st_value & 1))
> >  			--sym.st_value;
> >  
> > +		/*
> > +		 * PPC64 ELF ABIv2 encodes Local Entry Point offset in
> > +		 * the st_other field
> > +		 */
> > +		if ((map->type == MAP__FUNCTION) && sym.st_other)
> > +			sym.st_value += arch_elf_sym_decode_offset(&sym);
> 
> I guess no other arch has needed to do anything like this.
> 
> But if they did it's unlikely they'll want to do the exact same logic, ie.
> check st_other and add some value to st_value. To make it more generically
> useful you could just make it:
> 
> > +		if (map->type == MAP__FUNCTION)
> > +			arch_elf_sym_decode(&sym);
> 
> And do any other checks in the arch routine.

Sure. Makes sense.

- Naveen

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