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

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Message-ID: <1418206413.9279.9.camel@ellerman.id.au>
Date:	Wed, 10 Dec 2014 21:13:33 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
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 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

> +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?

> 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.

cheers


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