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    Subject[PATCH 23/40] perf report: Fix kernel symbol adjustment for s390x
    Date
    From: Thomas Richter <tmricht@linux.vnet.ibm.com>

    On s390x the kernel text segment starts at address 0x0. When perf
    report reads kernel symbols from vmlinux file it adds an offset of
    0x1000.

    For example see symbol set_reset_devices:

    [root@s8360047 linux-devel]# nm -A vmlinux| fgrep set_reset_devices
    vmlinux:0000000001379000 t set_reset_devices
    [root@s8360047 linux-devel]#

    [root@s8360047 linux-devel]# fgrep set_reset_devices /proc/kallsyms
    0000000001379000 t set_reset_devices
    [root@s8360047 linux-devel]#

    The kernel symbol table and the vmlinux file have the same address for
    symbol set_reset_devices namely 1379000.

    When perf report reads this symbols it displays it with address
    symbol__new: set_reset_devices 0x137a000-0x137a018

    There is a difference between perf report and vmlinux of 0x1000.

    The reason for the difference is at kernel symbol load time in function
    dso__load_sym(). The vmlinux file is investigated with its ELF header.
    Command readelf shows this:

    Section Headers:
    [Nr] Name Type Address Offset
    Size EntSize Flags Link Info Align
    [ 0] NULL 0000000000000000 00000000
    0000000000000000 0000000000000000 0 0 0
    [ 1] .text PROGBITS 0000000000000000 00001000
    0000000000b0e0c2 0000000000000000 AX 0 0 128

    This leads to an invalid calculation of the symbol start address, see
    file utit/symbol-elf.c line 974:

    /* Adjust symbol to map to file offset */
    if (adjust_kernel_syms)
    sym.st_value -= shdr.sh_addr - shdr.sh_offset;

    With shdr.sh_addr set to 0x0 and shdr.sh_offset set to 0x1000 as read
    from the ELF .text section 0x1000 is added to the symbol address.

    I would like to fix this by introducing an archticture specific function
    named elf__needs_adjust_symbols(). This is the same approach as done by
    PowerPC. The function currently does not exist for s390x and the
    default weak one is used. The s390x specific one returns false when
    symsrc_init() is invoked for kernel symbols and results in variable
    adjust_kernel_syms being false. This omits the adjustment and the
    correct address is displayed (when symbol resolvement does not work).

    The s390x specific function returns false for kernel symbol adjustment
    and returns true for kernel modules, processes and shared libraries.

    Signed-off-by: Thomas-Mich Richter <tmricht@linux.vnet.ibm.com>
    Cc: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
    Cc: Thomas-Mich Richter <tmricht@linux.vnet.ibm.com>
    LPU-Reference: 20170713130252.6167-1-tmricht@linux.vnet.ibm.com
    Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
    ---
    tools/perf/arch/s390/util/Build | 1 +
    tools/perf/arch/s390/util/sym-handling.c | 22 ++++++++++++++++++++++
    2 files changed, 23 insertions(+)
    create mode 100644 tools/perf/arch/s390/util/sym-handling.c

    diff --git a/tools/perf/arch/s390/util/Build b/tools/perf/arch/s390/util/Build
    index 5bd7b9260cc0..bd518b623d7a 100644
    --- a/tools/perf/arch/s390/util/Build
    +++ b/tools/perf/arch/s390/util/Build
    @@ -1,4 +1,5 @@
    libperf-y += header.o
    +libperf-y += sym-handling.o
    libperf-y += kvm-stat.o

    libperf-$(CONFIG_DWARF) += dwarf-regs.o
    diff --git a/tools/perf/arch/s390/util/sym-handling.c b/tools/perf/arch/s390/util/sym-handling.c
    new file mode 100644
    index 000000000000..b6cd056ccf71
    --- /dev/null
    +++ b/tools/perf/arch/s390/util/sym-handling.c
    @@ -0,0 +1,22 @@
    +/*
    + * Architecture specific ELF symbol handling and relocation mapping.
    + *
    + * Copyright 2017 IBM Corp.
    + * Author(s): Thomas Richter <tmricht@linux.vnet.ibm.com>
    + *
    + * This program is free software; you can redistribute it and/or modify
    + * it under the terms of the GNU General Public License (version 2 only)
    + * as published by the Free Software Foundation.
    + */
    +
    +#include "symbol.h"
    +
    +#ifdef HAVE_LIBELF_SUPPORT
    +bool elf__needs_adjust_symbols(GElf_Ehdr ehdr)
    +{
    + if (ehdr.e_type == ET_EXEC)
    + return false;
    + return ehdr.e_type == ET_REL || ehdr.e_type == ET_DYN;
    +}
    +
    +#endif
    --
    2.9.4
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