[PATCH] perf report: fix kernel symbol adjustment for s390x

[PATCH] perf report: fix kernel symbol adjustment for s390x

[Thomas Richter]

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 Richter <tmricht@linux.vnet.ibm.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 5bd7b92..bd518b6 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 0000000..b6cd056
--- /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
-- 
2.9.3

Re: [PATCH] perf report: fix kernel symbol adjustment for s390x

[Arnaldo Carvalho de Melo]

Em Thu, Jul 13, 2017 at 03:02:52PM +0200, Thomas Richter escreveu:
> 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.

Seems ok, arch specific one, same method used by another arch, applied.

- Arnaldo