Re: [PATCH] s390/perf: fix 'start' address of module's map

[Christian Borntraeger]

On 07/26/2016 09:50 PM, Arnaldo Carvalho de Melo wrote:
> Em Thu, Jul 21, 2016 at 11:10:51AM +0800, Song Shan Gong escreveu:
>> At preset, when creating module's map, perf gets 'start' address by parsing
>> '/proc/modules', but it's module base address, isn't the start address of
>> '.text' section. In most archs, it's OK. But for s390, it places 'GOT' and
>> 'PLT' relocations before '.text' section. So there exists an offset between
>> module base address and '.text' section, which will incur wrong symbol
>> resolution for modules.
> 
> I'll apply this as it fixes the problem for you and we need to get fixes
> in ASAP to get this into 4.8, but why can't we just use your method for
> all arches and get rid of this arch__ hook? I.e. if I look here in my
> x86_64 notebook I see:
> 
> [acme@jouet linux]$ cat /sys/module/tun/sections/.text 
> 0xffffffffc0af2000
> [acme@jouet linux]$ grep tun /proc/modules 
> tun 28672 4 vhost_net, Live 0xffffffffc0af2000
> [acme@jouet linux]$ 
> 
> So I could as well use what is in /sys/module/tun/sections/.text instead
> of reading it from /proc/modules and, in s390, reading it from
> /sys/module/tun/sections/.text.
> 
> Do you see any problem with using this approach for _all_ arches?

I think it should work well for _all_ arches but it will probably be
hard to test this without help. 
I wouldn't be surprised if other architectures than s390 actually have
the same issue, so doing this for everybody might atually fix this somewhere
else.

> 
> - Arnaldo
> 
>> Fix this bug by getting 'start' address of module's map from parsing
>> '/sys/module/[module name]/sections/.text', not from '/proc/modules'.
>>
>> Signed-off-by: Song Shan Gong <gongss@xxxxxxxxxxxxxxxxxx>
>> Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
>> ---
>>  tools/perf/arch/s390/util/Build     |  2 ++
>>  tools/perf/arch/s390/util/machine.c | 19 +++++++++++++++++++
>>  tools/perf/util/machine.c           |  8 ++++++++
>>  tools/perf/util/machine.h           |  1 +
>>  4 files changed, 30 insertions(+)
>>  create mode 100644 tools/perf/arch/s390/util/machine.c
>>
>> diff --git a/tools/perf/arch/s390/util/Build b/tools/perf/arch/s390/util/Build
>> index 8a61372..5bd7b92 100644
>> --- a/tools/perf/arch/s390/util/Build
>> +++ b/tools/perf/arch/s390/util/Build
>> @@ -2,3 +2,5 @@ libperf-y += header.o
>>  libperf-y += kvm-stat.o
>>  
>>  libperf-$(CONFIG_DWARF) += dwarf-regs.o
>> +
>> +libperf-y += machine.o
>> diff --git a/tools/perf/arch/s390/util/machine.c b/tools/perf/arch/s390/util/machine.c
>> new file mode 100644
>> index 0000000..b9a95a1
>> --- /dev/null
>> +++ b/tools/perf/arch/s390/util/machine.c
>> @@ -0,0 +1,19 @@
>> +#include <unistd.h>
>> +#include <stdio.h>
>> +#include <string.h>
>> +#include "util.h"
>> +#include "machine.h"
>> +#include "api/fs/fs.h"
>> +
>> +int arch__fix_module_text_start(u64 *start, const char *name)
>> +{
>> +	char path[PATH_MAX];
>> +
>> +	snprintf(path, PATH_MAX, "module/%.*s/sections/.text",
>> +				(int)strlen(name) - 2, name + 1);
>> +
>> +	if (sysfs__read_ull(path, (unsigned long long *)start) < 0)
>> +		return -1;
>> +
>> +	return 0;
>> +}
>> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
>> index b177218..97cc9f0 100644
>> --- a/tools/perf/util/machine.c
>> +++ b/tools/perf/util/machine.c
>> @@ -1091,12 +1091,20 @@ static int machine__set_modules_path(struct machine *machine)
>>  
>>  	return map_groups__set_modules_path_dir(&machine->kmaps, modules_path, 0);
>>  }
>> +int __weak arch__fix_module_text_start(u64 *start __maybe_unused,
>> +				const char *name __maybe_unused)
>> +{
>> +	return 0;
>> +}
>>  
>>  static int machine__create_module(void *arg, const char *name, u64 start)
>>  {
>>  	struct machine *machine = arg;
>>  	struct map *map;
>>  
>> +	if (arch__fix_module_text_start(&start, name) < 0)
>> +		return -1;
>> +
>>  	map = machine__findnew_module_map(machine, start, name);
>>  	if (map == NULL)
>>  		return -1;
>> diff --git a/tools/perf/util/machine.h b/tools/perf/util/machine.h
>> index 41ac9cf..20739f7 100644
>> --- a/tools/perf/util/machine.h
>> +++ b/tools/perf/util/machine.h
>> @@ -216,6 +216,7 @@ struct symbol *machine__find_kernel_function_by_name(struct machine *machine,
>>  
>>  struct map *machine__findnew_module_map(struct machine *machine, u64 start,
>>  					const char *filename);
>> +int arch__fix_module_text_start(u64 *start, const char *name);
>>  
>>  int __machine__load_kallsyms(struct machine *machine, const char *filename,
>>  			     enum map_type type, bool no_kcore, symbol_filter_t filter);
>> -- 
>> 2.3.0
>