Re: [PATCH] perf symbols/KCORE: Rebuild rbtree when adjusting symbols for kcore

[Adrian Hunter <adrian.hunter@intel.com>]

On 06/11/15 20:51, Arnaldo Carvalho de Melo wrote:
> Em Fri, Nov 06, 2015 at 03:59:29PM +0200, Adrian Hunter escreveu:
>> On 06/11/15 15:19, Arnaldo Carvalho de Melo wrote:
>>> Em Fri, Nov 06, 2015 at 09:46:12AM +0000, Wang Nan escreveu:
>>>> In dso__split_kallsyms_for_kcore(), current code adjusts symbol's
>>>> address but only reinsert it into rbtree if the symbol belongs to
>>>> another map. However, the expression for adjusting symbol (pos->start -=
>>>> curr_map->start - curr_map->pgoff) can change the relative order between
>>>> two symbols (even if the affected symbols are in different maps, in
>>>> kcore case they are possible to share one same dso), which damages the
>>>> rbtree.
>>>
>>> Right, some code does change the symbol values it gets from whatever
>>> symtab (kallsyms, ELF, JIT maps, etc) when it should instead use the per
>>> map data structure (struct map) and its ->{map,unmap}_ip, ->pgoff,
>>> ->reloc, members for that :-\
>>>
>>> I.e. 'struct dso' should be just what comes from the symtab, while
>>> 'struct map' should be about where that DSO is in memory.
>>>
>>> With that in mind, do you still think your fix is the correct one?
>>>
>>> Adrian?
>>
>> The problem is when the order in memory (in kallsyms) is different
>> to the order on the dso (kcore).
> 
> What order? Can you ellaborate a bit more?

Normally symbols are read from the DSO and adjusted, if need be, so that the
symbol start matches the file offset in the DSO file (we want the file
offset because that is what we know from MMAP events). That is done by
dso__load_sym() which inserts the symbols *after* adjusting them.

In the case of kcore, the symbols have been read from kallsyms and the
symbol start is the memory address. The symbols have to be adjusted to match
the kcore file offsets. dso__split_kallsyms_for_kcore() does that, but now
the adjustment is being done *after* the symbols have been inserted. It
appears dso__split_kallsyms_for_kcore() was assuming that changing the
symbol start would not change the order in the rbtree - which is, of course,
not guaranteed.

>                                            I thought more about keeping
> whatever address is in the symtab from where we read the symbols, and
> then create one map per kernel module all pointing to the same DSO, that
> would be the one loaded from kallsyms.
> 
> Any adjustments would be fone in the map, not the DSO.
> 
> I.e. we wouldn't be splitting anything, just creating struct map
> instances pointing to the same DSO.
> 
> - Arnaldo
> 
>> I think to make it more general it needs to insert to a new tree.
>> e.g.
> 
> 
>> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
>> index b4cc7662677e..09343a880c0b 100644
>> --- a/tools/perf/util/symbol.c
>> +++ b/tools/perf/util/symbol.c
>> @@ -654,19 +654,24 @@ static int dso__split_kallsyms_for_kcore(struct dso *dso, struct map *map,
>>  	struct map_groups *kmaps = map__kmaps(map);
>>  	struct map *curr_map;
>>  	struct symbol *pos;
>> -	int count = 0, moved = 0;
>> +	int count = 0;
>> +	struct rb_root old_root = dso->symbols[map->type];
>>  	struct rb_root *root = &dso->symbols[map->type];
>>  	struct rb_node *next = rb_first(root);
>>  
>>  	if (!kmaps)
>>  		return -1;
>>  
>> +	*root = RB_ROOT;
>> +
>>  	while (next) {
>>  		char *module;
>>  
>>  		pos = rb_entry(next, struct symbol, rb_node);
>>  		next = rb_next(&pos->rb_node);
>>  
>> +		rb_erase_init(&pos->rb_node, &old_root);
>> +
>>  		module = strchr(pos->name, '\t');
>>  		if (module)
>>  			*module = '\0';
>> @@ -674,28 +679,21 @@ static int dso__split_kallsyms_for_kcore(struct dso *dso, struct map *map,
>>  		curr_map = map_groups__find(kmaps, map->type, pos->start);
>>  
>>  		if (!curr_map || (filter && filter(curr_map, pos))) {
>> -			rb_erase_init(&pos->rb_node, root);
>>  			symbol__delete(pos);
>> -		} else {
>> -			pos->start -= curr_map->start - curr_map->pgoff;
>> -			if (pos->end)
>> -				pos->end -= curr_map->start - curr_map->pgoff;
>> -			if (curr_map->dso != map->dso) {
>> -				rb_erase_init(&pos->rb_node, root);
>> -				symbols__insert(
>> -					&curr_map->dso->symbols[curr_map->type],
>> -					pos);
>> -				++moved;
>> -			} else {
>> -				++count;
>> -			}
>> +			continue;
>>  		}
>> +
>> +		pos->start -= curr_map->start - curr_map->pgoff;
>> +		if (pos->end)
>> +			pos->end -= curr_map->start - curr_map->pgoff;
>> +		symbols__insert(&curr_map->dso->symbols[curr_map->type], pos);
>> +		++count;
>>  	}
>>  
>>  	/* Symbols have been adjusted */
>>  	dso->adjust_symbols = 1;
>>  
>> -	return count + moved;
>> +	return count;
>>  }
>>  
>>  /*
>