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

[Arnaldo Carvalho de Melo]

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?

- Arnaldo
 
> For example:
> 
> When using kcore:
> 
> # readelf -a /proc/kcore
> 
>   Type           Offset             VirtAddr           PhysAddr
>                  FileSiz            MemSiz              Flags  Align
>   ...
>   LOAD           0x0000000000002000 0xffffffc000000000 0x0000000000000000    <-- kernel
>                  0x000000007fc00000 0x000000007fc00000  RWE    1000
>   LOAD           0xfffffffffc002000 0xffffffbffc000000 0x0000000000000000    <-- module
>                  0x0000000004000000 0x0000000004000000  RWE    1000
> 
> For modules memory area:
>   map->start = 0xffffffbffc000000, map->pgoff = 0xfffffffffc002000
> For normal kernel memory area:
>   map->start = 0xffffffc000000000, map->pgoff = 0x0000000000002000
> 
> Function A is a normal kernel function at:	0xffffffc00021b428.
> Function B is a function in module at: 		0xffffffbffc000000.
> 
> &A > &B before calling dso__split_kallsyms_for_kcore(), and they are
> already in the rbtree.
> 
> During dso__split_kallsyms_for_kcore(), when adjusting symbols using
>  pos->start -= curr_map->start - curr_map->pgoff
> 
> pos->start for A become: (0xffffffc00021b428 - 0xffffffc000000000 + 0x0000000000002000) = 0x21d428
> pos->start for B become: (0xffffffbffc000000 - 0xffffffbffc000000 + 0xfffffffffc002000) = 0xfffffffffc002000
> 
> &A < &B, the order is changed.
> 
> This patch rebuild rbtree unconditionally to ensure the rbtree is
> always healthy.
> 
> Signed-off-by: Wang Nan <wangnan0@xxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: Masami Hiramatsu <masami.hiramatsu.pt@xxxxxxxxxxx>
> Cc: Namhyung Kim <namhyung@xxxxxxxxxx>
> Cc: Zefan Li <lizefan@xxxxxxxxxx>
> Cc: pi3orama@xxxxxxx
> ---
> 
> Here is my test result on my aarch64 system:
> 
> *Step 1: create kprobes*
> 
> [root@localhost ~]# ./perf_arm64 probe -m /tmp/kernel_module.ko my_func
> Added new event:
>   probe:my_func (on my_func in kernel_module)
> 
> You can now use it in all perf tools, such as:
> 
> 	perf record -e probe:my_func -aR sleep 1
> 
> [root@localhost ~]# ./perf_arm64 probe sys_write
> Added new event:
>   probe:sys_write      (on sys_write)
> 
> You can now use it in all perf tools, such as:
> 
> 	perf record -e probe:sys_write -aR sleep 1
> 
> [root@localhost ~]# cat /sys/kernel/debug/kprobes/list 
> ffffffbffc000000  k  my_func+0x0  kernel_module [DISABLED]
> ffffffc00021b428  k  SyS_write+0x0    [DISABLED]
> 
> 
> *Step 2: rebuild perf without commit 98d3b25*
> 
> $ git log --oneline
> 3321d2b Revert "perf tools: Fix find_perf_probe_point_from_map() which incorrectly returns success"
> e054731 perf stat: Make stat options global
> 0014de1 perf sched latency: Fix thread pid reuse issue
> 98d3b25 perf tools: Fix find_perf_probe_point_from_map() which incorrectly returns success
> 956959f perf trace: Fix documentation for -i
> 
> 
> *Step 3: test and get the buggy result*
> 
> [root@localhost ~]# PAGER=cat ./perf_arm64 probe -l 
>   Error: Failed to show event list.
> 
> [root@localhost ~]# PAGER=cat ./perf_arm64 probe -v -l 
> map_groups__set_modules_path_dir: cannot open /lib/modules/4.1.12+ dir
> Problems setting modules path maps, continuing anyway...
> Opening /sys/kernel/debug/tracing//kprobe_events write=0
> Opening /sys/kernel/debug/tracing//uprobe_events write=0
> Parsing probe_events: p:probe/my_func kernel_module:my_func
> Group:probe Event:my_func probe:p
> Looking at the vmlinux_path (7 entries long)
> symsrc__init: cannot get elf header.
> Using /proc/kcore for kernel object code
> Using /proc/kallsyms for symbols
> try to find information at 3ffc000000 in kernel_module
> Failed to find module kernel_module.
> Failed to find the path for kernel_module: [kernel_module]
> Failed to find corresponding probes from debuginfo.
> Failed to synthesize perf probe point: 0
>   Error: Failed to show event list. Reason: Invalid argument (Code: -22)
> 
> 
> *Step 4: Introduce this patch*
> 
> $ git log --oneline
> 36a8201 perf tools: Rebuild rbtree when adjusting symbols for kcore
> 3321d2b Revert "perf tools: Fix find_perf_probe_point_from_map() which incorrectly returns success"
> e054731 perf stat: Make stat options global
> 0014de1 perf sched latency: Fix thread pid reuse issue
> 98d3b25 perf tools: Fix find_perf_probe_point_from_map() which incorrectly returns success
> 
> 
> *Step 5: Try again*
> 
> [root@localhost ~]# PAGER=cat ./perf_arm64 probe -l 
>   probe:my_func (on my_func in kernel_module)
>   probe:sys_write      (on sys_write)
> [root@localhost ~]# PAGER=cat ./perf_arm64 probe -v -l 
> map_groups__set_modules_path_dir: cannot open /lib/modules/4.1.12+ dir
> Problems setting modules path maps, continuing anyway...
> Opening /sys/kernel/debug/tracing//kprobe_events write=0
> Opening /sys/kernel/debug/tracing//uprobe_events write=0
> Parsing probe_events: p:probe/my_func kernel_module:my_func
> Group:probe Event:my_func probe:p
> Looking at the vmlinux_path (7 entries long)
> symsrc__init: cannot get elf header.
> Using /proc/kcore for kernel object code
> Using /proc/kallsyms for symbols
> Failed to find corresponding probes from debuginfo.
> Failed to find probe point from both of dwarf and map.
>   probe:my_func (on my_func in kernel_module)
> Parsing probe_events: p:probe/sys_write _text+1684520
> Group:probe Event:sys_write probe:p
> try to find information at 19b428 in kernel
> Looking at the vmlinux_path (7 entries long)
> symsrc__init: cannot get elf header.
> Failed to find the path for kernel: Invalid ELF file
> Failed to find corresponding probes from debuginfo.
>   probe:sys_write      (on sys_write)
> 
> ---
>  tools/perf/util/symbol.c | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
> 
> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
> index b4cc766..09bb6e8 100644
> --- a/tools/perf/util/symbol.c
> +++ b/tools/perf/util/symbol.c
> @@ -654,7 +654,7 @@ 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 *root = &dso->symbols[map->type];
>  	struct rb_node *next = rb_first(root);
>  
> @@ -677,25 +677,23 @@ static int dso__split_kallsyms_for_kcore(struct dso *dso, struct map *map,
>  			rb_erase_init(&pos->rb_node, root);
>  			symbol__delete(pos);
>  		} else {
> +			rb_erase_init(&pos->rb_node, root);
> +
>  			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;
> -			}
> +
> +			symbols__insert(
> +				&curr_map->dso->symbols[curr_map->type],
> +				pos);
> +			++count;
>  		}
>  	}
>  
>  	/* Symbols have been adjusted */
>  	dso->adjust_symbols = 1;
>  
> -	return count + moved;
> +	return count;
>  }
>  
>  /*
> -- 
> 1.8.3.4
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