[PATCH] perf symbol: correction while adjusting symbol

[PATCH] perf symbol: correction while adjusting symbol

[Ajay Kaher]

perf doesn't provide proper symbol information for specially crafted
.debug files.

Sometimes .debug file may not have similar program header as runtime
ELF file. For example if we generate .debug file using objcopy
--only-keep-debug resulting file will not contain .text, .data and
other runtime sections. That means corresponding program headers will
have zero FileSiz and modified Offset.

Example: program header of text section of libxxx.so:

Type           Offset             VirtAddr           PhysAddr
               FileSiz            MemSiz              Flags  Align
LOAD        0x00000000003d3000 0x00000000003d3000 0x00000000003d3000
            0x000000000055ae80 0x000000000055ae80  R E    0x1000

Same program header after executing:
objcopy --only-keep-debug libxxx.so libxxx.so.debug

LOAD        0x0000000000001000 0x00000000003d3000 0x00000000003d3000
            0x0000000000000000 0x000000000055ae80  R E    0x1000

Offset and FileSiz have been changed. 

Following formula will not provide correct value, if program header
taken from .debug file (syms_ss):

    sym.st_value -= phdr.p_vaddr - phdr.p_offset;

Correct program header information is located inside runtime ELF
file (runtime_ss).

Fixes: 2d86612aacb7 ("perf symbol: Correct address for bss symbols")
Signed-off-by: Ajay Kaher <akaher@vmware.com>
---
 tools/perf/util/symbol-elf.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index 5fba57c..8dde436 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -1129,7 +1129,7 @@ int dso__load_sym(struct dso *dso, struct map *map, struct symsrc *syms_ss,
 			   (!used_opd && syms_ss->adjust_symbols)) {
 			GElf_Phdr phdr;
 
-			if (elf_read_program_header(syms_ss->elf,
+			if (elf_read_program_header(runtime_ss->elf,
 						    (u64)sym.st_value, &phdr)) {
 				pr_warning("%s: failed to find program header for "
 					   "symbol: %s st_value: %#" PRIx64 "\n",
-- 
2.7.4

Re: [PATCH] perf symbol: correction while adjusting symbol

[Leo Yan]

Hi Ajay,

On Wed, Nov 23, 2022 at 03:48:16PM +0530, Ajay Kaher wrote:
> perf doesn't provide proper symbol information for specially crafted
> .debug files.
> 
> Sometimes .debug file may not have similar program header as runtime
> ELF file. For example if we generate .debug file using objcopy
> --only-keep-debug resulting file will not contain .text, .data and
> other runtime sections. That means corresponding program headers will
> have zero FileSiz and modified Offset.
> 
> Example: program header of text section of libxxx.so:
> 
> Type           Offset             VirtAddr           PhysAddr
>                FileSiz            MemSiz              Flags  Align
> LOAD        0x00000000003d3000 0x00000000003d3000 0x00000000003d3000
>             0x000000000055ae80 0x000000000055ae80  R E    0x1000
> 
> Same program header after executing:
> objcopy --only-keep-debug libxxx.so libxxx.so.debug
> 
> LOAD        0x0000000000001000 0x00000000003d3000 0x00000000003d3000
>             0x0000000000000000 0x000000000055ae80  R E    0x1000
> 
> Offset and FileSiz have been changed. 
> 
> Following formula will not provide correct value, if program header
> taken from .debug file (syms_ss):
> 
>     sym.st_value -= phdr.p_vaddr - phdr.p_offset;
> 
> Correct program header information is located inside runtime ELF
> file (runtime_ss).
> 
> Fixes: 2d86612aacb7 ("perf symbol: Correct address for bss symbols")
> Signed-off-by: Ajay Kaher <akaher@vmware.com>


Just curious how you can produce this issue?  IIUC, the runtime symbol
files are copied into .debug folder and they can be found by perf tool
by matching build ID.  Seems to me, you manully use
"objcopy --only-keep-debug" command to strip runtime info from elf files
under .debug folder.  Do I understand correctly?

Though I have above question, this patch itself looks good to me,
thanks for the fixing!

Reviewed-by: Leo Yan <leo.yan@linaro.org>

Re: [PATCH] perf symbol: correction while adjusting symbol

[Ajay Kaher]

> On 24/11/22, 3:20 PM, "Leo Yan" <leo.yan@linaro.org> wrote:
>>
>> Fixes: 2d86612aacb7 ("perf symbol: Correct address for bss symbols")
>> Signed-off-by: Ajay Kaher <akaher@vmware.com>
>
> Just curious how you can produce this issue?  IIUC, the runtime symbol
> files are copied into .debug folder and they can be found by perf tool
> by matching build ID.  Seems to me, you manully use
> "objcopy --only-keep-debug" command to strip runtime info from elf files
> under .debug folder.  Do I understand correctly?

While creating debuginfo rpm we can have extra stripping.

> Though I have above question, this patch itself looks good to me,
> thanks for the fixing!
>
> Reviewed-by: Leo Yan <leo.yan@linaro.org>

Thanks.
- Ajay

Re: [PATCH] perf symbol: correction while adjusting symbol

[Arnaldo Carvalho de Melo]

Em Thu, Nov 24, 2022 at 05:50:23PM +0800, Leo Yan escreveu:
> Hi Ajay,
> 
> On Wed, Nov 23, 2022 at 03:48:16PM +0530, Ajay Kaher wrote:
> > perf doesn't provide proper symbol information for specially crafted
> > .debug files.
> > 
> > Sometimes .debug file may not have similar program header as runtime
> > ELF file. For example if we generate .debug file using objcopy
> > --only-keep-debug resulting file will not contain .text, .data and
> > other runtime sections. That means corresponding program headers will
> > have zero FileSiz and modified Offset.
> > 
> > Example: program header of text section of libxxx.so:
> > 
> > Type           Offset             VirtAddr           PhysAddr
> >                FileSiz            MemSiz              Flags  Align
> > LOAD        0x00000000003d3000 0x00000000003d3000 0x00000000003d3000
> >             0x000000000055ae80 0x000000000055ae80  R E    0x1000
> > 
> > Same program header after executing:
> > objcopy --only-keep-debug libxxx.so libxxx.so.debug
> > 
> > LOAD        0x0000000000001000 0x00000000003d3000 0x00000000003d3000
> >             0x0000000000000000 0x000000000055ae80  R E    0x1000
> > 
> > Offset and FileSiz have been changed. 
> > 
> > Following formula will not provide correct value, if program header
> > taken from .debug file (syms_ss):
> > 
> >     sym.st_value -= phdr.p_vaddr - phdr.p_offset;
> > 
> > Correct program header information is located inside runtime ELF
> > file (runtime_ss).
> > 
> > Fixes: 2d86612aacb7 ("perf symbol: Correct address for bss symbols")
> > Signed-off-by: Ajay Kaher <akaher@vmware.com>
> 
> 
> Just curious how you can produce this issue?  IIUC, the runtime symbol
> files are copied into .debug folder and they can be found by perf tool
> by matching build ID.  Seems to me, you manully use
> "objcopy --only-keep-debug" command to strip runtime info from elf files
> under .debug folder.  Do I understand correctly?
> 
> Though I have above question, this patch itself looks good to me,
> thanks for the fixing!
> 
> Reviewed-by: Leo Yan <leo.yan@linaro.org>

Had to apply it manually, as it was done on a codebase older than this:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6d518ac7be6223811ab947897273b1bbef846180

author	Ian Rogers <irogers@google.com>	2022-07-31 09:49:23 -0700
committer	Arnaldo Carvalho de Melo <acme@redhat.com>	2022-08-01 09:30:36 -0300

@@ -1305,16 +1305,29 @@ dso__load_sym_internal(struct dso *dso, struct map *map, struct symsrc *syms_ss,

 			if (elf_read_program_header(syms_ss->elf,
 						    (u64)sym.st_value, &phdr)) {
-				pr_warning("%s: failed to find program header for "
+				pr_debug4("%s: failed to find program header for "
 					   "symbol: %s st_value: %#" PRIx64 "\n",
 					   __func__, elf_name, (u64)sym.st_value);