Re: [PATCH v3 1/2] perf symbol: Correct address for bss symbols

[Leo Yan]

Hi Ian,

On Fri, Jul 29, 2022 at 10:13:04PM -0700, Ian Rogers wrote:

[...]

> I am seeing a problem with this patch with jvmti. To repro:
> 
> 1) download a Java workload dacapo-9.12-MR1-bach.jar from
> https://sourceforge.net/projects/dacapobench/
> 2) build perf such as "make -C tools/perf O=/tmp/perf NO_LIBBFD=1" it
> should detect Java and create /tmp/perf/libperf-jvmti.so
> 3) run perf with the jvmti agent:
> /tmp/perf/perf record -k 1 java -agentpath:/tmp/perf/libperf-jvmti.so
> -jar dacapo-9.12-MR1-bach.jar -n 10 fop
> 4) run perf inject:
> /tmp/perf/perf inject -i perf.data -o perf-injected.data -j
> 5) run perf report
> /tmp/perf/perf report -i  perf-injected.data | grep org.apache.fop
> 
> With this patch reverted I see lots of symbols like:
>      0.00%  java             jitted-388040-4656.so  [.]
> org.apache.fop.fo.FObj.bind(org.apache.fop.fo.PropertyList)
> 
> With the patch I see lots of:
> dso__load_sym_internal: failed to find program header for symbol:
> Lorg/apache/fop/fo/FObj;bind(Lorg/apache/fop/fo/PropertyList;)V
> st_value: 0x40

Thanks for sharing the steps, I can reproduce the issue.

I tried to add more logs to dump and hope can find specific pattern for
these symbols, one thing I observed that if a symbol fails to find
program header, it has the same values for st_value, shdr.sh_addr and
shdr.sh_offset: all of them are 0x40.  So that means if with you
proposed change in below, then we will get the file address is:

  file_addr = st_value - shdr.sh_addr + shdr.sh_offset = 0x40

Seems to me this is not reasonable: perf tries to add many symbols
with the same file address 0x40.

> Combining the old and new behaviors fixes the issue for me, wdyt?

So far we don't answer a question is what's the purpose for these JAVA
symbols.  I checked these symbols and concluded as:

- They are not label, this is because sym.st_info is 0x2, so its
  symbol type is STT_FUNC;
- They are from ".text" section;
- Symbol visibility is STV_DEFAULT;
- Symbol's section index number is 0x1, which is different from some
  special sections (STV_DEFAULT/SHN_COMMON/SHN_UNDEF/SHN_XINDEX).

This is a rough summary, these symbols are likewise the normal function
symbols, but they have special st_value (0x40) and has no matched the
program header for them.

If we rollback to use old offsets to calculate the symbol file address,
it still is incorrect.

I list all relevant symbols in: https://termbin.com/s0fb, for a reliable
fixing, could anyone with java experience shed some lights for handling
the symbols?

On the other hand, I can accept to simply change pr_warning() to
pr_debug4() to avoid warning flood, the log still can help us to find
potential symbol parsing issue, so far they are not false-positive
reporting.

Thanks,
Leo

> ```
> --- a/tools/perf/util/symbol-elf.c
> +++ b/tools/perf/util/symbol-elf.c
> @@ -1305,16 +1305,21 @@ 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);
> -                               continue;
> +                               pr_debug4("%s: adjusting symbol:
> st_value: %#" PRIx64 " "
> +                                       "sh_addr: %#" PRIx64 "
> sh_offset: %#" PRIx64 "\n",
> +                                       __func__, (u64)sym.st_value,
> (u64)shdr.sh_addr,
> +                                       (u64)shdr.sh_offset);
> +                               sym.st_value -= shdr.sh_addr - shdr.sh_offset;
> +                       } else {
> +                               pr_debug4("%s: adjusting symbol:
> st_value: %#" PRIx64 " "
> +                                       "p_vaddr: %#" PRIx64 "
> p_offset: %#" PRIx64 "\n",
> +                                       __func__, (u64)sym.st_value,
> (u64)phdr.p_vaddr,
> +                                       (u64)phdr.p_offset);
> +                               sym.st_value -= phdr.p_vaddr - phdr.p_offset;
>                        }
> -                       pr_debug4("%s: adjusting symbol: st_value: %#"
> PRIx64 " "
> -                                 "p_vaddr: %#" PRIx64 " p_offset: %#"
> PRIx64 "\n",
> -                                 __func__, (u64)sym.st_value,
> (u64)phdr.p_vaddr,
> -                                 (u64)phdr.p_offset);
> -                       sym.st_value -= phdr.p_vaddr - phdr.p_offset;
>                }
> 
>                demangled = demangle_sym(dso, kmodule, elf_name);
> ```
> 
> Thanks,
> Ian
> 
> >
> >                 demangled = demangle_sym(dso, kmodule, elf_name);
> > --
> > 2.25.1
> >