Re: [PATCH v3] scripts/link-vmlinux.sh: Add alias to duplicate symbols for kallsyms

[Alessandro Carminati <alessandro.carminati@gmail.com>]

Il giorno sab 2 set 2023 alle ore 08:36 Masahiro Yamada
<masahiroy@kernel.org> ha scritto:
>
> On Mon, Aug 28, 2023 at 8:45 PM Alessandro Carminati (Red Hat)
> <alessandro.carminati@gmail.com> wrote:
> >
> > From: Alessandro Carminati <alessandro.carminati@gmail.com>
> >
> > It is not uncommon for drivers or modules related to similar peripherals
> > to have symbols with the exact same name.
> > While this is not a problem for the kernel's binary itself, it becomes an
> > issue when attempting to trace or probe specific functions using
> > infrastructure like ftrace or kprobe.
> >
> > The tracing subsystem relies on the `nm -n vmlinux` output, which provides
> > symbol information from the kernel's ELF binary. However, when multiple
> > symbols share the same name, the standard nm output does not differentiate
> > between them. This can lead to confusion and difficulty when trying to
> > probe the intended symbol.
> >
> >  ~ # cat /proc/kallsyms | grep " name_show"
> >  ffffffff8c4f76d0 t name_show
> >  ffffffff8c9cccb0 t name_show
> >  ffffffff8cb0ac20 t name_show
> >  ffffffff8cc728c0 t name_show
> >  ffffffff8ce0efd0 t name_show
> >  ffffffff8ce126c0 t name_show
> >  ffffffff8ce1dd20 t name_show
> >  ffffffff8ce24e70 t name_show
> >  ffffffff8d1104c0 t name_show
> >  ffffffff8d1fe480 t name_show
> >
> > **kas_alias** addresses this challenge by extending the symbol names with
> > unique suffixes during the kernel build process.
> > The newly created aliases for these duplicated symbols are unique names
> > that can be fed to the ftracefs interface. By doing so, it enables
> > previously unreachable symbols to be probed.
> >
> >  ~ # cat /proc/kallsyms | grep " name_show"
> >  ffffffff974f76d0 t name_show
> >  ffffffff974f76d0 t name_show__alias__6340
> >  ffffffff979cccb0 t name_show
> >  ffffffff979cccb0 t name_show__alias__6341
> >  ffffffff97b0ac20 t name_show
> >  ffffffff97b0ac20 t name_show__alias__6342
> >  ffffffff97c728c0 t name_show
> >  ffffffff97c728c0 t name_show__alias__6343
> >  ffffffff97e0efd0 t name_show
> >  ffffffff97e0efd0 t name_show__alias__6344
> >  ffffffff97e126c0 t name_show
> >  ffffffff97e126c0 t name_show__alias__6345
> >  ffffffff97e1dd20 t name_show
> >  ffffffff97e1dd20 t name_show__alias__6346
> >  ffffffff97e24e70 t name_show
> >  ffffffff97e24e70 t name_show__alias__6347
> >  ffffffff981104c0 t name_show
> >  ffffffff981104c0 t name_show__alias__6348
> >  ffffffff981fe480 t name_show
> >  ffffffff981fe480 t name_show__alias__6349
> >  ~ # echo "p:kprobes/evnt1 name_show__alias__6349" \
> >  > >/sys/kernel/tracing/kprobe_events
> >  ~ # cat /sys/kernel/tracing/kprobe_events
> >  p:kprobes/evnt1 name_show__alias__6349
> >
> > Changes from v1:
> > - Integrated changes requested by Masami to exclude symbols with prefixes
> >   "_cfi" and "_pfx".
> > - Introduced a small framework to handle patterns that need to be excluded
> >   from the alias production.
> > - Excluded other symbols using the framework.
> > - Introduced the ability to discriminate between text and data symbols.
> > - Added two new config symbols in this version: CONFIG_KALLSYMS_ALIAS_DATA,
> >   which allows data for data, and CONFIG_KALLSYMS_ALIAS_DATA_ALL, which
> >   excludes all filters and provides an alias for each duplicated symbol.
> >
> > https://lore.kernel.org/all/20230711151925.1092080-1-alessandro.carminati@gmail.com/
> >
> > Changes from v2:
> > - Alias tags are created by querying DWARF information from the vmlinux.
> > - The filename + line number is normalized and appended to the original name.
> > - The tag begins with '@' to indicate the symbol source.
> > - Not a change, but worth mentioning, since the alias is added to the existing
> >   list, the old duplicated name is preserved, and the livepatch way of dealing
> >   with duplicates is maintained.
> > - Acknowledging the existence of scenarios where inlined functions declared in
> >   header files may result in multiple copies due to compiler behavior, though
> >    it is not actionable as it does not pose an operational issue.
> > - Highlighting a single exception where the same name refers to different
> >   functions: the case of "compat_binfmt_elf.c," which directly includes
> >   "binfmt_elf.c" producing identical function copies in two separate
> >   modules.
> >
> > sample from new v3
> >
> >  ~ # cat /proc/kallsyms | grep gic_mask_irq
> >  ffffd0b03c04dae4 t gic_mask_irq
> >  ffffd0b03c04dae4 t gic_mask_irq@_drivers_irqchip_irq-gic_c_167
> >  ffffd0b03c050960 t gic_mask_irq
> >  ffffd0b03c050960 t gic_mask_irq@_drivers_irqchip_irq-gic-v3_c_404
> >  ~ #
> >
> > https://lore.kernel.org/all/20230714150326.1152359-1-alessandro.carminati@gmail.com/
> >
> > Signed-off-by: Alessandro Carminati (Red Hat) <alessandro.carminati@gmail.com>
> > ---
> >  init/Kconfig                        |  36 ++++
> >  scripts/Makefile                    |   4 +
> >  scripts/kas_alias/Makefile          |   4 +
> >  scripts/kas_alias/a2l.c             | 268 ++++++++++++++++++++++++++++
> >  scripts/kas_alias/a2l.h             |  32 ++++
> >  scripts/kas_alias/duplicates_list.c |  70 ++++++++
> >  scripts/kas_alias/duplicates_list.h |  15 ++
> >  scripts/kas_alias/item_list.c       | 230 ++++++++++++++++++++++++
> >  scripts/kas_alias/item_list.h       |  26 +++
> >  scripts/kas_alias/kas_alias.c       | 217 ++++++++++++++++++++++
> >  scripts/link-vmlinux.sh             |  11 +-
> >  11 files changed, 910 insertions(+), 3 deletions(-)
>
>
> I added some review comments in another thread, but
> one of the biggest concerns might be "910 insertions".

Based on the feedback I received in the reviews, I need to overhaul the
code, potentially reducing its size. What would be a reasonable number
of lines for this feature?

>
>
> What this program does is quite simple,
> "find duplicated names, and call addr2line".
>
>
>
> You wrote a lot of code to self-implement these:
>
>  - sort function
>  - parse PATH env variable to find addr2line
>  - fork addr2line to establish pipe communications

Some of these functions might become obsolete in the upcoming v4, which
will certainly reduce the line count.

>
>
>
> Have you considered writing the code in Python (or Perl)?
> Is it too slow?

From my perspective, there is a concern that using Python or Perl might
result in slower performance. My proficiency in Python and Perl is
limited, so I did not initially consider them as viable options for
implementing this solution.

>
> Most of the functions you implemented are already
> available in script languages.
>
>
>
> I am not sure if "@<file-path>" is a good solution,
> but the amount of the added code looks too much to me.
>

I had reservations about using the '@' symbol to decorate the alias because
it's not a character commonly found in the kallsyms output. However, after
careful consideration, I arrived at the conclusion that it was suitable for
the task because it would make the alias stand-out and be easily
identifiable.
I'm open to any suggestions or alternative approaches you may have on this
matter.

>
>
>
> --
> Best Regards
> Masahiro Yamada

Thank you