Re: [PATCH v4] scripts/link-vmlinux.sh: Add alias to duplicate symbols for kallsyms

[Alessandro Carminati]

Hello Masami,

Thank you for the feedback.


Il giorno mer 20 set 2023 alle ore 01:52 Masami Hiramatsu
<mhiramat@xxxxxxxxxx> ha scritto:
>
> On Tue, 19 Sep 2023 19:39:48 +0000
> "Alessandro Carminati (Red Hat)" <alessandro.carminati@xxxxxxxxx> wrote:
>
> > 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 enhancing symbol names with
> > meaningful suffixes generated from the source file and line number
> > during the kernel build process.
> > These newly generated aliases provide tracers with the ability to
> > comprehend the symbols they are interacting with when utilizing the
> > ftracefs interface.
> > This approach may also allow for the probing by name of previously
> > inaccessible symbols.
> >
> >  ~ # cat /proc/kallsyms | grep gic_mask_irq
> >  ffffd15671e505ac t gic_mask_irq
> >  ffffd15671e505ac t gic_mask_irq@drivers_irqchip_irq_gic_c_167
> >  ffffd15671e532a4 t gic_mask_irq
> >  ffffd15671e532a4 t gic_mask_irq@drivers_irqchip_irq_gic_v3_c_407
> >  ~ #
> >
> > 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@xxxxxxxxx/
> >
> > 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.
> >
> > https://lore.kernel.org/all/20230714150326.1152359-1-alessandro.carminati@xxxxxxxxx/
> >
> > Changes from v3:
> > - kas_alias was rewritten in Python to create a more concise and
> >   maintainable codebase.
> > - The previous automation process used by kas_alias to locate the vmlinux
> >   and the addr2line has been replaced with an explicit command-line switch
> >   for specifying these requirements.
> > - addr2line has been added into the main Makefile.
> > - A new command-line switch has been introduced, enabling users to extend
> >   the alias to global data names.
> >
> > https://lore.kernel.org/all/20230828080423.3539686-1-alessandro.carminati@xxxxxxxxx/
> >
> > NOTE:
> > About the symbols name duplication that happens as consequence of the
> > inclusion compat_binfmt_elf.c does, it is evident that this corner is
> > inherently challenging the addr2line approach.
> > Attempting to conceal this limitation would be counterproductive.
> >
> > compat_binfmt_elf.c includes directly binfmt_elf.c, addr2line can't help
> > but report all functions and data declared by that file, coming from
> > binfmt_elf.c.
> >
> > My position is that, rather than producing a more complicated pipeline
> > to handle this corner case, it is better to fix the compat_binfmt_elf.c
> > anomaly.
> >
> > This patch does not deal with the two potentially problematic symbols
> > defined by compat_binfmt_elf.c
>
> Hi, it looks good but if I build with O=<build dir>, I have this error.
>
> /ksrc/linux/scripts/link-vmlinux.sh: 176: scripts/kas_alias.py: not found
>
> Maybe something wrong with setting the path?

You're spot on.
Switching from C to Python for kas_alias has this little quirk where it
doesn't end up in the target script directory anymore. So, if you're using
a specific target build directory, you'll have to hunt down the script in
the source tree. I'll sort this out once I've collected all the feedback.

Regards
Alessandro