Message-ID: <CAK7LNARZ07hnAH_LT0QDKuHmn23Mgs-v124B3rpVjvBD=j21fA@mail.gmail.com>
Date: Wed, 3 Jan 2024 14:16:33 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Alessandro Carminati <alessandro.carminati@...il.com>
Cc: Luis Chamberlain <mcgrof@...nel.org>, Petr Mladek <pmladek@...e.com>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, 
	Daniel Bristot de Oliveira <bristot@...nel.org>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Nathan Chancellor <nathan@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, 
	Nicolas Schier <nicolas@...sle.eu>, Alexander Lobakin <aleksander.lobakin@...el.com>, 
	Nick Alcock <nick.alcock@...cle.com>, Kris Van Hees <kris.van.hees@...cle.com>, 
	Eugene Loh <eugene.loh@...cle.com>, Francis Laniel <flaniel@...ux.microsoft.com>, 
	Viktor Malik <vmalik@...hat.com>, Tom Rix <trix@...hat.com>, linux-kbuild@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	llvm@...ts.linux.dev
Subject: Re: [PATCH v7] scripts/link-vmlinux.sh: Add alias to duplicate
 symbols for kallsyms

On Mon, Jan 1, 2024 at 8:11 PM Alessandro Carminati
<alessandro.carminati@...il.com> wrote:
>
> Hello,
>
>
>
> Il giorno dom 31 dic 2023 alle ore 06:35 Masahiro Yamada
> <masahiroy@...nel.org> ha scritto:
> >
> > On Tue, Dec 5, 2023 at 6:48 AM Alessandro Carminati (Red Hat)
> > <alessandro.carminati@...il.com> wrote:
> > >
> > > In the kernel environment, situations frequently arise where identical
> > > names are shared among symbols within both the core image and modules.
> > > While this doesn't pose issues for the kernel's binary itself, it
> > > complicates trace or probe operations using tools like kprobe.
> > >
> > > This patch introduces "kas_alias" to address this challenge.
> > >
> > > During the kernel's build process, just before linking the vmlinux
> > > image in the "scripts/link-vmlinux.sh", symbol name frequencies are
> > > collected.
> > > This collection includes both the core kernel components and modules.
> > > Subsequently, within the same action, the nm data relative to vmlinux
> > > is modified by adding aliases based on the comprehensive symbol
> > > information gathered.
> > >
> > > The collection process occurs in two phases:
> > >
> > > 1. First phase: Executed during the linking of vmlinux, "kas_alias" scans
> > >    all symbols provided by the 'nm' data against the vmlinux core image
> > >    and all objects used for module linkage. This phase requires all
> > >    modules objects to be produced at this stage, thereby adding a vmlinux
> > >    dependency for linking modules in 'scripts/Makefile.modfinal'.
> > >
> > > 2. Second phase: In a subsequent run in the same build, "kas_alias"
> > >    processes module objects and injects aliases into the objects' symbol
> > >    tables where necessary. This operation is done by modifying
> > >    'scripts/Makefile.modfinal' to include an action for each processed
> > >    module.
> > >
> > > Example:
> > >
> > > Consider the symbol "device_show", you can expect an output like the
> > > following:
> > >
> > >  ~ # cat /proc/kallsyms | grep " name_show"
> > > ffffcaa2bb4f01c8 t name_show
> > > ffffcaa2bb4f01c8 t name_show@...nel_irq_irqdesc_c_264
> > > ffffcaa2bb9c1a30 t name_show
> > > ffffcaa2bb9c1a30 t name_show@...vers_pnp_card_c_186
> > > ffffcaa2bbac4754 t name_show
> > > ffffcaa2bbac4754 t name_show@...vers_regulator_core_c_678
> > > ffffcaa2bbba4900 t name_show
> > > ffffcaa2bbba4900 t name_show@...vers_base_power_wakeup_stats_c_93
> > > ffffcaa2bbec4038 t name_show
> > > ffffcaa2bbec4038 t name_show@...vers_rtc_sysfs_c_26
> > > ffffcaa2bbecc920 t name_show
> > > ffffcaa2bbecc920 t name_show@...vers_i2c_i2c_core_base_c_660
> > > ffffcaa2bbed3840 t name_show
> > > ffffcaa2bbed3840 t name_show@...vers_i2c_i2c_dev_c_100
> > > ffffcaa2bbef7210 t name_show
> > > ffffcaa2bbef7210 t name_show@...vers_pps_sysfs_c_66
> > > ffffcaa2bbf03328 t name_show
> > > ffffcaa2bbf03328 t name_show@...vers_hwmon_hwmon_c_72
> > > ffffcaa2bbff6f3c t name_show
> > > ffffcaa2bbff6f3c t name_show@...vers_remoteproc_remoteproc_sysfs_c_215
> > > ffffcaa2bbff8d78 t name_show
> > > ffffcaa2bbff8d78 t name_show@...vers_rpmsg_rpmsg_core_c_455
> > > ffffcaa2bbfff7a4 t name_show
> > > ffffcaa2bbfff7a4 t name_show@...vers_devfreq_devfreq_c_1395
> > > ffffcaa2bc001f60 t name_show
> > > ffffcaa2bc001f60 t name_show@...vers_extcon_extcon_c_389
> > > ffffcaa2bc009890 t name_show
> > > ffffcaa2bc009890 t name_show@...vers_iio_industrialio_core_c_1396
> > > ffffcaa2bc01212c t name_show
> > > ffffcaa2bc01212c t name_show@...vers_iio_industrialio_trigger_c_51
> > > ffffcaa2bc025e2c t name_show
> > > ffffcaa2bc025e2c t name_show@...vers_fpga_fpga_mgr_c_618
> > > ffffcaa2a052102c t name_show    [hello]
> > > ffffcaa2a052102c t name_show@...lo_hello_c_8    [hello]
> > > ffffcaa2a051955c t name_show    [rpmsg_char]
> > > ffffcaa2a051955c t name_show@...vers_rpmsg_rpmsg_char_c_365     [rpmsg_char]
> > >
> > > where hello, is a plain helloworld module built OOT.
> > >
> > > Tested-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> > > Signed-off-by: Alessandro Carminati (Red Hat) <alessandro.carminati@...il.com>
> > >
> > > ---
> > >
> > > NOTE1:
> > > 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
> > >
> > > 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.
> > >
> > > https://lore.kernel.org/all/20230714150326.1152359-1-alessandro.carminati@gmail.com/
> > >
> > > 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@gmail.com/
> > >
> > > Changes from v4:
> > > * Fixed the O=<build dir> build issue
> > > * The tool halts execution upon encountering major issues, thereby ensuring
> > >   the pipeline is interrupted.
> > > * A cmdline option to specify the source directory added.
> > > * Minor code adjusments.
> > > * Tested on mips32 and i386
> > >
> > > https://lore.kernel.org/all/20230919193948.465340-1-alessandro.carminati@gmail.com/
> > >
> > > Changes from v5:
> > > * Regex filter extended to all symbols
> > > * Alias creation extended to module objects
> > > * Code cleaned and commented
> > > * kas_alias verbose execution via KAS_ALIAS_DEBUG env variable
> > > * CONFIG_KALLSYMS_ALIAS_SRCLINE selects KBUILD_BUILTIN to ensure no races
> > >   during modules build
> > > * Tested on x86_64, aarch64 and i386
> > >
> > > https://lore.kernel.org/all/20230927173516.1456594-1-alessandro.carminati@gmail.com/
> > >
> > > Changes from v6:
> > > * Generate a file to facilitate the construction of custom OOT modules using aliases.
> > >   In this context, it is needed to export the modules.symbfreq file, a product of
> > >   the build.
> > > * Previously, kas_alias was intended to run once, executing all tasks within the same
> > >   session. In this version, two targets have been introduced to split the work into
> > >   distinct phases: one for creating aliases for the core image and another for
> > >   generating aliases for modules.
> > > * The module aliases production statement has been inserted in the
> > >   scripts/Makefile.modfinal. Instead of being executed individually for each module,
> > >   it now runs once for all modules.
> > > * The approach for excluding symbols in .init and .exit ELF sections has undergone a
> > >   rewrite.
> > > * kas_alias is now capable of applying aliases to OOT modules. However, for this to
> > >   occur, the 'modules.symbfreq' file needs to be accessible during the build.
> > >   If it is not found, aliases won't be added.
> > >
> > > https://lore.kernel.org/all/20231024201157.748254-1-alessandro.carminati@gmail.com/
> >
> >
> >
> >
> >
> > I am concerned with your code.
> > At least, I'd like you to understand the Makefile principle.
> >
> >
> >
> >
> >
> > <output>: <input>
> >        <command>
> >
> >
> >
> > The <command> reads <input> and generates <output>.
> >
> > The <command> must not modify <input>.
> >
> >
> > If you deviate from this principle,
> > we will never know where build artifacts are updated.
> >
> >
> >
> >
> > Your code looks like this:
> >
> >
> > %.ko:  %.o %.mod.o
> >         Your script inserts aliases to %.o
> >         Link %.o and %.mod.o into %.ko
> >
> >
> >
> > Your script updates the input file, %.o.
> > This is never acceptable.
> >
> >
> >
>
> I truly appreciate your guidance regarding the Makefile principle. I realize
> now that my self-taught approach overlooked key aspects, particularly the
> importance of not modifying the input file, and I'm committed to aligning my
> code with this principle.
>
> Before I proceed, I seek a better understanding of how the kernel community
> aims to address the duplicate symbols issue.
>
> > Anyway, @FILENAME_LINE seems a cheesy workaround after all,
> > and I do not see a good reason to grow the ugliness.
>
> Regarding the FILENAME_LINE implementation, my assumption stemmed from
> notable comments by
>
> Luis Chamberlain
> https://lore.kernel.org/all/ZLVxUQiC5iF+xTPQ@bombadil.infradead.org/
>
> Petr Mladek
> https://lore.kernel.org/all/ZQFtdJEKJ9taYpA0@alley/
>
> within the threads related to my work, but the idea itself originated
> elsewhere: the erlies message I'm aware of, carrying this contents is a
> Petr's message as answer to another thread.
> https://lore.kernel.org/all/ZEeO_rWL4bDh5vui@alley/
>
> If FILENAME_LINE doesn't serve the community's purpose, and my interpretation
> was mistaken, I apologize for any inconvenience caused and will cease work on
> this aspect.


There is no common opinion in the community.
It is just that every person has a different thought.



When I use GDB for debugging, the debug info is optional;
even without debug info, I can use minimal functionality;
assembler-level debugging, break-point setting by specifying
an address.

By loading the symbol table, I can have better experiences;
for example, source code debugging, break-point setting by
a function name, etc.




I wonder if we can do something similar for this case.
We can use ftrace/kprobe without CONFIG_DEBUG_INFO.
This covers most use cases because kallsyms records
(address, symbol-name) tuples.

With debug info loaded, every symbol address is associated
with more info such as file name, line number, etc.


In many distributions, the debug info files of vmlinux
and modules are available under /usr/lib/debug/.


Your patch already depends on CONFIG_DEBUG_INFO,
so you have the ones in your build directory at least.

Adding <func>@<filename>_<line> aliases seems a
niche workaround, as this is duplicated data
transformed from what you already have in debug info.




--
Best Regards
Masahiro Yamada

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