Re: [PATCH] depmod: create and use System.map.no_namespaces

Matthias Maennich
2019-10-07T06:46:37Z

Hi!

On Sat, Oct 05, 2019 at 04:53:56PM +0900, Masahiro Yamada wrote:
>On Sat, Oct 5, 2019 at 3:25 AM Lucas De Marchi
><lucas.de.marchi@gmail.com> wrote:
>>
>> On Fri, Oct 4, 2019 at 2:57 AM Matthias Maennich <maennich@google.com> wrote:
>> >
>> > depmod in its current version is not aware of symbol namespace in
>> > ksymtab entries introduced with 8651ec01daed ("module: add support for
>> > symbol namespaces."). They have the form
>> >
>> > __ksymtab_NAMESPACE.symbol_name
>> >
>> > A fix for kmod's depmod has been proposed [1]. In order to support older
>> > versions of depmod as well, create a System.map.no_namespaces during
>> > scripts/depmod.sh that has the pre-namespaces format. That way users do
>> > not immediately upgrade the userspace tool.
>> >
>> > [1] https://lore.kernel.org/linux-modules/20191004094136.166621-1-maennich@google.com/
>> >
>> > Reported-by: Stefan Wahren <stefan.wahren@i2se.com>
>> > Fixes: 8651ec01daed ("module: add support for symbol namespaces.")
>> > Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
>> > Cc: Lucas De Marchi <lucas.de.marchi@gmail.com>
>> > Cc: Jessica Yu <jeyu@kernel.org>
>> > Cc: Martijn Coenen <maco@android.com>
>> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> > Cc: linux-modules@vger.kernel.org
>> > Signed-off-by: Matthias Maennich <maennich@google.com>
>> > ---
>> >
>> > Please note this depends on the new ksymtab entry format proposed in
>> > https://lore.kernel.org/lkml/20191003075826.7478-2-yamada.masahiro@socionext.com/
>>
>> I don't really agree with that thought, more below.
>>
>> >
>> > That is likely to be merged soon as well as it fixes problems in 5.4-rc*, hence
>> > this patch depends on it.
>> >
>> > Cheers,
>> > Matthias
>> >
>> > .gitignore | 1 +
>> > scripts/depmod.sh | 8 +++++++-
>> > 2 files changed, 8 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/.gitignore b/.gitignore
>> > index 70580bdd352c..5ed58a7cb433 100644
>> > --- a/.gitignore
>> > +++ b/.gitignore
>> > @@ -59,6 +59,7 @@ modules.order
>> > /vmlinux-gdb.py
>> > /vmlinuz
>> > /System.map
>> > +/System.map.no_namespaces
>> > /Module.markers
>> > /modules.builtin.modinfo
>> >
>> > diff --git a/scripts/depmod.sh b/scripts/depmod.sh
>> > index e083bcae343f..602e1af072c7 100755
>> > --- a/scripts/depmod.sh
>> > +++ b/scripts/depmod.sh
>> > @@ -39,7 +39,13 @@ if $depmod_hack_needed; then
>> > KERNELRELEASE=99.98.$KERNELRELEASE
>> > fi
>> >
>> > -set -- -ae -F System.map
>> > +# Older versions of depmod do not support symbol namespaces in ksymtab entries,
>> > +# hence create an alternative System.map with namespace patched out to use for
>> > +# depmod. I.e. transform entries as follows:
>> > +# __ksymtab_NAMESPACE.symbol_name -> __ksymtab_symbol_name
>> > +sed 's/__ksymtab_.*\./__ksymtab_/' System.map > System.map.no_namespaces
>>
>> So people with old kmod will have to know they need to pass
>> System.map.no_namespaces rather than the usual
>> System.map. Also, distros will need to be update to also copy the new
>> file to the kernel package (or upgrade/patch kmod).
>>
>> I'd rather maintain the current format and fix the bug that patch is
>> fixing. The namespace
>> in the end IMO is just a small annoyance with a reason to exist.
>
>I agree, this fix is bad.
>We should not bother kmod or any tools.
>And System.map.no_namespaces is a cheesy workaround.

Thanks for the feedback!

Based on the discussion from last year about changing kmod/depmod for
this change to the ksymtab entries,
(https://lore.kernel.org/lkml/CAKi4VA+wvVxPxVSP6ruSR++TYvavKPNAQ5XhREF_9ZxeOKQirg@mail.gmail.com/),
I assumed this approach would be acceptable. The workaround (yeah, it is
a hack) is a way to mitigate the issue for people that can't update
depmod so easily. System.map.no_namespaces was not intended to be part
of any distribution, but rather used locally in depmod.sh. I could have
made this more clear.

>
>BTW, I expressed my negative opinion in the review process
>for the patch set. I am still not convinced with the
>namespace feature, but anyway it was merged
>(with poor review and test).
>
>
>
>Get back on track, probably the right fix would be to
>stop using __ksymtab_<namespace>.<symbol>.
>
>It is not used for any purposes but passing
><namespace> / <symbol> pairs to modpost.
>
>
>For example, __kstrtabns_##sym points to
>the namespace string, so it would be possible
>to parse it from modpost?
>
>Then, asm("__ksymtab_" #ns NS_SEPARATOR #sym)
>will go away.

I will give this a try.

Cheers,
Matthias

>
>
>Masahiro
>
>
>
>
>> Lucas De Marchi
>>
>> > +
>> > +set -- -ae -F System.map.no_namespaces
>> > if test -n "$INSTALL_MOD_PATH"; then
>> > set -- "$@" -b "$INSTALL_MOD_PATH"
>> > fi
>> > --
>> > 2.23.0.581.g78d2f28ef7-goog
>> >
>>
>>
>> --
>> Lucas De Marchi
>
>
>
>--
>Best Regards
>Masahiro Yamada